"David Nebenzahl"
Phil Allison spake thus:

If you wouldn't mind indulging my relative ignorance for a minute,
that "concertina" stage business got my curiosity up. So I found
the two triode (12AX7) stages before the EL84s, and it looks to me
like the first stage (V3A) is a phase inverter,

** With an imagination like that - you should be writing fiction for a
living.

A " phase inverter" ( or phase splitter) has equal and opposite polarity
outputs.

OK, my bad: poor choice of terminology. What I meant to say is that both
stages are inverting stages, being common cathode. Correct?


** I already explained the circuit - but you decide to snip the lot.

Here is the schem:

http://www.el34world.com/charts/Sche..._classic30.pdf

Here is the missing explanation:

" V3A is a voltage amplifier ( gain of about 50 before NFB) while V3B is
the
"concertina" phase inverter. Concertina circuits always have equal plate and
cathode resistors and in operation, the voltage between the plate and
cathode grows and falls with audio drive like a concertina being pumped in
and out.
The cathode of V3A acts as a negative ( opposite phase) input for feedback
from the output transformer secondary. "

Since one pair of EL84s seems to be driven from V3As plate

** You looking at the schem above or not ?

V3A drives V3B which delivers TWO anti-phase signals (one from the plate and
one from the cathode) so the push-pull output stage can work.

V3B has a gain of 2 as the output is across the plate and cathode.




..... Phil