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Default Experiment (valve).

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?





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Default Experiment (valve).

On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?



You're making it too hard.

Just slip a 2N7000 under your 900V device (making a cascode), tie the
900V device gate to the +30V, and bias the 2N7000 as desired.

Personally I've done this where the 900V device was a toob, and, in
the cathode circuit was an NPN with an OpAmp wrapped around it ;-)

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

The only thing bipartisan in this country is hypocrisy
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Default Experiment (valve).


"Jim Thompson" wrote in
message ...
On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising
and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in
an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?



You're making it too hard.

Just slip a 2N7000 under your 900V device (making a cascode), tie the
900V device gate to the +30V, and bias the 2N7000 as desired.

Personally I've done this where the 900V device was a toob, and, in
the cathode circuit was an NPN with an OpAmp wrapped around it ;-)



Alternatively I could just use a depletion mode power MOSFET - but none of
the suppliers I use have ever heard of the few that are on the market.

And that would spoil all the fun of figuring it out with easy to get parts.

The main problem I was trying to solve is that designing a self biasing
enhancement MOSFET stage for this sort of application can be a bit tricky.


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"flipper" wrote in message
...
On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising
and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in
an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?


What's the goal? To 'fix' a broken radio? Why not just buy the proper
valve and plug it in?


To some extent its just a bit of fun.

One significant inconvenience is that most UK radios had series heater
chains, so a dropper resistor would have to be wired in to replace the
valve's heater.



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Default Experiment (valve).

Have ian field wrote:
Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?




Have those tube replacements been on the (military) market??
Or does my memory play tricks on me?
Google: fet tube replacements
.....
1000.000 hits. Rescued by google.


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Default Experiment (valve).

On Thu, 29 Apr 2010 17:04:28 +0100, "ian field"
wrote:


"Jim Thompson" wrote in
message ...
On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising
and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in
an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?



You're making it too hard.

Just slip a 2N7000 under your 900V device (making a cascode), tie the
900V device gate to the +30V, and bias the 2N7000 as desired.

Personally I've done this where the 900V device was a toob, and, in
the cathode circuit was an NPN with an OpAmp wrapped around it ;-)



Alternatively I could just use a depletion mode power MOSFET - but none of
the suppliers I use have ever heard of the few that are on the market.

And that would spoil all the fun of figuring it out with easy to get parts.

The main problem I was trying to solve is that designing a self biasing
enhancement MOSFET stage for this sort of application can be a bit tricky.


You could bias the gate up, to +8 maybe, and put a source resistor to
ground, bypassed maybe. 1 volt p-p of AC coupled gate drive would be
in the ballpark.

Mosfets have higher transconductances than toobs ever dreamed of, so
you'll have lots of gain.

John

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Default Experiment (valve).


"flipper" wrote in message
...
On Thu, 29 Apr 2010 18:50:04 +0100, "ian field"
wrote:


"flipper" wrote in message
. ..
On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve
radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising
and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in
an
old valve radio, it has 3 stages to maintain its invertingness in case
the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang
it
will make?

What's the goal? To 'fix' a broken radio? Why not just buy the proper
valve and plug it in?


To some extent its just a bit of fun.

One significant inconvenience is that most UK radios had series heater
chains, so a dropper resistor would have to be wired in to replace the
valve's heater.



Here's a quick throw together idea.


What's the typical anode current on an average table top set?


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Default Experiment (valve).


"flipper" wrote in message
...
On Fri, 30 Apr 2010 22:22:03 +0100, "ian field"
wrote:


"flipper" wrote in message
. ..
On Thu, 29 Apr 2010 18:50:04 +0100, "ian field"
wrote:


"flipper" wrote in message
m...
On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I
would
like to experiment with, but unfortunately I don't have an old valve
radio
to use as a testbed.

The component values that are pencilled in will doubtless need
revising
and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode
in
an
old valve radio, it has 3 stages to maintain its invertingness in case
the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang
it
will make?

What's the goal? To 'fix' a broken radio? Why not just buy the proper
valve and plug it in?

To some extent its just a bit of fun.

One significant inconvenience is that most UK radios had series heater
chains, so a dropper resistor would have to be wired in to replace the
valve's heater.



Here's a quick throw together idea.


What's the typical anode current on an average table top set?


I don't work on them so I'm not sure but I would imagine it's close to
the 'typical operation' given in the respective tube datasheet. Like,
for the 35L6 (octal AA5 output, 150mA heaters, circa 1939), it's given
as 40mA (idle) at 110V (tube rectified, typically a 35Z4, U.S. AC line



Last night I had a flick through my old Mazda valve data book and the
popular audio output valves range from Ia about 30 to 70mA, the BF246B would
be good enough for all but the most powerful types but the 246C would cover
almost any eventuality.

The best I have in the parts drawer is BF245C, I'd have to select for
highest Idss and still gang at least 2 devices - and tweak the individual
decoupled source resistors to balance the dissipation.


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Default Experiment (valve).


"flipper" wrote in message
...
On Sat, 1 May 2010 13:53:37 +0100, "ian field"
wrote:


"flipper" wrote in message
. ..
On Fri, 30 Apr 2010 22:22:03 +0100, "ian field"
wrote:


"flipper" wrote in message
m...
On Thu, 29 Apr 2010 18:50:04 +0100, "ian field"
wrote:


"flipper" wrote in message
news:bedjt5t8ds9vopi85fvebp8us54h90835c@4ax. com...
On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I
would
like to experiment with, but unfortunately I don't have an old valve
radio
to use as a testbed.

The component values that are pencilled in will doubtless need
revising
and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P
pentode
in
an
old valve radio, it has 3 stages to maintain its invertingness in
case
the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a
bang
it
will make?

What's the goal? To 'fix' a broken radio? Why not just buy the
proper
valve and plug it in?

To some extent its just a bit of fun.

One significant inconvenience is that most UK radios had series heater
chains, so a dropper resistor would have to be wired in to replace the
valve's heater.



Here's a quick throw together idea.

What's the typical anode current on an average table top set?

I don't work on them so I'm not sure but I would imagine it's close to
the 'typical operation' given in the respective tube datasheet. Like,
for the 35L6 (octal AA5 output, 150mA heaters, circa 1939), it's given
as 40mA (idle) at 110V (tube rectified, typically a 35Z4, U.S. AC line



Last night I had a flick through my old Mazda valve data book and the
popular audio output valves range from Ia about 30 to 70mA, the BF246B
would
be good enough for all but the most powerful types but the 246C would
cover
almost any eventuality.

The best I have in the parts drawer is BF245C, I'd have to select for
highest Idss and still gang at least 2 devices - and tweak the individual
decoupled source resistors to balance the dissipation.


Are you trying to just make a test device or a 'universal device' of
some sort?

I didn't mean to suggest the BF246C was the best thing to use. It's
just the best ready made jfet model I had (and did seem 'good enough'
for a test shot anyway. Btw, 'good enough' was before I realized the
zener needed to be increased.)

My opinion is you need one with comparatively high Vgs so the source
resistor is large enough for some feedback (as well as a comparable
input voltage range, depending on how everything else turns out). A
j105 looks interesting with 500mA max but it's TO-92 again.



Any in that series; J105 - 107 should do, but from Farnell they're a tad
pricey, it seems dissipation may be an issue so ganged BF245C or 246C might
be the better solution.


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Default Experiment (valve).


