HF squeal - does polarity matter for SE OT connections?

[Groover]

I have read in various places that sometimes if an amp is oscillating the OT wires may need to be swapped. I think they were referring to the hot leads of the primary, (not the center tap of course) but I don't recall for sure.

I just replaced the Fender OEM Vibro-Champ transformer in my 5F2a experimental build with another SE OT with an 8 ohm output impedance (5k primary). The primary colors were the same as the Fender (red & blue) so I connected them the same: red to B+, blue to 6V6 plate. But the secondary wires are yellow and black on the Fender (yellow to hot, black to ground) and they are green and black on the replacement OT, so I connected them green to hot, black to ground.

I am now getting high squeals when I turn the adjustable NFB knob too far in the more NFB direction, and/or when I activate the gain boost (cathode bypass cap on 2nd stage.) I am also getting some noise (buzz/static) I don't recall having before the swap.

Any ideas?

[martin manning]

If you have a feedback loop, SE or push-pull, you can get oscillation if you have positive FB. Try reversing the transformer's secondary leads, leaving everything else as is. You could reverse the primaries, but you might as well leave the color coding as it was originally.

MPM

[Groover]

Is it asking for trouble to secure the (twisted) OT primary and (twisted) OT secondary leads to the (twisted) heater wires with cable ties for neatness's sake?

[martin manning]

I would not tie those together, twisted or not. If that's what you've got, separate them and see if that helps.

MPM

[Groover]

Swapping the secondary leads did cure the problem. I did un-bundle the OT leads from the heater wires, but did not do a before & after test of that, just did it when I swapped the OT leads.

Thanks for your help.