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
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