"flipper" wrote in message
news
On Sat, 1 May 2010 20:38:32 +0100, "ian field"
wrote:


"flipper" wrote in message
. ..
On Sat, 1 May 2010 13:53:37 +0100, "ian field"
wrote:


"flipper" wrote in message
m...
On Fri, 30 Apr 2010 22:22:03 +0100, "ian field"
wrote:


"flipper" wrote in message
news:q68kt5l5noh0o5pupj3uhcvbdcq7u0bavi@4ax. com...
On Thu, 29 Apr 2010 18:50:04 +0100, "ian field"
wrote:


"flipper" wrote in message
news:bedjt5t8ds9vopi85fvebp8us54h90835c@4a x.com...
On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I
would
like to experiment with, but unfortunately I don't have an old
valve
radio
to use as a testbed.

The component values that are pencilled in will doubtless need
revising
and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P
pentode
in
an
old valve radio, it has 3 stages to maintain its invertingness in
case
the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a
bang
it
will make?

What's the goal? To 'fix' a broken radio? Why not just buy the
proper
valve and plug it in?

To some extent its just a bit of fun.

One significant inconvenience is that most UK radios had series
heater
chains, so a dropper resistor would have to be wired in to replace
the
valve's heater.



Here's a quick throw together idea.

What's the typical anode current on an average table top set?

I don't work on them so I'm not sure but I would imagine it's close to
the 'typical operation' given in the respective tube datasheet. Like,
for the 35L6 (octal AA5 output, 150mA heaters, circa 1939), it's given
as 40mA (idle) at 110V (tube rectified, typically a 35Z4, U.S. AC line


Last night I had a flick through my old Mazda valve data book and the
popular audio output valves range from Ia about 30 to 70mA, the BF246B
would
be good enough for all but the most powerful types but the 246C would
cover
almost any eventuality.

The best I have in the parts drawer is BF245C, I'd have to select for
highest Idss and still gang at least 2 devices - and tweak the
individual
decoupled source resistors to balance the dissipation.


Are you trying to just make a test device or a 'universal device' of
some sort?

I didn't mean to suggest the BF246C was the best thing to use. It's
just the best ready made jfet model I had (and did seem 'good enough'
for a test shot anyway. Btw, 'good enough' was before I realized the
zener needed to be increased.)

My opinion is you need one with comparatively high Vgs so the source
resistor is large enough for some feedback (as well as a comparable
input voltage range, depending on how everything else turns out). A
j105 looks interesting with 500mA max but it's TO-92 again.



Any in that series; J105 - 107 should do, but from Farnell they're a tad
pricey, it seems dissipation may be an issue so ganged BF245C or 246C
might
be the better solution.


Well, we'll just have to fix that power thingie, then.

Try this idea on for size. 4mW in the jfet and idle is about 660 mW in
the MJE340.

Btw, that IRFU410 is there simply by reflex as it's a close spice sub
for the 600V STP2NK60Z I like (built in gate protection and [used to
be] cheap). Same with the MJE340. Certainly don't need a 20 W (well,
with heatsink) 300V transistor around a 25 V jfet but it'll sure as
hell survive. It would still need some heatsinking because they're
going into a hot environment but so will the MOSFET so you could use
one and put 'em both on it.

You could maybe build it on an octal base but it isn't a pin for pin
straight 'plug in' kind of thing like a fetron because, for one, it
needs an extra ground pin for the zener reference (could put the plus
end on 'screen'). Also, existing Rk wouldn't work.

Btw, adding the power bypass ends any chance of faking a soft clip as
follower gain smashes any low Vds 'triode region' in the jfet flat as
a pancake.



Thanks - that looks about it, but it won't please the critics who commented
my original 3 MOSFET contraption was too difficult.




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Default Experiment (valve).

On Thu, 29 Apr 2010 11:33:10 -0500, flipper wrote:

On Thu, 29 Apr 2010 08:41:52 -0700, Jim Thompson
wrote:

On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?



You're making it too hard.

Just slip a 2N7000 under your 900V device (making a cascode), tie the
900V device gate to the +30V, and bias the 2N7000 as desired.

Personally I've done this where the 900V device was a toob, and, in
the cathode circuit was an NPN with an OpAmp wrapped around it ;-)


Well, in the toob world there are those who would say the same thing,
that "You're making it too hard," since the toob will self bias with a
simple resistor (audio bypassed for maximum gain) under the cathode,
grid to ground, similar to what is done with a simple jfet current
source.


Except for when it doesn't work, to bias the final / power output stage.
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Default Experiment (valve).


"flipper" wrote in message
...
On Sat, 1 May 2010 22:42:45 +0100, "ian field"
wrote:


"flipper" wrote in message
news
On Sat, 1 May 2010 20:38:32 +0100, "ian field"
wrote:


"flipper" wrote in message
m...
On Sat, 1 May 2010 13:53:37 +0100, "ian field"
wrote:


"flipper" wrote in message
news:2b8nt593rafnfocp63ceg5g5f8steq8l6r@4ax. com...
On Fri, 30 Apr 2010 22:22:03 +0100, "ian field"
wrote:


"flipper" wrote in message
news:q68kt5l5noh0o5pupj3uhcvbdcq7u0bavi@4a x.com...
On Thu, 29 Apr 2010 18:50:04 +0100, "ian field"
wrote:


"flipper" wrote in message
news:bedjt5t8ds9vopi85fvebp8us54h90835c@ 4ax.com...
On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that
I
would
like to experiment with, but unfortunately I don't have an old
valve
radio
to use as a testbed.

The component values that are pencilled in will doubtless need
revising
and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P
pentode
in
an
old valve radio, it has 3 stages to maintain its invertingness
in
case
the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud
a
bang
it
will make?

What's the goal? To 'fix' a broken radio? Why not just buy the
proper
valve and plug it in?

To some extent its just a bit of fun.

One significant inconvenience is that most UK radios had series
heater
chains, so a dropper resistor would have to be wired in to replace
the
valve's heater.



Here's a quick throw together idea.

What's the typical anode current on an average table top set?

I don't work on them so I'm not sure but I would imagine it's close
to
the 'typical operation' given in the respective tube datasheet.
Like,
for the 35L6 (octal AA5 output, 150mA heaters, circa 1939), it's
given
as 40mA (idle) at 110V (tube rectified, typically a 35Z4, U.S. AC
line


Last night I had a flick through my old Mazda valve data book and the
popular audio output valves range from Ia about 30 to 70mA, the BF246B
would
be good enough for all but the most powerful types but the 246C would
cover
almost any eventuality.

The best I have in the parts drawer is BF245C, I'd have to select for
highest Idss and still gang at least 2 devices - and tweak the
individual
decoupled source resistors to balance the dissipation.


Are you trying to just make a test device or a 'universal device' of
some sort?

I didn't mean to suggest the BF246C was the best thing to use. It's
just the best ready made jfet model I had (and did seem 'good enough'
for a test shot anyway. Btw, 'good enough' was before I realized the
zener needed to be increased.)

My opinion is you need one with comparatively high Vgs so the source
resistor is large enough for some feedback (as well as a comparable
input voltage range, depending on how everything else turns out). A
j105 looks interesting with 500mA max but it's TO-92 again.


Any in that series; J105 - 107 should do, but from Farnell they're a tad
pricey, it seems dissipation may be an issue so ganged BF245C or 246C
might
be the better solution.


Well, we'll just have to fix that power thingie, then.

Try this idea on for size. 4mW in the jfet and idle is about 660 mW in
the MJE340.

Btw, that IRFU410 is there simply by reflex as it's a close spice sub
for the 600V STP2NK60Z I like (built in gate protection and [used to
be] cheap). Same with the MJE340. Certainly don't need a 20 W (well,
with heatsink) 300V transistor around a 25 V jfet but it'll sure as
hell survive. It would still need some heatsinking because they're
going into a hot environment but so will the MOSFET so you could use
one and put 'em both on it.

You could maybe build it on an octal base but it isn't a pin for pin
straight 'plug in' kind of thing like a fetron because, for one, it
needs an extra ground pin for the zener reference (could put the plus
end on 'screen'). Also, existing Rk wouldn't work.

