NFB question

[conisat]

Having read a few threads stating when NFB is out of phase the OP primaries should be swaped and not the secondaries. Is this just to keep the speaker negative to ground or is there another reason??

[tubeswell]

My understanding of this is that negative feedback is supposed to be out-of-phase with the AC swing on the electrode/cathode in the stage you are inserting the NFB loop into because it is supposed to reduce gain. That is the way it works. If it is in phase, it results in positive feedback, and concommittent squealing/howl etc. (That is why if the amp squeals from having the NFB loop sourced from the 'wrong' side of the OT secondary, you have to swap it to the side that is out-of-phase with the signal you are applying the NFB to. This can be done from swapping the primaries around on SE OTs).

[defaced]

If you have an OT with multiple secondaries, swapping the secondary that the NFB is taken from with ground can screw up your other secondary taps impedance.

[Enzo]

In a simple single secondary transformer, yes, you could reverse the secondary instead. As defaced points out though, in multitap secondaries, the various taps are with respect to one end. SO if you grounded to opposite end, your feedback oscillation would be cured, but instead of a 4/8/16 output. you would wind up with something like 16/12/8. So swapping the primaries is simpler.

[pdf64]

Or swap over the wires to the power tube grids. Whichever is easiest to do - Peter.