On Tue, 11 May 2010 00:20:39 -0500, flipper wrote:

On Mon, 10 May 2010 19:10:40 -0700,
wrote:

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.

I'm not sure where you get the 'low PSRR' and 'poor linearity' ideas.
Compared to what? PSRR is primarily a function of hum into the plates
(and screens) and is going to be essentially the same whether cathode
or fixed bias. And if things are well balanced it'll cancel in the
push pull output transformer.

The 'linearity' issue is inherent to the A to B transition and is
there with fixed bias too.

Where self bias has a 'problem' is in sustained 'B' operation as bias
voltage will rise due to the higher 'average current'. But, in
practice, that isn't a problem because 'music' is not a single
mono-tone max volume note and the 'B' region only handles transient
peaks. That is, as long as there is enough 'A' in the AB to well cover
the average music level.

That isn't the case in heavily driven Class AB guitar amps so they are
most commonly fixed bias but most commodity, as well as low to medium
level 'hi-fi', AB amps used self bias for the (power and) simplicity
(cost) and convenience of no user adjustment required. Meaning you
didn't have to worry about Blondie buying replacements out of the
corner Piggly Wiggly tube tester and setting Dagwood's phonograph on
fire because she didn't know to re-bias the amp.


Nor does self bias work well with class A single ended power output
stages.

That's where it works best because, in idealized theory, there is no
'change' in average current. In reality, distortion causes a slight
change but virtually every AA5 radio made, and the vast majority of TV
sets, employed single ended, self bias, audio outputs.

Class A, btw, is also where simple bias servos work best, and for the
same reason: current is relatively constant. When you think about it,
most bias servos are essentially glorified 'high gain' variations of
self bias.

Hum is considerably more difficult to deal with SE, because there's no
inherent cancellation, but the less overall complexity more than makes
up for it (in the context of commodity table radio and TV set cost).

I regularly saw combined bias in power output cases, often with negative
grid supplies.

What would be a "combined bias" without a negative supply?

The holy grail of AB (tube) bias has always been to devise a 'fixed
bias' that 'self biases' and Lincoln Walsh may have come closest with
the Brook Bias.

http://www.tubecad.com/2007/12/15/Hi...%20Control.pdf

The idea is, if you look at the sum of the dynamic current through
both output tubes in an AB amp the 'minimum' observed value represents
twice the idle current. (Spice it. It works).

His circuit merely detects that minimum and then adjusts grid voltage
accordingly. The problem, however, is in accurately catching the
minimum of a variable frequency waveform so it tends to fluctuate as
well, but not so much as 'plain' self bias.

Patrick Turner has a different approach where 'excess' (meaning
essentially the 'B' portion of AB) current is shunted around the 'self
bias' resistor.

http://www.turneraudio.com.au/300w-5...stabilizer.htm


Am i getting this straight? You are proposing servoed bias as simple PA
bias? That servoed bias is comparable to self bias? Somehow i suspect
not. The examples you show argue for my case that self bias (and fixed
bias to a lesser extent) does not work well for power output stages.