"Phil Allison" wrote in message
...

"David Nebenzahl"
Phil Allison spake thus:

Also, the power output will of course drop in triode mode and each EL84
will dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not
be symmetrical.

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.


with the other triode acting as *another* phase inverter cascaded from
the first stage, presumably with unity gain. Is that correct?

** Nope.

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.

Because of the easy drive requirements of an EL84, such a concertina
circuit is able to send them deep into overdrive - while maintaining good
symmetry. Not so for 6L6GC or EL34 stages which need several times more
drive voltage.


.... Phil



So is it the fact that the circuit relies to some extent on NFB for correct
symmetrical overdrive operation, that leads you to believe that this might
be upset by operating in a pseudo-triode mode ?

Interesting analysis. I'll report back when I've tried it

Arfa