Out Tranny primary leads

[trem]

We have all heard the squeal when we hook up an OT with the plate leads reversed from how they were originally wired.

Why does that matter..... what is going on with that scenario.?
Thank You

[glebert]

Reversing the phase will turn negative feedback into positive feedback.

[mozz]

You wont hear any squeal if your amp doesn't use negative feedback.

[g1]

Different amps may do it in other ways, but in the classic AB763 Fender circuit, there is a resistor from the output jack connecting back to the PI stage. This is the NFB (negative feedback) loop.
It is the opposite phase of what the regular signal is doing at that part of the PI. Also called degenerative feedback, it cancels out part of the signal there.
If the OT phase gets reversed, instead of cancelling out part of the signal, it boosts it. This can cause various forms of instability, one of which is the squeal type of feedback noise.

[The Dude]

Agree with above and add: You are basically turning an amplifier circuit into an oscillator circuit. If you look at most any oscillator, positive feedback is a key component.

[trem]

Oh Yeah..... NFB
I forgot all about that

Thanks All

[Tom Phillips]

It is also worth noting that positive feedback does not always result in a loud squeal. Sometimes it's just an instability that gives a bad response when playing the amp. A good test is to break the feedback path, play a tone and re-connect the feedback path. If the sound level reduces when the feedback path is connected then the feedback is negative. If the level increases then the feedback is positive. This is a test that you can do without using any test equipment.

[trem]

It is also worth noting that positive feedback does not always result in a loud squeal. Sometimes it's just an instability that gives a bad response when playing the amp. A good test is to break the feedback path, play a tone and re-connect the feedback path. If the sound level reduces when the feedback path is connected then the feedback is negative. If the level increases then the feedback is positive. This is a test that you can do without using any test equipment.

In all honesty... not because i am so together, just some luck and paying attention... i do not think i created that squeal more than once. It is something you do mot forget very easily

Plus, disconnecting the OT primary is not the most common procedure in guitar amp "fixing"