View Single Post
  #12   Report Post  
Posted to alt.binaries.schematics.electronic
ian field[_2_] ian field[_2_] is offline
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 ?