The OT I'm using has a center-tapped secondary. In its original application, it was grounded at the CT. Is there any reason to choose that over grounding at one end? It would just change the ground reference, which doesn't matter to the speaker, right?
Either connection is ok. Sometimes partial cathode coupling is used, where the output tubes' cathodes
are connected to the 16 ohm and 0 ohm ends of the secondary, with the center tap (4 ohm tap) grounded.
Also with the 4 ohm tap grounded, balanced nfb can be used from each end of the secondary.
are connected to the 16 ohm and 0 ohm ends of the secondary, with the center tap (4 ohm tap) grounded.
Also with the 4 ohm tap grounded, balanced nfb can be used from each end of the secondary.
Another reason to ground some other tap is if you need to take negative feedback inverted from normal phase.
For example grounding the 4 ohm tap also gives you a balanced pair of feedback signals.
For example grounding the 4 ohm tap also gives you a balanced pair of feedback signals.
Amplifiers which drove looooong cables, such as factories or school buildings, were reputed to have less instability with firmly balanced (CT) speaker lines.
I don't think this is true even of theater amps. So maybe <100', whatever, >200', you better CT.
And of course the clever tricks above, notably local NFB to a push-pull stage.
This is going to be driving a 3ft cable, and that's just because I don't have anything shorter. That's interesting, though. I don't have any use for balanced feedback on this amp, but I've got another of these OTs that I may experiment with.
The original amp, in its role amplifying a Hammond M3, didn't do anything clever with it, and it was driving no more than maybe 2ft. Go figure.