Btw, adding the power bypass ends any chance of faking a soft clip as
follower gain smashes any low Vds 'triode region' in the jfet flat as
a pancake.



Thanks - that looks about it, but it won't please the critics who
commented
my original 3 MOSFET contraption was too difficult.


Well, maybe not.

Not knowing your thinking I couldn't quite figure out how you intended
yours to work so I simply fell back to what I'd done before. Maybe
yours would be a gentler clip. I dunno.



The clipping characteristics would be more of a concern if it was intended
for use as a guitar amp, it is hoped that the increased gain and current
handling will mean it never goes anywhere near clipping.

BTW: why does the bipolar need to be an MJE340 ?


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Default Experiment (valve).

On Sun, 02 May 2010 06:56:01 -0700,
wrote:

On Thu, 29 Apr 2010 11:33:10 -0500, flipper wrote:

On Thu, 29 Apr 2010 08:41:52 -0700, Jim Thompson
wrote:

On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?



You're making it too hard.

Just slip a 2N7000 under your 900V device (making a cascode), tie the
900V device gate to the +30V, and bias the 2N7000 as desired.

Personally I've done this where the 900V device was a toob, and, in
the cathode circuit was an NPN with an OpAmp wrapped around it ;-)


Well, in the toob world there are those who would say the same thing,
that "You're making it too hard," since the toob will self bias with a
simple resistor (audio bypassed for maximum gain) under the cathode,
grid to ground, similar to what is done with a simple jfet current
source.


Except for when it doesn't work, to bias the final / power output stage.


My "toob+tranny" was incredibly linear as compared to a toob alone.

You can even make a triode behave with the linearity of a pentode...
handy in HV conditions.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

The only thing bipartisan in this country is hypocrisy
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Default Experiment (valve).

Mine saturates to a lower voltage.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"flipper" wrote in message
...
On Sun, 2 May 2010 15:20:11 +0100, "ian field"
wrote:

trim snip





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On Sun, 02 May 2010 14:48:19 -0500, flipper wrote:

On Sun, 2 May 2010 15:20:11 +0100, "ian field"
wrote:

trim snip


The clipping characteristics would be more of a concern if it was intended
for use as a guitar amp, it is hoped that the increased gain and current
handling will mean it never goes anywhere near clipping.


I thought you were talking about a radio output stage substitute. If
so, then you're constrained by everything else already in the radio:
the OPT, B+, and so on. Neither gain nor 'more current' is going to
make much of a, if any, difference because you have an already defined
B+ into an already defined output impedance. You can't swing more
voltage across it than is there and with no more voltage you swing no
more current.

Increased idle current would likely affect the B+ filter and the
preamp stages already expect a certain output gain so I'd think the
goal would be to 'match' the designed for tube as close as practical.

BTW: why does the bipolar need to be an MJE340 ?


I said it was there simply by reflex and probably overkill.

I wasn't trying to optimize it, just demonstrate the idea.

The MJE340 is a nice 'all around' HV transistor and I do use where the
300V is needed, so that puts it in my parts bin. 300V also makes it
robust in 'low voltage' circuits under fault conditions, like say a
tube short, but, as I mentioned, that's probably moot in this circuit.

Hmm, speaking of robust, it might not be a bad idea to put a reverse
protection diode in there.

Anyway, it's just a transistor I probably don't have to 'worry much
about' so slap it in for testing, since I'll have one in the bin (as
well as the STP2NK60Z) to breadboard with, and optimize things later,
etc.

Feel free to juggle things around to suit your needs.

Which reminds me, splitting R4 into two, mentioned in my last post,
also widens the range of jfets you can use since Vgs can be catered
for by adjusting the 'gate boost' ratio. I mean, the main reason I
suggested a large Vgs device was to get some source R under it for
feedback (reducing gain & distortion) and boosting Vgate accomplishes
the same thing for a low Vgs device: you increase the source resistor
(actually the one under the MJE340 in this case) to counter the
'boost'. Should be able to tailor it to almost any desired gain by
selecting appropriate jfets and suitable 'boost' ratio/source
(emitter) resistor. Also acts as an input impedance partial boostrap.
How's that for serendipity?

The R4 split might also be the best place to put a 'bias adjust'
trimmer. Once set it shouldn't need readjusting so that doesn't need
to be user accessible.

I kinda like the 'bias boost' idea because it keeps the bias trimmer
out of high current and makes for quite a bit of flexibility.
(attachment)

I didn't look at Vgs variability in picking that adjustment range.
Again, it's 'for show'. The BF244 is lower Vgs than the previous BF246
but with 'bias boost' we end up with a 20V zener again, equivalent
source resistor feedback, and similar gain (clip point). Also note we
don't need high current jfets anymore because the NPN is doing that
work. Btw, that could be a (MOS)FET too but it's Vgs would increase
the reference voltage needed, cutting into output swing. That's why I
went bipolar.

Something I'd keep in mind when testing is potential noise from that
zener on the gate. I'm gambling it's small enough to not matter on an
output stage but if it's a problem then filter it or create a voltage
reference by other means. One might be tempted to use a simple
resistor divider but, while I haven't done an analysis, I suspect
component tolerances and B+ variation are likely too much to ensure
things work properly on the low end while staying safe for the jfet on
the high end, plus that's a hum injection point, hence the zener for a
quickie first shot.

Maybe we should patent this thing



Why not just do this? No adjustments needed.

John




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John Larkin wrote:
On Sun, 02 May 2010 14:48:19 -0500, flipper wrote:

On Sun, 2 May 2010 15:20:11 +0100, "ian field"
wrote:

trim snip


The clipping characteristics would be more of a concern if it was intended
for use as a guitar amp, it is hoped that the increased gain and current

cut
Maybe we should patent this thing



Why not just do this? No adjustments needed.

John




A 400?? v vcr protecting the fet and you have a dandy replacement
for a tube!!
Removing the speaker while working might kill the fet.
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On Sun, 02 May 2010 18:39:11 -0500, flipper wrote:

On Sun, 02 May 2010 14:59:19 -0700, John Larkin
wrote:

On Sun, 02 May 2010 14:48:19 -0500, flipper wrote:

On Sun, 2 May 2010 15:20:11 +0100, "ian field"
wrote:

trim snip


The clipping characteristics would be more of a concern if it was intended
for use as a guitar amp, it is hoped that the increased gain and current
handling will mean it never goes anywhere near clipping.

I thought you were talking about a radio output stage substitute. If
so, then you're constrained by everything else already in the radio:
the OPT, B+, and so on. Neither gain nor 'more current' is going to
make much of a, if any, difference because you have an already defined
B+ into an already defined output impedance. You can't swing more
voltage across it than is there and with no more voltage you swing no
more current.

Increased idle current would likely affect the B+ filter and the
preamp stages already expect a certain output gain so I'd think the
goal would be to 'match' the designed for tube as close as practical.

BTW: why does the bipolar need to be an MJE340 ?

I said it was there simply by reflex and probably overkill.

I wasn't trying to optimize it, just demonstrate the idea.

The MJE340 is a nice 'all around' HV transistor and I do use where the
300V is needed, so that puts it in my parts bin. 300V also makes it
robust in 'low voltage' circuits under fault conditions, like say a
tube short, but, as I mentioned, that's probably moot in this circuit.

Hmm, speaking of robust, it might not be a bad idea to put a reverse
protection diode in there.

Anyway, it's just a transistor I probably don't have to 'worry much
about' so slap it in for testing, since I'll have one in the bin (as
well as the STP2NK60Z) to breadboard with, and optimize things later,
etc.

Feel free to juggle things around to suit your needs.

