"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.