Thanks, Ronald. No I don't read German (wish I did...only foreign
language I know even a bit of is Latin, unfortunately. I blame the
Jesuits.) but I can read the pictures and the flow of equations :-)
Now I remember what a SEPP is and hey, I have the parts to build one
right now, with ECC81 and a spare pair of 6CH6. PS would be awkward
though. No chance of driving these speakers either, says the humble
Choky.

But where does the "double choke come in"? Still don't quite know
what I'm on about. Looking through the thread I can't see a current
link to that circuit. Coupling the screens? At a guess only
problem might be capacitance.

I found with my simulated parafeed circlotron, and my various
simulated totem-poles including SEPP and mu-follower, that they
seemed very wonderful in almost every respect, but there is always a
quirk. Some are too easily provoked into current limiting, and some
have output impedance that behaves strangely. And some seem like
they can source current but can't sink it so good. Perhaps I just
need more volts. Transformers that may seem adequate with casual
investigation can turn out to be not so.

One mechanism is where low-frequency response is extended by
feedback...particularly cathode feedback which reduces Vak...so
early clipping results at low frequencies if you aren't careful, and
also possibly slew-rate limiting for roughly the same reason. A
related problem with cheap trannies is saturation. Another is
where leakage doesn't seem important until you consider
high-frequency output impedance.

None of which may apply to the mysterious double chokes.

I don't mean to detract from the fun of experimentally discovering
such things, and trying to work out why. I just don't like the
smell of solder ;-) Simulation is fun too. And easy and cheap and
safe.

In both cases, however, if you don't grasp the maths, you're
stumbling. Transformers have too many equations bundled together to
be encompassed by a single thought, and sometimes I get lost.

best wishes

Ian