Which reminds me, splitting R4 into two, mentioned in my last post,
also widens the range of jfets you can use since Vgs can be catered
for by adjusting the 'gate boost' ratio. I mean, the main reason I
suggested a large Vgs device was to get some source R under it for
feedback (reducing gain & distortion) and boosting Vgate accomplishes
the same thing for a low Vgs device: you increase the source resistor
(actually the one under the MJE340 in this case) to counter the
'boost'. Should be able to tailor it to almost any desired gain by
selecting appropriate jfets and suitable 'boost' ratio/source
(emitter) resistor. Also acts as an input impedance partial boostrap.
How's that for serendipity?

The R4 split might also be the best place to put a 'bias adjust'
trimmer. Once set it shouldn't need readjusting so that doesn't need
to be user accessible.

I kinda like the 'bias boost' idea because it keeps the bias trimmer
out of high current and makes for quite a bit of flexibility.
(attachment)

I didn't look at Vgs variability in picking that adjustment range.
Again, it's 'for show'. The BF244 is lower Vgs than the previous BF246
but with 'bias boost' we end up with a 20V zener again, equivalent
source resistor feedback, and similar gain (clip point). Also note we
don't need high current jfets anymore because the NPN is doing that
work. Btw, that could be a (MOS)FET too but it's Vgs would increase
the reference voltage needed, cutting into output swing. That's why I
went bipolar.

Something I'd keep in mind when testing is potential noise from that
zener on the gate. I'm gambling it's small enough to not matter on an
output stage but if it's a problem then filter it or create a voltage
reference by other means. One might be tempted to use a simple
resistor divider but, while I haven't done an analysis, I suspect
component tolerances and B+ variation are likely too much to ensure
things work properly on the low end while staying safe for the jfet on
the high end, plus that's a hum injection point, hence the zener for a
quickie first shot.

Maybe we should patent this thing



Why not just do this? No adjustments needed.

John



I dunno. Give me some component values and we'll see.


Make the upper resistor whatever it needs to be to get +10 on the
gate. Make the source resistor whatever you like to get your preferred
idle current.


Just off hand, the 100k makes input impedance rather low and if that's
a MOSFET then gate capacitance is a problem.


Depends on what you drive it from. 100K is not going to load many
driver circuits.

If you delete the source bypass cap, Cin goes way down. At that point,
drive levels will be closer to what a toob would need, and linearity
is better.

Of course, drain impedance is high, so speaker damping is poor. Some
people like that. Local NFB is easy (just connect the upper resistor
to the drain) and helps that situation.

I was just showing a bias scheme, not a full amplifier.

John

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On Sun, 02 May 2010 23:43:14 -0500, flipper wrote:

On Sun, 02 May 2010 17:08:30 -0700, John Larkin
wrote:

On Sun, 02 May 2010 18:39:11 -0500, flipper wrote:

On Sun, 02 May 2010 14:59:19 -0700, John Larkin
wrote:

snip


Why not just do this? No adjustments needed.

John



I dunno. Give me some component values and we'll see.


Make the upper resistor whatever it needs to be to get +10 on the
gate. Make the source resistor whatever you like to get your preferred
idle current.


Actually, I was wondering if you had a magic FET but that's now moot.


Just off hand, the 100k makes input impedance rather low and if that's
a MOSFET then gate capacitance is a problem.


Depends on what you drive it from. 100K is not going to load many
driver circuits.


It's not all that great in the tube world and AA5 radios normally have
470k to 1M grid leaks but that's solvable. The trick here is to make a
separate reference, not tied to the gate, and then take gate to it
through an equivalent 470k to 1M 'gate leak'.


If you delete the source bypass cap, Cin goes way down. At that point,
drive levels will be closer to what a toob would need, and linearity
is better.


First thing I looked at was just like what you drew but bandwidth
stunk so I moved on. Source feedback is the trick to it and looks like
there's enough to make it work.

Of course, drain impedance is high, so speaker damping is poor. Some
people like that. Local NFB is easy (just connect the upper resistor
to the drain) and helps that situation.


I think adding some local drain feedback might be a little more
complicated than that because bias would go away with large output
swings.

Anyway, a 'plain Jane version might look something like the
attachment. A zener, rather than the resistor divider, keeps hum off
the gate. Or one could filter the divider.

[snip]

What will Cdg feedback do to an uncascoded circuit such as yours?
Limit the high end?

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

The only thing bipartisan in this country is hypocrisy
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"flipper" wrote in message
...
On Sun, 2 May 2010 15:20:11 +0100, "ian field"
wrote:

trim snip


The clipping characteristics would be more of a concern if it was intended
for use as a guitar amp, it is hoped that the increased gain and current
handling will mean it never goes anywhere near clipping.


I thought you were talking about a radio output stage substitute. If
so, then you're constrained by everything else already in the radio:
the OPT, B+, and so on. Neither gain nor 'more current' is going to
make much of a, if any, difference because you have an already defined
B+ into an already defined output impedance. You can't swing more
voltage across it than is there and with no more voltage you swing no
more current.

Increased idle current would likely affect the B+ filter and the
preamp stages already expect a certain output gain so I'd think the
goal would be to 'match' the designed for tube as close as practical.

BTW: why does the bipolar need to be an MJE340 ?


I said it was there simply by reflex and probably overkill.

I wasn't trying to optimize it, just demonstrate the idea.

The MJE340 is a nice 'all around' HV transistor and I do use where the
300V is needed, so that puts it in my parts bin. 300V also makes it
robust in 'low voltage' circuits under fault conditions, like say a
tube short, but, as I mentioned, that's probably moot in this circuit.

Hmm, speaking of robust, it might not be a bad idea to put a reverse
protection diode in there.

Anyway, it's just a transistor I probably don't have to 'worry much
about' so slap it in for testing, since I'll have one in the bin (as
well as the STP2NK60Z) to breadboard with, and optimize things later,
etc.

Feel free to juggle things around to suit your needs.

Which reminds me, splitting R4 into two, mentioned in my last post,
also widens the range of jfets you can use since Vgs can be catered
for by adjusting the 'gate boost' ratio. I mean, the main reason I
suggested a large Vgs device was to get some source R under it for
feedback (reducing gain & distortion) and boosting Vgate accomplishes
the same thing for a low Vgs device: you increase the source resistor
(actually the one under the MJE340 in this case) to counter the
'boost'. Should be able to tailor it to almost any desired gain by
selecting appropriate jfets and suitable 'boost' ratio/source
(emitter) resistor. Also acts as an input impedance partial boostrap.
How's that for serendipity?

The R4 split might also be the best place to put a 'bias adjust'
trimmer. Once set it shouldn't need readjusting so that doesn't need
to be user accessible.

I kinda like the 'bias boost' idea because it keeps the bias trimmer
out of high current and makes for quite a bit of flexibility.
(attachment)

I didn't look at Vgs variability in picking that adjustment range.
Again, it's 'for show'. The BF244 is lower Vgs than the previous BF246
but with 'bias boost' we end up with a 20V zener again, equivalent
source resistor feedback, and similar gain (clip point). Also note we
don't need high current jfets anymore because the NPN is doing that
work. Btw, that could be a (MOS)FET too but it's Vgs would increase
the reference voltage needed, cutting into output swing. That's why I
went bipolar.

Something I'd keep in mind when testing is potential noise from that
zener on the gate. I'm gambling it's small enough to not matter on an
output stage but if it's a problem then filter it or create a voltage
reference by other means. One might be tempted to use a simple
resistor divider but, while I haven't done an analysis, I suspect
component tolerances and B+ variation are likely too much to ensure
things work properly on the low end while staying safe for the jfet on
the high end, plus that's a hum injection point, hence the zener for a
quickie first shot.

Maybe we should patent this thing



One thing I was wondering, can the JFET have a much lower Idss since its
buffered with a bipolar?


