Vibro Champ problem

[mresistor]

Well this is kind of like Unclerny's problem below. I have a home built VC AA764 clone. Its in a 12 " cab and has Classic Tone iron. OT has 8 ohm out as well as 4 ohm taps, and I'm using the 8 ohm tap. I had a Jensen 12" in there but yanked it in favor of a 1957 alnico 12" out of a Baldwin organ. The speaker measure 6.8 ohms static. After changing the speaker I get a nasty ulta high pitched squeal with the volume down and all the way up. 2.5 to 9.5 on the knob is OK. Then I notice that I could turn down the intensity pot and it would go away completely. Not sure what to make of that. I pulled one leg of the feedback resistor and the problem is totally gone. So I subbed in a 33K resistor for feedback and the problem is gone. But that is a pretty high value for a feedback resistor in this circuit. It has nothing to do with 12ax7 tubes as I've swapped them out with known good. I'm pretty sure this isn't the common case of OT output phase rev, as the amp was working fine with the Jensen in there. Don't know what to think about this. It's weird.

[sluckey]

I'm pretty sure this isn't the common case of OT output phase rev

I think it is. Very easy to prove/disprove.

[mresistor]

here's a link to the OT I'm using http://www.classictone.net/40-18031.pdf

I've got red wire going to 1k-1W resistor, blue to 6V6 plate.  Green wire going to speaker jack center and black to ground.

[lego4040]

Out of curiosity, before you added the resistor did you put original speaker in a see if it happened again or went away? Do you know if the speaker out of the Baldwin was working good? I know there are some different ohm speakers  and using the wrong one can cause a amp to overwork., I am no amp guru for sure, just curious. Those Vibro Champs don't get the respect they deserve. For amp to work fine and then not when you change one part is a little suspicious.

[sluckey]

Swap the red and blue wires. Or swap the green and black wires. Whichever is easier. Does that help?

[eleventeen]

It is not a perfect certainty that one mfr's "red" (talking about the B+ lead on the primary) exists in the same phase relationship to the "green" speaker side output as another mfr's transformer. If you were replacing a Fender OT with another, same-part Fender OT, you be pretty darn sure you could replace the tranny using the same wire colors, wire for wire, and have a "go" condition. It is that relationship that governs whether you have negative feedback (desired) or positive feedback (causes oscillation)


But whether something should be one way or not, if you have that oscillation, there is so little in a Champ to go wrong that you have to investigate what's what with this.

[mresistor]

yup Steve -  that was it. I wouldn't have thought, but I should've tried a wire swap prior to stressing out over it..   ;-)

Guess the C12Q Jensen isn't as efficient nor has the freq response of this Alnico monster, which btw sounds fantastic.

Thanks guys.