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Experiment (valve).



 
 
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  #11  
Old May 2nd 10, 02:56 PM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 182
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.
Ads
  #12  
Old May 2nd 10, 03:20 PM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 800
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 ?


  #13  
Old May 2nd 10, 04:37 PM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 1,941
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
  #14  
Old May 2nd 10, 09:49 PM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 254
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





Attached Images
File Type: bmp Tube Half Bridge.bmp (33.2 KB, 32 views)
  #15  
Old May 2nd 10, 10:59 PM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 1,420
Default Experiment (valve). - NFET.jpg

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




Attached Thumbnails
Experiment (valve).-nfet-jpg  
  #16  
Old May 3rd 10, 12:07 AM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 384
Default Experiment (valve). - NFET.jpg

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.
  #17  
Old May 3rd 10, 01:08 AM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 1,420
Default Experiment (valve). - NFET.jpg

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

  #18  
Old May 3rd 10, 02:59 PM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 1,941
Default Experiment (valve). - NFET.jpg

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
  #19  
Old May 3rd 10, 04:31 PM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 800
Default Experiment (valve).


"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?


  #20  
Old May 3rd 10, 11:25 PM posted to alt.binaries.schematics.electronic
external usenet poster
 
Posts: 254
Default Experiment (valve). - NFET.jpg

"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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