6G2 Build. Negative Feedback Question

[keithb7]

I recently plugged in my new Fender style 6G2 build. I built it pretty much stock like Fender did back in 1963. However I used an OT with 4/8/16 ohm tap options. It sounds great. I left the negative feedback wire off on purpose. I am planning to make it switchable. The amp is quiet and behaves like it should. Tonight I decided to hook up the negative feedback wire. The amp squeals and my dog hides with it hooked up. With or without the on/off switch, I
was planning to use. Do I need to reverse the wires from my OT to my speaker jack? Without any NFB the amp sounds just fine. NFB resistor is stock at 56K.

I hooked up two separate OT jacks using 4 ohm and 8 ohm OT taps only. Jacks are 1/4" Switchcraft mono 2 wire type, the non-switching style. Positive goes from each OT tap (4 and 8 ohm) to each individual jack. The common wire from the OT is shared with both jacks.

Thanks, Keith

[Justin Thomas]

The usual fix is to reverse the OT Primary wires that go to the power tube plates, but I believe your solution should work, too. Fender did it on the AB165 & Silverface Bassmans when they changed the location where the NFB was inserted...

Justin

[loudthud]

The OT has a single (common) wire that is grounded so that the 4 and 8 Ohm taps work as intended. You can't really reverse wires at the speaker jack(s). You need to reverse the wires at the power tube plates (pin 3) or reverse the wires feeding the power tube grids (pin 5). Don't do both!

[Justin Thomas]

Point taken, LT. All my Bassmans only have a single secondary winding, so they could be reversed. I'll try to remember that for future use!

Justin

[keithb7]

Thanks very much folks. Glad I asked. With other single tap OTs I suppose you can just flip the wires on the jack. Not with a muti-tap. Good to know!

[loudthud]

Yes the multi-tap OT causes complications. Sometimes one of the plate wires from the OT is cut too short to reach one of the tubes when they need to be reversed. Rather than splice an extension to the short plate wire, the wires to the grids can be reversed and if one is too short simply replace it.

[Chuck H]

I don't mind adding a splice to the OT leads. Aesthetics don't matter much to the circuit I do, however, leave a little extra lead OT now on new builds so I can swap leads without needing to splice (just trim and shorten if it's already correct). For the few amps I've actually built it's surprising how often the OT primary leads (often brown and blue) are inverse to what would be vintage, or "stock". So even as a reasonably experienced builder I still get that squeal occasionally on first run.