"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.