Fender Bandmaster Reverb output stage - negative feedback issue

[Chevy]

Gentlemen,

I've just replaced my Bandmaster Reverb output transformer with a new Hammond Bassman xformer, wanting to get a little more meat out of the low end if possible. It seems to work ok, but the sound is a little "spiky". A 400Hz test signal goes right thru to the output tube grids just great, (and even the plates as well) looks excellent on the scope right up till the PI distorts at too high a level. So far so good.
But on the output side, the signal seems a little more sawtooth-looking than the sine wave that is being input. The really weird part is, when I disconnect the negative feedback loop to the stock 820 ohm PI resistor, the amp's output goes DOWN, and the "sawtooth" appearance diminishes. When it's reconnected, the output signal goes UP. Being negative feedback, that doesn't make any sense to me. I assume that if I had the leads reversed on either primary or secondary, it would just howl (oscillate) like mad. On the output I'm using the green wire as the hot, black as the ground. The primary side is wired with the same color scheme as the original transformer. It seems that there isn't any proper negative feedback. Dunno... Any ideas on what's going on here?
Thanks much,
Chevy

[pdf64]

Reverse those OT leads!
There isn't always sufficient positive feedback to cause oscillation, but you do get exactly the symptoms you describe.

[Jazz P Bass]

That is exactly "how" you test the OPT polarity.
If you disconnect the feedback & the output goes up, then it is correct.

[Tom Phillips]

... I assume that if I had the leads reversed on either primary or secondary, it would just howl (oscillate) like mad....

That is what people mostly seem to say. However, as has already been stated, it just isn't so all the time. I think this is another one of those "facts" that just gets repeated in discussion forums. Often passed on by people who don't have first hand experience.

Tom

[Chevy]

Well, how about that?

I reversed the OT leads and voila! Captain, thar be tones!
Live and learn, huh? I was so worried (by what I'd read long ago on this forum) that I may do some damage or something... seems like the color code of the original OT isn't the same as the new one... well there you go. I'm now a bonafied transformer putter-inner.

Thanks for your comments, guys!

[Chuck H]

Great. The first time it happened to me was with a "replacement" tranny for a Fender amp that also had the blue/brown leads opposite from the stock OT. I was green enough at the time to wonder if I had somehow spoiled the layout and induced an oscillation due to proximity. A quick Q here put me right. That was a long time ago. I still love this place.

[Jazz P Bass]

Years ago, I inadvertantly reversed the connector on a Peavey Classic 50 repair.
Oscillated like crazy.
Not knowing what was what, I called Peavey.
"Reverse the output connector" was there immediate reply.