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"flipper" wrote in message
...
Anyone know if this is an 'inherent' relationship: that as Gfs
increases so does gate capacitance sort of correspondingly so we're
fighting a loosing battle trying to trade one for the other?


1 / (2*pi*R*C) is the cutoff frequency. Put in R = 1/Gfs and you get
something like fT. This isn't so much a physically realizable frequency as
it is a quality factor (tubes were similarly rated in terms of Gm vs. C).

1 mho is an awful lot of transconductance. At 170pF, that's 936MHz. The
equivalent transresistance is quite small (= 1 ohm), which suggests actual
resistances (like gate spreading resistance) or inductances (figure 5nH
typical source inductance) will dominate. (Ls is 29 ohms at that frequency,
way past cutoff, and Rg might be 1 ohm or so. Cg resonates with Ls at
173MHz, which would be series resonant with the input, but tweaked by drain
current, giving rise to funny phase shifts as seen in the s-parameters.)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms




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"flipper" wrote in message
...
It's source resistance into gate capacitance that's the problem, not
Rds on.


Source impedance is approximately 1/Gm ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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"flipper" wrote in message
...
So you really don't think it matters whether one tries to drive the
gate through a 10 ohm or 100 megohm resistor?


I said cutoff frequency, which means voltage source. Do you have a 0 ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim

--
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Website: http://webpages.charter.net/dawill/tmoranwms


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On Tue, 04 May 2010 01:03:43 -0500, flipper wrote:

On Mon, 3 May 2010 23:02:27 -0500, "Tim Williams"
wrote:

"flipper" wrote in message
. ..
So you really don't think it matters whether one tries to drive the
gate through a 10 ohm or 100 megohm resistor?


I said cutoff frequency, which means voltage source. Do you have a 0 ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim


I know we won't see it so I have no idea what the heck your tap dance
is for.


Gm/Co was a popular figure of merit for tubes used in video amps and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.

Mosfets have impressive Gm/Co values, at the cost of high input
capacitance.

John

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"John Larkin" wrote in message
...
Gm/Co was a popular figure of merit for tubes used in video amps and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.


7KY6 has four times the transconductance of 2N3819, and plenty more voltage
and power capacity.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms




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On Tue, 4 May 2010 14:31:11 -0500, "Tim Williams"
wrote:

"John Larkin" wrote in message
.. .
Gm/Co was a popular figure of merit for tubes used in video amps and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.


7KY6 has four times the transconductance of 2N3819, and plenty more voltage
and power capacity.

Tim


Sure, but a 3819 is a dog. Jfets are wimpy, although a BF862 has a bit
more Gm than the 7KY6.

For serious transconductance, phemts and GaN fets are the winners,
with whole digits of siemens to the left of the decimal and absurdly
low capacitances.

John



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"flipper" wrote in message
...
On Tue, 04 May 2010 09:11:47 -0700, John Larkin
wrote:

On Tue, 04 May 2010 01:03:43 -0500, flipper wrote:

On Mon, 3 May 2010 23:02:27 -0500, "Tim Williams"
wrote:

"flipper" wrote in message
m...
So you really don't think it matters whether one tries to drive the
gate through a 10 ohm or 100 megohm resistor?

I said cutoff frequency, which means voltage source. Do you have a 0
ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim

I know we won't see it so I have no idea what the heck your tap dance
is for.


Gm/Co was a popular figure of merit for tubes used in video amps and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.

Mosfets have impressive Gm/Co values, at the cost of high input
capacitance.

John


I understand but that doesn't answer the question at hand because we
do not have unlimited gate drive capability. FOM, Rdson x Qg, isn't
the issue as the device is 'capable' of more bandwidth if we could
drive it but, because the 'driver' already exists, we do not have that
choice. At least not without buffering it with something and if we do
that we might as well use the jfet under the source version. But, in
either case, it's no longer 'plain Jane'.

For a given MOSFET technology FOM is roughly constant and I was asking
if the same kind of thing applied to our problem of trading off gm,
with source feedback, vs Ciss, because that is what the driving
impedance works into.

Unfortunately, Mouser doesn't let me search by Ciss, or gate charge
either, so it's not a simple task to find a low one but I seem to
remember that when I originally latched onto the STP2NK60Z it was
'pretty low'... but I wasn't taking gm into account.

As I said, it's probably moot for a (AA5) 'table radio', because we
don't need much bandwidth to begin with, but I was pondering it's
potential suitability as a more general replacement. I suspect we need
the more complex version if one wants 'full capability'.



Just a thought, but if the common source section of the cascode is boosted
by a bipolar, then a really low Idss FET can be used - like a 40673 or a
BF991, even with both gates tied together its only a few pF.

Being depletion MOSFETs, they should be even better.


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"flipper" wrote in message
...
On Tue, 4 May 2010 21:20:41 +0100, "ian field"
wrote:


"flipper" wrote in message
. ..
On Tue, 04 May 2010 09:11:47 -0700, John Larkin
wrote:

On Tue, 04 May 2010 01:03:43 -0500, flipper wrote:

On Mon, 3 May 2010 23:02:27 -0500, "Tim Williams"
wrote:

"flipper" wrote in message
news:1k4vt5pctqpqush7g57aappqgt1e4ooubd@4ax. com...
So you really don't think it matters whether one tries to drive the
gate through a 10 ohm or 100 megohm resistor?

I said cutoff frequency, which means voltage source. Do you have a 0
ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim

I know we won't see it so I have no idea what the heck your tap dance
is for.

Gm/Co was a popular figure of merit for tubes used in video amps and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.

Mosfets have impressive Gm/Co values, at the cost of high input
capacitance.

John

I understand but that doesn't answer the question at hand because we
do not have unlimited gate drive capability. FOM, Rdson x Qg, isn't
the issue as the device is 'capable' of more bandwidth if we could
drive it but, because the 'driver' already exists, we do not have that
choice. At least not without buffering it with something and if we do
that we might as well use the jfet under the source version. But, in
either case, it's no longer 'plain Jane'.

For a given MOSFET technology FOM is roughly constant and I was asking
if the same kind of thing applied to our problem of trading off gm,
with source feedback, vs Ciss, because that is what the driving
impedance works into.

Unfortunately, Mouser doesn't let me search by Ciss, or gate charge
either, so it's not a simple task to find a low one but I seem to
remember that when I originally latched onto the STP2NK60Z it was
'pretty low'... but I wasn't taking gm into account.

As I said, it's probably moot for a (AA5) 'table radio', because we
don't need much bandwidth to begin with, but I was pondering it's
potential suitability as a more general replacement. I suspect we need
the more complex version if one wants 'full capability'.



Just a thought, but if the common source section of the cascode is boosted
by a bipolar, then a really low Idss FET can be used - like a 40673 or a
BF991, even with both gates tied together its only a few pF.


Sure, you can.

A 'typical' (if there is such a thing) tube in this power range looks
to be about 11 or 12 pF and the BF244 looks to be about 3 pF so I
wouldn't think that's a problem. The BF246 looks to be about 15pF so
it's not as good.

Being depletion MOSFETs, they should be even better.


Better than what in what way?


More similar in character to a grid.


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"flipper" wrote in message
news
On Wed, 5 May 2010 15:53:19 +0100, "ian field"
wrote:


"flipper" wrote in message
. ..
On Tue, 4 May 2010 21:20:41 +0100, "ian field"
wrote:


"flipper" wrote in message
m...
On Tue, 04 May 2010 09:11:47 -0700, John Larkin
wrote:

On Tue, 04 May 2010 01:03:43 -0500, flipper wrote:

On Mon, 3 May 2010 23:02:27 -0500, "Tim Williams"
wrote:

"flipper" wrote in message
news:1k4vt5pctqpqush7g57aappqgt1e4ooubd@4a x.com...
So you really don't think it matters whether one tries to drive
the
gate through a 10 ohm or 100 megohm resistor?

