5E7 oscillation depending on the treble pot position

[Strat76]

Hi everybody,
I built recently a 5E7. No mod in the circuit except the Kilback mod for the bias.
The tensions are good, the tone is ok, I checked the connections several times but there is always a moderate HF squeal when the treble pot is under 3.
It looks like an oscillation.
FYI I use a OT from M. Components with a 2.6 tap, 3X4Ohms speakers and a 56K nfb resistor.

Any help is welcome.
Thanks in advance

[Tom Phillips]

Can you post photos of the wiring dress and component layout?

Re: "It looks like oscillation" Does that mean that you have an oscilloscope?

[Chuck H]

It's mentioned that it happens when the trble pot is set lower??? Is this correct? Try lifting the one end of the 56k resistor that goes to the presence pot. If this stops the oscillation you can reconnect it and swap the power tube plate leads from the OT.

[frus]

FYI I use a OT from M. Components with a 2.6 tap, 3X4Ohms speakers and a 56K nfb resistor.

that would be three 8 ohm speakers I hope

[Chuck H]

Good catch. +1

[Strat76]

that would be three 8 ohm speakers I hope

Sorry I made a mistake. These are 8 ohms speakers of course.

I will check without the nfb resistor.

[Strat76]

Can you post photos of the wiring dress and component layout?

Re: "It looks like oscillation" Does that mean that you have an oscilloscope?

Sorry, I must say " it sounds like ..." as I didn't see the phenomen on a scope.

[Strat76]

Fixed !!!

I put another 56K resistor in parallel with the NFB 56K resistor to decrease the overall value and the amp squealed whatever I do. It was obvious that I had a NFB problem.
I changed the wires of the OT and no more oscillation and the amp sounds and behaves better. It now sounds very very good.

But I have another question. I tried this amp with another OT before, and I had the same oscillation problem, that I fixed in the same way. So it's not a bad wiring OT. As I said, I checked several times the connections, and I really believe there is not a wiring mistake.

So what could be the reason of this polarity inversion ? Can the reason be that I didn't put screen resistors (like the original schematic) and the amp is, because that, very vulnerable of the oscillations ?

I don't know if you can understand be, but believe me, I try to do my best in English

Thanks

[Chuck H]

Transformer makers simply get the wires reversed sometimes. Blue and brown (probably) are the ends of the primary winding. No way to know which end is which until it's wired up and squealing.

[Strat76]

Transformer makers simply get the wires reversed sometimes. Blue and brown (probably) are the ends of the primary winding. No way to know which end is which until it's wired up and squealing.

Thanks Chuck ... I feel more comfortable now ...

Thanks for everybody

[Jazz P Bass]

A nice little trick to check the output transformer polarity, as far as negative feedback is concerned.
Send a signal into the front of the amp & monitor the output voltage.
Now disconnect the feedback resistor.
If the output voltage goes up, then all is well.
If the output voltage goes down, then the primary wires are reversed.
(positive feedback)