Steve O'Neill wrote:

Hi

You stated at the end of your post:

"Luckily for those radios that escaped the factory
the bottom tube will not get in the way since it's plate reflects
a CC load to the output transformer"

Curious how you arrive at the conclusion that the bottom tube plate acts as
a CC load to the OPT. I built the TRIODEEL compact amp output stage using
recommended circuit values and measured the thing. By the measurements I
made, the bottom tube did not behave as a CC load. The whole thing acted
like a poorly balanced phase inverter: nothing more, nothing less. Did I
measure something wrongly or are we speaking of different circuits here?

--
Steve

I finally got a look at that circuit by way of a link posted by Fabio Berutti.
It is simple enough alright but doomed to poor performance. It may appeal
to the SE crowd since it is bound to generate lots of even order harmonics
with its very shot tail.

I managed to find a drawing of the amp I built using this circuit. It is dated
Feb 3, 1958. At the time I was trying to find a way of building a very low
cost amp for hifi. I had come across a good supply of cheap power
transformers with 170-0-170 HT, so bought several. The basic amps
were various combos of PP pairs of 25L6, 35L6 or 50L6's, so the heaters
could be run straight off the power line. I used a 6SJ7 to drive the PP
amp with the short tail.

I quickly came to the same conclusion that you did, although with less
sophisticated
test equipment. It looked terrible on the scope trace & did not sound much
better.
Why anyone would think this circuit would deliver hifi I'm not sure. We are
told time & again that in this type of phase inverter that for balance the tail

needs to be at least as large as the plate loads & preferable several times
the plate load. This amp finished up with a 6SF5 split load phase inverter.

Pentodes & Beam Tetrodes are inherentely CC devices when operated
in their linear mode. If the short tail were bypassed then you would have
a CC load applied to one end of the output transformer. However, in
this instance, the lower of the PP pair is partially driven, so perhaps not
as CC as when not driven. It would be interesting to get into the circuit
to get some idea of what impedance we are talking about. However,
who wants or needs to spend time it what amounts to trying
to push a rope?

BTW, I have posted both John B' s phase inverter & some other
inverters (one referred too by Fred N), all from RDH4 on ABSE
for everyones enjoyment. See Phase Inverters. John Stewart