I said cutoff frequency, which means voltage source. Do you have a
0
ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim

I know we won't see it so I have no idea what the heck your tap dance
is for.

Gm/Co was a popular figure of merit for tubes used in video amps and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.

Mosfets have impressive Gm/Co values, at the cost of high input
capacitance.

John

I understand but that doesn't answer the question at hand because we
do not have unlimited gate drive capability. FOM, Rdson x Qg, isn't
the issue as the device is 'capable' of more bandwidth if we could
drive it but, because the 'driver' already exists, we do not have that
choice. At least not without buffering it with something and if we do
that we might as well use the jfet under the source version. But, in
either case, it's no longer 'plain Jane'.

For a given MOSFET technology FOM is roughly constant and I was asking
if the same kind of thing applied to our problem of trading off gm,
with source feedback, vs Ciss, because that is what the driving
impedance works into.

Unfortunately, Mouser doesn't let me search by Ciss, or gate charge
either, so it's not a simple task to find a low one but I seem to
remember that when I originally latched onto the STP2NK60Z it was
'pretty low'... but I wasn't taking gm into account.

As I said, it's probably moot for a (AA5) 'table radio', because we
don't need much bandwidth to begin with, but I was pondering it's
potential suitability as a more general replacement. I suspect we need
the more complex version if one wants 'full capability'.


Just a thought, but if the common source section of the cascode is
boosted
by a bipolar, then a really low Idss FET can be used - like a 40673 or a
BF991, even with both gates tied together its only a few pF.

Sure, you can.

A 'typical' (if there is such a thing) tube in this power range looks
to be about 11 or 12 pF and the BF244 looks to be about 3 pF so I
wouldn't think that's a problem. The BF246 looks to be about 15pF so
it's not as good.

Being depletion MOSFETs, they should be even better.

Better than what in what way?


More similar in character to a grid.


If you mean it seems 'more similar' physically then I suggest the SS
device, as a whole, is sufficiently different as to make focusing on
one bit not very illuminating but if you like them there's nothing to
prevent it's use. At least, I don't see one off hand.

I think it better to focus on device parameters, like the BF244 has
higher breakdown but the BF991 has more gain. And then there's that
one is through hole and the other is surface mount.


I wanted to buy some of the old BF9xx "pill" devices but couldn't find any,
I got hold of a few BF991 SMDs and the only UHF types had Vds 7V abs-max,
there's some very old TV tuners in a tea chest in the garage, so I do have a
very small selection of "pill" types.

I found that if you cut a 4-hole square of Veroboard and carefully V-groove
across the 2 tracks with a scalpel, making 4 copper lands with a hole
through each, the 4-pin SMD sits between the lands nicely, add 22SWG lead
wires through the holes and the device is ready for general experimenting..

There's no reason the same can't be done directly on the underside of a
project stripboard, it just requires a bit of carefull planning of the
layout.


  #29   Report Post  
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Posts: 800
Default Experiment (valve). - NFET.jpg


"flipper" wrote in message
...
On Wed, 5 May 2010 20:58:30 +0100, "ian field"
wrote:


"flipper" wrote in message
news
On Wed, 5 May 2010 15:53:19 +0100, "ian field"
wrote:


"flipper" wrote in message
m...
On Tue, 4 May 2010 21:20:41 +0100, "ian field"
wrote:


"flipper" wrote in message
news:8vo0u5lcehppt1if7msgt9gujo0qe87ag7@4ax. com...
On Tue, 04 May 2010 09:11:47 -0700, John Larkin
wrote:

On Tue, 04 May 2010 01:03:43 -0500, flipper
wrote:

On Mon, 3 May 2010 23:02:27 -0500, "Tim Williams"
wrote:

"flipper" wrote in message
news:1k4vt5pctqpqush7g57aappqgt1e4ooubd@ 4ax.com...
So you really don't think it matters whether one tries to drive
the
gate through a 10 ohm or 100 megohm resistor?

I said cutoff frequency, which means voltage source. Do you have
a
0
ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim

I know we won't see it so I have no idea what the heck your tap
dance
is for.

Gm/Co was a popular figure of merit for tubes used in video amps and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.

Mosfets have impressive Gm/Co values, at the cost of high input
capacitance.

John

I understand but that doesn't answer the question at hand because we
do not have unlimited gate drive capability. FOM, Rdson x Qg, isn't
the issue as the device is 'capable' of more bandwidth if we could
drive it but, because the 'driver' already exists, we do not have
that
choice. At least not without buffering it with something and if we
do
that we might as well use the jfet under the source version. But, in
either case, it's no longer 'plain Jane'.

For a given MOSFET technology FOM is roughly constant and I was
asking
if the same kind of thing applied to our problem of trading off gm,
with source feedback, vs Ciss, because that is what the driving
impedance works into.

Unfortunately, Mouser doesn't let me search by Ciss, or gate charge
either, so it's not a simple task to find a low one but I seem to
remember that when I originally latched onto the STP2NK60Z it was
'pretty low'... but I wasn't taking gm into account.

As I said, it's probably moot for a (AA5) 'table radio', because we
don't need much bandwidth to begin with, but I was pondering it's
potential suitability as a more general replacement. I suspect we
need
the more complex version if one wants 'full capability'.


Just a thought, but if the common source section of the cascode is
boosted
by a bipolar, then a really low Idss FET can be used - like a 40673 or
a
BF991, even with both gates tied together its only a few pF.

Sure, you can.

A 'typical' (if there is such a thing) tube in this power range looks
to be about 11 or 12 pF and the BF244 looks to be about 3 pF so I
wouldn't think that's a problem. The BF246 looks to be about 15pF so
it's not as good.

Being depletion MOSFETs, they should be even better.

Better than what in what way?

More similar in character to a grid.


If you mean it seems 'more similar' physically then I suggest the SS
device, as a whole, is sufficiently different as to make focusing on
one bit not very illuminating but if you like them there's nothing to
prevent it's use. At least, I don't see one off hand.

I think it better to focus on device parameters, like the BF244 has
higher breakdown but the BF991 has more gain. And then there's that
one is through hole and the other is surface mount.


I wanted to buy some of the old BF9xx "pill" devices but couldn't find
any,
I got hold of a few BF991 SMDs and the only UHF types had Vds 7V abs-max,
there's some very old TV tuners in a tea chest in the garage, so I do have
a
very small selection of "pill" types.


7V? Boy, that's not a lot to play with. I don't know what values would
be needed to make that one work but, for example, in the BF244 v8
circuit your 7V job wouldn't survive because there's up to 16V across
the jfet (a bit over 9V even just at idle).


IIRC only the UHF type has that limitation and I wouldn't bother using a UHF
dg MOSFET for audio work when I have VHF types with a more useful voltage
rating.

Some of the tuners have "pill" type UHF MOSFETS with voltage ratings in the
general direction of 20V, so I'm not at all cornered.


  #30   Report Post  
Posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 800
Default Experiment (valve). - NFET.jpg


"flipper" wrote in message
...
On Wed, 05 May 2010 16:01:07 -0500, flipper wrote:

On Wed, 5 May 2010 20:58:30 +0100, "ian field"
wrote:


"flipper" wrote in message
news On Wed, 5 May 2010 15:53:19 +0100, "ian field"
wrote:


"flipper" wrote in message
om...
On Tue, 4 May 2010 21:20:41 +0100, "ian field"
wrote:


"flipper" wrote in message
news:8vo0u5lcehppt1if7msgt9gujo0qe87ag7@4ax .com...
On Tue, 04 May 2010 09:11:47 -0700, John Larkin
wrote:

On Tue, 04 May 2010 01:03:43 -0500, flipper
wrote:

On Mon, 3 May 2010 23:02:27 -0500, "Tim Williams"
wrote:

"flipper" wrote in message
news:1k4vt5pctqpqush7g57aappqgt1e4ooubd @4ax.com...
So you really don't think it matters whether one tries to drive
the
gate through a 10 ohm or 100 megohm resistor?

