[Big Bad Bob]

On 02/01/16 10:10, John L Stewart so wittily quipped:
I would expect screen resistance to change based on G-K volts,
according
to some kind of general curve, and it's probably NON-linear at that,
based on the actual screen volts, plate volts, plate current, blah blah
blah.

But yeah, get it wrong and you howl and screech like a poorly
configured
regenerative receiver.

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OK Bob, you bring up a good point. But things are not as ominous as
that. See below-

Terman’s Explanation of Change of rs with Change of Screen Current

(etc.)

this probably plays a big role in determining the series resistance
needed for a proper ultra-linear config. As for me, I think I prefer
using a fixed and well regulated (and short-circuit protected) voltage
for G2 in the power stages. It also tends to give you a bit more power
out of the same tubes, with somewhat higher distortion depending on the
amplifier configuration [if you're doing AB2, it's probably less
significant, and G1 current probably becomes the primary source of
distortion and whatnot]. My focus is normally on 'final stage' beam
power and power pentodes.

In low power amplification stages with pentodes, this is a completely
different thing.

Biggest problems with pentodes overall is the nonlinearity. It's great
for mixer stages, great for frequency synth in radios, great for IF with
AGC, not so good for audio amplification. Basically, a pentode can act
as nonlinearly as a bipolar transistor. Correcting for this in audio
circuits requires lots of negative feedback, and so you point out the
problems with feedback and G2 series resistors and bypass capacitors [etc.].

Most pentode tube usage I've seen is in RF, not AF, and RF is where they
do the best, particularly the 'sharp cutoff' variety [a side effect of
their nonlinearity] for AGC and similiar circuits.

I'm not a fan of using them in AF circuitry. A dual triode typically
gives you as much open loop amplification using 2 stages, with
significantly lower distortion.

Anyway, this whole thing about G2's biasing and filtering could become
the topic of an entire series of articles or even a book.