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.
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For sure a pair of triodes will get more gain than a pentode. But the 2nd triode of the pair amplifies all the distortion of the first as well as the fundamental resulting in several even higher order harmonics, not exactly what we need.
As it turns out a pentode does have lower D than it does as a triode over a useful section of its output voltage range. Refer to the attached work done many years ago.
That range corresponds to the region where most hi G power pentodes such as EL34, KT66, 6550 & son on can be driven to full power. And running thru a split load phase invertor can easily drive a PP pair to full power, something Dyna & others took advantage of. Fewer stages translates to fewer high order harmonics before NFB is applied. And much better stability margin & reliability.
In the normal listening range the pentode looks much better than it does hooked up as a triode.
Cheers to all, John L Stewart
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