[Alex Pogossov]

The recent thread about the SET 300B based amplifier got me to question: Why
triode?

1) Obviously, a triode 300B boasts low distortion say 2% at 6...8W output
and reasonable damping WITHOUT GNFB, but the penalties a low plate
efficiency, complexity of filament supply, cost, possible self-destruction
if shorted load, full volume and fixed bias.

2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in
parallel) design needs GNFB to achieve the same benchmarks, but more
efficient, cheap and will not blow the tubes. A penalty here is probably the
GNFB loop which encompasses the OPT and would require shelving or other
elaborate compensation, which only Mr Turner can properly master to achieve
stability over the wide range of the load impedance.

3) Yet there is another possibility -- use pentodes (or beam tetrodes) with
local feedback. In effect the goal here is to reduce the gain (from G1 to
plate) of the stage to about the typical mu of a rival triode. In other
words, replace the internal electric field local NFB in a triode by an
external local feedback around a pentode.

One way is to use cathode feedback with the cathode winding having about 1/4
of number of turns of the plate winding. A drawback here is a special
transformer to be made.

Another simpler method is to arrange a resistive local feedback. Throw say
R1=470K from the output tube plate to grid, through a DC blocking cap of
course. Feed the input signal to the grid via a series R2=150K resistor.
Thus the output stage will resemble an inverting op-amp, the gain of which
is defined by the ratio of R2 / R1. This is because an inherent gain of a
pentode from grid to plate is high. Say for two EL34s in parallel it can be
up to 50. (More practical to throw R1 from the output tube plate to the
driver tube plate without a DC blocking cap.)

Thus the local feedback can reach 15...20dB. As a result:
a) Drive voltage will be comparable to 300B;
b) Distortion will be same if not lower than with 300B;
c) The local NFB will be unconditionally stable, since it does not include
OPT;
d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1)
+ 1). With a paig of hi-gm tubes as EL34s it will be even lower than with
300B;
e) Will yield higher efficiency with the same DC input power;
f) No real need for resonance damping circuits across OPT primary;
g) No GNFB will be needed;
h) Not prone to self destruction with shorted load.

A drawback though is a low input impedance (about R2). However, with a
cathode follower in the driver stage it will not be an issue.

I am wondering if such "op-amped" pentode stages are common. What am I
missing?

Regards,
Alex