On Sun, 02 May 2010 12:28:29 -0500, flipper wrote:

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.

You'll have to explain what you mean by "for when it doesn't work"
because self biased was the most common means of biasing output
stages, with fixed bias next.

I wouldn't say it's necessarily the 'best', depending on what the
goals are and, so, how 'best' is measured, but it's arguably the
'least hard'.

It is mostly a pointer to push-pull output class AB finals. Self bias
works very poorly in that case. The issues start with low PSRR and poor
linearity.
Nor does self bias work well with class A single ended power output
stages.
I regularly saw combined bias in power output cases, often with negative
grid supplies.