odd nfb squeel

[worth]

I've been collecting Workers' Comp for the last 4 months, so I've had time to hear a lot of circuits that I've never played through myself. I took an amp I built years ago, that originally was a single-channel 40-ish watt Fender with reverb, and converted it to.. a Princeton Reverb, a Brown Bassman , a Brown Vibrolux, a 5F6-A Bassman, A Marshall JTM-45, AND FINALLY , a 5F4 Super , ( which is my favorite so far ). I've used the same OT for all of these builds... but on the Super , I had to switch the OT wires because of NFB squeel !!? What is different about the 5F4 circuit ?

[sluckey]

The PR and Super both have the same split load PI circuit, BUT..., the layout is different. When comparing the two amps, the two phases from the PI are crossed, ie, feed different power tubes. If you don't swap OT plate wires one amp will squeal, the other wont.

The other amps all use LTP PI.

[HotBluePlates]

Said another way: where the negative feedback is injected earlier in the circuit will determine whether it is positive or negative with respect to the speaker output.