[Big Bad Bob]

(I said)
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.


then, On 02/04/16 06:43, John L Stewart so wittily quipped:

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.

yeah, that's somewhat unavoidable. so you suggest that single stage has
lower overall distortion? Hard to tell, but I suppose it would depend
on the overall circuit design, operating range, etc.. and whether you're
using 'mu factor' gain or an unbypassed cathode resistor to provide a
small amount of negative feedback in the stage.

[my favorite includes partial bypass to give you proper cathode bias but
still control the gain, maybe 1/2 or 1/3 of the mu factor per stage]

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.

saw that, measured IM distortion but for the same tube [not an actual
triode, just triode-configured pentode]. But it doesn't address the
higher noise typically found in pentode amplifiers due to the additional
grids, etc..

So maybe it makes the case for a pentode at the mid-point, then? Low
signal input levels sort of demand a triode to improve S:N (and high
quality low noise grid resistors with values below ~200k).

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.

OK - that would make sense. dual pentodes driving power output tubes,
maybe, or single-ended pentode driving a phase-split transformer. Since
off-the-shelf pentodes typically run at higher currents than triodes,
AB2 amplifiers would possibly work better.

In the normal listening range the pentode looks much better than it does
hooked up as a triode.

I'd still like to see the noise measurements, comparing actual triodes
to pentode, maybe 12AX7 [which would be the typical triode to use for
low level signals]. I suppose I'd run my own o-scope test for that one,
if I had any pentodes laying about to test. There are a number of
things to look at, from grid resistor values to capacitor materials to
power supply rejection. Those would have to be factored out somehow for
a fair comparison.