Fender Pro Jr. Negative Feedback Weirdness

[tdlunsfo]

So a customer brought me two Fender Pro Jr's both with typical new age Fender issues (leaky caps, burned boards, cold joints, etc). I get them fixed up and out the door and a week later he calls me back and tells me one of the emits a loud hum through his external speaker cab but the other one doesn't. He showed up yesterday and sure enough, as soon as you play a note you get blasted by 10hz and it doesn't respond to volume, tone or anything. The only way to make it stop is to pull one of the preamp tubes or turn it off. So, I plug in the internal speaker and it's fine. I plug in my shop speaker and it's fine. I find out that if I pull the negative feedback the noise stops, so that leads me to the OT being wired backwards and positive feedback. Sure enough, I swap the leads and reinstall the NFB and all is well in all the speakers. So, why did I only get positive feedback through this particular speaker (Eminence Wizard 8ohm)? I assume it has something to do with the amount of NFB plus the amount of field certain speaker coils and magnets are giving off, but I am not smart enough to figure all that out.

[Helmholtz]

...you get blasted by 10hz

Do you mean 10kHz?

Schematic?

[tdlunsfo]

No low frequency.. almost like motorboating except it was constant. Stated frequency is an approximation based on the beat made by feeding it a signal with my audio generator. It didn't let it carry on long enough to measure it with the DMM.

Fender-ProJunior-III-Schematic-Rev-F.pdf (thetubestore.com)

[Helmholtz]

All I can say is that amp stability is influenced by speaker impedance.
Low frequency speaker impedance can differ a lot between speakers having the same nominal impedance.

Now reversing OT leads changes negative feedback into positive feedback (and vice versa).
Means there should be a noticeable difference in power amp gain (and sound).

Pulling which preamp tube did stop the oscillation?

[tdlunsfo]

Either one.

[The Dude]

Swapping speakers wouldn't change feedback polarity. I suspect something else is going on. Have you checked the suspect speaker? What is the DCR of that speaker?

[Chuck H]

I think what tdlunsfo is implying is that a positive feedback issue only presented with that one speaker, but the amp may have been stable enough with other speakers. Which is hard to imagine but I don't have much experience with those amps. On another note...

The Wizard is an odd speaker. It's reported efficiency is very high at 102.8dB. Which is a lower spec than it use to have because I remember when Eminence advertised 104dB. It's also reported to have a lot of low end. I've always wanted to try one but I've never prioritized it. But it could be that under the conditions used by the owner this very efficient speaker is the only one that demonstrated instability inherent with positive feedback.

That said, why should the OT leads have been reversed? Is this suspected to have been stock? Was the OT replaced at some point? etc. More to the point, has gain been measured with the feedback lead both installed and disconnected? That would be the tell tale indication of the polarity of the feedback loop.

[The Dude]

FWIW: Here is the response curve of the Eminence Wizard.


It falls off pretty hard below 100Hz. I wouldn't expect it to respond well at 10Hz. I agree with your NFB disconnection test, Chuck. That would tell us for sure if it actually is an NFB problem. One thing I wonder about is shorted voice coil windings causing signal to be reflected back into the OT, which is why I asked for a DCR measurement.

[tdlunsfo]

DCR is 6.6ohms. OT looks stock but will attempt to verify. I'll get some NFB shots tomorrow to look at gain. I can also test the OT for shorts.

[tdlunsfo]

OT is stock. I setup the amp for 1Vrms output with NFB connected, once I disconnect the NFB it jumps to 1.41Vrms. Unable to test for shorts as my shorted winding neon tester doesn't work on such a small secondary winding.

[g1]

Since it's motorboating, and some of these style Fender's have problems with preamp filter caps, try bridging a good cap across C18 and C17.

[tdlunsfo]

Caps are new stock value F&Ts.

[Helmholtz]

OT is stock. I setup the amp for 1Vrms output with NFB connected, once I disconnect the NFB it jumps to 1.41Vrms.

That's what I expected (post #4).
As feedback lowers gain, it's negative, so everything should be fine now.

Positive feedback (by reversed OT primary leads) may or may not cause oscillation. If it does depends among other things on load impedance and phase angle at some critical frequency.
As the amp didn't oscillate with the internal speaker, it might have passed factory QC with OT wires reversed (though their criteria seem questionable).

BTW, the NFB will produce some presence boost, so sound should be different as well.

[tdlunsfo]

It plays fine now other than the dreaded pro jr buzz. These are ridiculously noisy amps.