Output Transformer Primary Wires

[deuce42]

Hi guys, could you let me now how I determine which wire orientation to use for a Hammond 15watt AB763 style output transformer?

Normally the output transformer's colour code has a brown and blue wire (other than the centre tap) for the primaries. I realise however that unless you have the original transformer, the colour code is irrelevant and the consensus is to simply try both primary wire orientations and see which works.

On a previous amp build I found that the amp squealed when I had the primary wires one way so reversing them was easy. On my current build however, no squealing occurs (amp uses normal AB763 NFB) with either orientation. However, I notice that with one way the amp has :

1) a lot more hum/buzz, but
2) a lot more "sparkle" or treble sound to it like an EQ device was added to the signal or the treble pot is turned up higher; and
3) the amp signal is louder and stronger.

Using the brown and blue wires the other way reduces an enormous amount of hum and buzz so the amp is mostly quiet, but seems to sound a bit flat and dull.

Can anyone tell me how I work out what orientation should be used?

Thanks

[sluckey]

Using the brown and blue wires the other way reduces an enormous amount of hum and buzz so the amp is mostly quiet, but seems to sound a bit flat and dull.

This is the correct phasing.

[deuce42]

Thanks Sluckey. So is it the case that any background hum/buzz can also make clear the phasing is wrong, just like the squealing in some amps is a give away?

[sluckey]

3) the amp signal is louder and stronger.

This is what I keyed on.

[deuce42]

ah so is the louder and stronger signal the wrong phasing and the correct one is the less noisy but less strong?

[Paul G.]

You're in phase with the signal being injected as negative feedback. Negative feedback reduces noise, and gain. Because you're in phase, it's no longer negative feedback. It doesn't have to squeal to be wrong, more gain and noise are also symptoms.

[pdf64]

Hammond 15watt AB763 style output transformer

What model is that?

I’m struggling trying to recall EM theory.
Do the winding ends marked with a dot to indicate the start of the winding (?) have the same or opposing signal polarities? Pretty sure but not certain they’re the same polarity. But if that’s the case, then Hammond’s brown / blue colour code polarity seems to the opposite to a typical Fender layout. https://www.hammfg.com/files/parts/pdf/1760J.pdf

[TUBEDUDE]

If after playing it for a while you still think it's flat and dull, try fine tuning the feedback circuit. Reducing it will give you some liveliness/feel.