I said cutoff frequency, which means voltage source. Do you have
a
0
ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim

I know we won't see it so I have no idea what the heck your tap
dance
is for.

Gm/Co was a popular figure of merit for tubes used in video amps
and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.

Mosfets have impressive Gm/Co values, at the cost of high input
capacitance.

John

I understand but that doesn't answer the question at hand because
we
do not have unlimited gate drive capability. FOM, Rdson x Qg, isn't
the issue as the device is 'capable' of more bandwidth if we could
drive it but, because the 'driver' already exists, we do not have
that
choice. At least not without buffering it with something and if we
do
that we might as well use the jfet under the source version. But,
in
either case, it's no longer 'plain Jane'.

For a given MOSFET technology FOM is roughly constant and I was
asking
if the same kind of thing applied to our problem of trading off gm,
with source feedback, vs Ciss, because that is what the driving
impedance works into.

Unfortunately, Mouser doesn't let me search by Ciss, or gate charge
either, so it's not a simple task to find a low one but I seem to
remember that when I originally latched onto the STP2NK60Z it was
'pretty low'... but I wasn't taking gm into account.

As I said, it's probably moot for a (AA5) 'table radio', because we
don't need much bandwidth to begin with, but I was pondering it's
potential suitability as a more general replacement. I suspect we
need
the more complex version if one wants 'full capability'.


Just a thought, but if the common source section of the cascode is
boosted
by a bipolar, then a really low Idss FET can be used - like a 40673
or a
BF991, even with both gates tied together its only a few pF.

Sure, you can.

A 'typical' (if there is such a thing) tube in this power range looks
to be about 11 or 12 pF and the BF244 looks to be about 3 pF so I
wouldn't think that's a problem. The BF246 looks to be about 15pF so
it's not as good.

Being depletion MOSFETs, they should be even better.

Better than what in what way?

More similar in character to a grid.


If you mean it seems 'more similar' physically then I suggest the SS
device, as a whole, is sufficiently different as to make focusing on
one bit not very illuminating but if you like them there's nothing to
prevent it's use. At least, I don't see one off hand.

I think it better to focus on device parameters, like the BF244 has
higher breakdown but the BF991 has more gain. And then there's that
one is through hole and the other is surface mount.

I wanted to buy some of the old BF9xx "pill" devices but couldn't find
any,
I got hold of a few BF991 SMDs and the only UHF types had Vds 7V abs-max,
there's some very old TV tuners in a tea chest in the garage, so I do
have a
very small selection of "pill" types.


7V? Boy, that's not a lot to play with. I don't know what values would
be needed to make that one work but, for example, in the BF244 v8
circuit your 7V job wouldn't survive because there's up to 16V across
the jfet (a bit over 9V even just at idle).

Remember, the MOSFET gate is fixed and there has to be enough
headroom for the jfet *plus* current swing through it's source
resistor (plus the NPN).

One might try a swinging gate on the power MOSFET but that creates
stability and frequency response problems for the same reason we went
cascode to begin with.

I found that if you cut a 4-hole square of Veroboard and carefully
V-groove
across the 2 tracks with a scalpel, making 4 copper lands with a hole
through each, the 4-pin SMD sits between the lands nicely, add 22SWG lead
wires through the holes and the device is ready for general
experimenting..


That's clever but I found TO-92 devices just sort of come "ready for
general experimenting." hehe

On the other hand it's irritating to pay shipping for a 10 cent
device.


There's no reason the same can't be done directly on the underside of a
project stripboard, it just requires a bit of carefull planning of the
layout.


P.S. If you want to just try something why not start with the simpler
'plain Jane' version?


As explained at the start, I don't have an old radio to experiment on, so
its a purely theoretical excercise.

Thanks to the suggestions in the replies, I shouldn't have any trouble
getting started should the opportunity for practical arise.




  #31   Report Post  
Posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 182
Default Experiment (valve).

On Sun, 02 May 2010 12:28:29 -0500, flipper wrote:

On Sun, 02 May 2010 06:56:01 -0700,
wrote:

On Thu, 29 Apr 2010 11:33:10 -0500, flipper wrote:

On Thu, 29 Apr 2010 08:41:52 -0700, Jim Thompson
wrote:

On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?



You're making it too hard.

Just slip a 2N7000 under your 900V device (making a cascode), tie the
900V device gate to the +30V, and bias the 2N7000 as desired.

Personally I've done this where the 900V device was a toob, and, in
the cathode circuit was an NPN with an OpAmp wrapped around it ;-)

Well, in the toob world there are those who would say the same thing,
that "You're making it too hard," since the toob will self bias with a
simple resistor (audio bypassed for maximum gain) under the cathode,
grid to ground, similar to what is done with a simple jfet current
source.


Except for when it doesn't work, to bias the final / power output stage.


You'll have to explain what you mean by "for when it doesn't work"
because self biased was the most common means of biasing output
stages, with fixed bias next.

I wouldn't say it's necessarily the 'best', depending on what the
goals are and, so, how 'best' is measured, but it's arguably the
'least hard'.


It is mostly a pointer to push-pull output class AB finals. Self bias
works very poorly in that case. The issues start with low PSRR and poor
linearity.
Nor does self bias work well with class A single ended power output
stages.
I regularly saw combined bias in power output cases, often with negative
grid supplies.
  #32   Report Post  
Posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 800
Default Experiment (valve). - NFET.jpg


"flipper" wrote in message
...
On Wed, 05 May 2010 16:01:07 -0500, flipper wrote:

On Wed, 5 May 2010 20:58:30 +0100, "ian field"
wrote:


"flipper" wrote in message
news On Wed, 5 May 2010 15:53:19 +0100, "ian field"
wrote:


"flipper" wrote in message
om...
On Tue, 4 May 2010 21:20:41 +0100, "ian field"
wrote:


"flipper" wrote in message
news:8vo0u5lcehppt1if7msgt9gujo0qe87ag7@4ax .com...
On Tue, 04 May 2010 09:11:47 -0700, John Larkin
wrote:

On Tue, 04 May 2010 01:03:43 -0500, flipper
wrote:

On Mon, 3 May 2010 23:02:27 -0500, "Tim Williams"
wrote:

"flipper" wrote in message
news:1k4vt5pctqpqush7g57aappqgt1e4ooubd @4ax.com...
So you really don't think it matters whether one tries to drive
the
gate through a 10 ohm or 100 megohm resistor?

I said cutoff frequency, which means voltage source. Do you have
a
0
ohm
source at ~1GHz? No? Then you won't get to see fT. ;-)

Tim

I know we won't see it so I have no idea what the heck your tap
dance
is for.

Gm/Co was a popular figure of merit for tubes used in video amps
and
oscilloscopes. They took a hockey-stick turn upward when frame-grid
tubes were invented, just before semiconductors took over.

Mosfets have impressive Gm/Co values, at the cost of high input
capacitance.

John

I understand but that doesn't answer the question at hand because
we
do not have unlimited gate drive capability. FOM, Rdson x Qg, isn't
the issue as the device is 'capable' of more bandwidth if we could
drive it but, because the 'driver' already exists, we do not have
that
choice. At least not without buffering it with something and if we
do
that we might as well use the jfet under the source version. But,
in
either case, it's no longer 'plain Jane'.

For a given MOSFET technology FOM is roughly constant and I was
asking
if the same kind of thing applied to our problem of trading off gm,
with source feedback, vs Ciss, because that is what the driving
impedance works into.

