Output transformer for both SE and PP

[Paul P]

In my amp design I have separated everything past the PI : dual
bias supplies, cathode resistors and caps, etc. This is mostly to
make it easy for me to go from single-ended to push-pull (AB). Up
until now I've thought that to go from one configuration to another
would require changing the output transformer but now I'm wondering
if I couldn't use a push-pull OT for both, and only use half of the
OT for single-ended use.

Is this possible ? The OT I plan to use for push-pull (Allen TO25)
is 7000 to 4/8 ohms and the OT for single-ended (Allen TO11C) is
4000 to 8/16 ohms. Would using only one half of the push-pull still
give 4/8 ohms at the secondary ?

Or could I use two SE transformers and somehow combine their
secondaries ?

I suppose I could mount both a SE and a PP OT and switch from one
to the other but this would mean switching high voltages (?).

Victoria Amps produce an amp that does this sort of thing but it apparently
uses a special OT :

"The Regal II is designed around a custom bifilar output transformer
that is essentially two discrete transformers driving a common
speaker load."

(see http://www.victoriaamp.com/visitor/show_model/14)


Paul P

[MWJB]

*Ideally* SE OTs require an air gap to avoid DC saturation, in reality using a PP OT for a single ended application will "work". However, for the T025 I'd suggest that you use 1x6L6/5881 power tube (as you're only using half the primary) instead of a 6V6 to restore impedance match).

Or try and buy a regal OT from Victoria?

[Paul P]

Originally Posted by MWJB

Or try and buy a regal OT from Victoria?

I just got an answer back. Mark says he'll be happy to sell me one for 2800$.

He did offer to throw in the rest of the amp at no extra charge...

Thanks for the bit about the air gap. Yet one more thing to throw onto the
pile of things one should know.

Paul P