Unfortunately, Mouser doesn't let me search by Ciss, or gate charge
either, so it's not a simple task to find a low one but I seem to
remember that when I originally latched onto the STP2NK60Z it was
'pretty low'... but I wasn't taking gm into account.

As I said, it's probably moot for a (AA5) 'table radio', because we
don't need much bandwidth to begin with, but I was pondering it's
potential suitability as a more general replacement. I suspect we
need
the more complex version if one wants 'full capability'.


Just a thought, but if the common source section of the cascode is
boosted
by a bipolar, then a really low Idss FET can be used - like a 40673
or a
BF991, even with both gates tied together its only a few pF.

Sure, you can.

A 'typical' (if there is such a thing) tube in this power range looks
to be about 11 or 12 pF and the BF244 looks to be about 3 pF so I
wouldn't think that's a problem. The BF246 looks to be about 15pF so
it's not as good.

Being depletion MOSFETs, they should be even better.

Better than what in what way?

More similar in character to a grid.


If you mean it seems 'more similar' physically then I suggest the SS
device, as a whole, is sufficiently different as to make focusing on
one bit not very illuminating but if you like them there's nothing to
prevent it's use. At least, I don't see one off hand.

I think it better to focus on device parameters, like the BF244 has
higher breakdown but the BF991 has more gain. And then there's that
one is through hole and the other is surface mount.

I wanted to buy some of the old BF9xx "pill" devices but couldn't find
any,
I got hold of a few BF991 SMDs and the only UHF types had Vds 7V abs-max,
there's some very old TV tuners in a tea chest in the garage, so I do
have a
very small selection of "pill" types.


7V? Boy, that's not a lot to play with. I don't know what values would
be needed to make that one work but, for example, in the BF244 v8
circuit your 7V job wouldn't survive because there's up to 16V across
the jfet (a bit over 9V even just at idle).

Remember, the MOSFET gate is fixed and there has to be enough
headroom for the jfet *plus* current swing through it's source
resistor (plus the NPN).

One might try a swinging gate on the power MOSFET but that creates
stability and frequency response problems for the same reason we went
cascode to begin with.

I found that if you cut a 4-hole square of Veroboard and carefully
V-groove
across the 2 tracks with a scalpel, making 4 copper lands with a hole
through each, the 4-pin SMD sits between the lands nicely, add 22SWG lead
wires through the holes and the device is ready for general
experimenting..


That's clever but I found TO-92 devices just sort of come "ready for
general experimenting." hehe

On the other hand it's irritating to pay shipping for a 10 cent
device.


There's no reason the same can't be done directly on the underside of a
project stripboard, it just requires a bit of carefull planning of the
layout.


P.S. If you want to just try something why not start with the simpler
'plain Jane' version?


Just out of curiosity, regarding the basic JFET/MOSFET cascode with the JFET
buffered by a bipolar - would there be any particular advantage from using a
single BJT current source in place of the source/emitter resistor?


  #33   Report Post  
Posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 182
Default Experiment (valve).

On Tue, 11 May 2010 00:20:39 -0500, flipper wrote:

On Mon, 10 May 2010 19:10:40 -0700,
wrote:

On Sun, 02 May 2010 12:28:29 -0500, flipper wrote:

On Sun, 02 May 2010 06:56:01 -0700,
wrote:

On Thu, 29 Apr 2010 11:33:10 -0500, flipper wrote:

On Thu, 29 Apr 2010 08:41:52 -0700, Jim Thompson
wrote:

On Thu, 29 Apr 2010 16:22:59 +0100, "ian field"
wrote:

Here is a circuit that I scribbled on an odd scrap of paper that I would
like to experiment with, but unfortunately I don't have an old valve radio
to use as a testbed.

The component values that are pencilled in will doubtless need revising and
the rest by trial and error.

The plan is of course to make a MOSFET substitute for the O/P pentode in an
old valve radio, it has 3 stages to maintain its invertingness in case the
audio amplifier has a nfb loop.

Anyone care to cast a critical eye over it and predict how loud a bang it
will make?



You're making it too hard.

Just slip a 2N7000 under your 900V device (making a cascode), tie the
900V device gate to the +30V, and bias the 2N7000 as desired.

Personally I've done this where the 900V device was a toob, and, in
the cathode circuit was an NPN with an OpAmp wrapped around it ;-)

Well, in the toob world there are those who would say the same thing,
that "You're making it too hard," since the toob will self bias with a
simple resistor (audio bypassed for maximum gain) under the cathode,
grid to ground, similar to what is done with a simple jfet current
source.


Except for when it doesn't work, to bias the final / power output stage.

You'll have to explain what you mean by "for when it doesn't work"
because self biased was the most common means of biasing output
stages, with fixed bias next.

I wouldn't say it's necessarily the 'best', depending on what the
goals are and, so, how 'best' is measured, but it's arguably the
'least hard'.


It is mostly a pointer to push-pull output class AB finals. Self bias
works very poorly in that case. The issues start with low PSRR and poor
linearity.


I'm not sure where you get the 'low PSRR' and 'poor linearity' ideas.
Compared to what? PSRR is primarily a function of hum into the plates
(and screens) and is going to be essentially the same whether cathode
or fixed bias. And if things are well balanced it'll cancel in the
push pull output transformer.

The 'linearity' issue is inherent to the A to B transition and is
there with fixed bias too.

Where self bias has a 'problem' is in sustained 'B' operation as bias
voltage will rise due to the higher 'average current'. But, in
practice, that isn't a problem because 'music' is not a single
mono-tone max volume note and the 'B' region only handles transient
peaks. That is, as long as there is enough 'A' in the AB to well cover
the average music level.

That isn't the case in heavily driven Class AB guitar amps so they are
most commonly fixed bias but most commodity, as well as low to medium
level 'hi-fi', AB amps used self bias for the (power and) simplicity
(cost) and convenience of no user adjustment required. Meaning you
didn't have to worry about Blondie buying replacements out of the
corner Piggly Wiggly tube tester and setting Dagwood's phonograph on
fire because she didn't know to re-bias the amp.


Nor does self bias work well with class A single ended power output
stages.


That's where it works best because, in idealized theory, there is no
'change' in average current. In reality, distortion causes a slight
change but virtually every AA5 radio made, and the vast majority of TV
sets, employed single ended, self bias, audio outputs.

Class A, btw, is also where simple bias servos work best, and for the
same reason: current is relatively constant. When you think about it,
most bias servos are essentially glorified 'high gain' variations of
self bias.

Hum is considerably more difficult to deal with SE, because there's no
inherent cancellation, but the less overall complexity more than makes
up for it (in the context of commodity table radio and TV set cost).

I regularly saw combined bias in power output cases, often with negative
grid supplies.


What would be a "combined bias" without a negative supply?

The holy grail of AB (tube) bias has always been to devise a 'fixed
bias' that 'self biases' and Lincoln Walsh may have come closest with
the Brook Bias.

http://www.tubecad.com/2007/12/15/Hi...%20Control.pdf

The idea is, if you look at the sum of the dynamic current through
both output tubes in an AB amp the 'minimum' observed value represents
twice the idle current. (Spice it. It works).

His circuit merely detects that minimum and then adjusts grid voltage
accordingly. The problem, however, is in accurately catching the
minimum of a variable frequency waveform so it tends to fluctuate as
well, but not so much as 'plain' self bias.

Patrick Turner has a different approach where 'excess' (meaning
essentially the 'B' portion of AB) current is shunted around the 'self
bias' resistor.

http://www.turneraudio.com.au/300w-5...stabilizer.htm


Am i getting this straight? You are proposing servoed bias as simple PA
bias? That servoed bias is comparable to self bias? Somehow i suspect
not. The examples you show argue for my case that self bias (and fixed
bias to a lesser extent) does not work well for power output stages.
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