JM Signature hum with neg feedback on (pics added)

[Guy77]

Hi everyone. I just finished building another JM signature amp. The weirdest thing is happening. When I turn on my negative feedback (with a switch) and I play my heavy E string I get a loud hum and the loudness of this hum cannot be controlled with my volume pot so turning down the volume does not make the hum go away. When I turn off my neg feedback everything is fine and hum goes away. The only thing different with this build is that I am using cliff jacks.

Any help appreciated.

Cheers

Guy

[sluckey]

The NFB phase is probably wrong. Reverse the OT primary plate leads and see if this corrects the issue.

[Guy77]

Thanks Sluckey! I will try that tonight when I get home from work. Strange that it happens only on the E string. Usually when I have the NFB phase wrong hitting any string would cause a loud hum.
I remember pulling out the speaker jack while the hum was happening and I could still hear a hum in the circuit with no speaker connected.
I will report back tonight after I switch the leads.

Thanks again .

Cheers

Guy

[pompeiisneaks]

I think I recall Sluckey telling me in the past that sometimes the NFB being reversed can be a very subtle problem, and not the squeal we're accustomed to. If removing the NFB fixes it, it's almost always a phase problem.

I myself have had a bit of 'weird' behavior on my dumble build where it was sounding 'okay' but weird, and then switching the OT leads cleaned it right up, No obvious howling or odd on all strings like you mention.

~Phil

[10thTx]

I think I recall Sluckey telling me in the past that sometimes the NFB being reversed can be a very subtle problem, and not the squeal we're accustomed to

I've that exact thing happen. Because there was no "squeal" as one normally would experience, it took some time to figure it out.

With respect, 10thtx

[Guy77]

Thanks for all the assistance everyone. I wish I could leave work early but will have to wait until my shift is over later tonight and then I will switch the OT leads .

I will post later tonight.

Cheers

Guy

[Guy77]

Finally got to work on my amp tonight and indeed swapping the output transformer primary leads fixed the problem!
What a beautiful sound these amps make. It is my favourite sounding amp. I made the tone bypass boost switch with a 3M resistor to ground this time and found it more usable than using a 5M.
Thanks again for everyone's help. Here is a few pics of her in early stages and later stages and the combo cab that is being built .



Cheers

Guy

[joe6v6]

I've that exact thing happen. Because there was no "squeal" as one normally would experience, it took some time to figure it out.

Same here, no squeal just loud humm. I dis connected the negative feed back first just to verify, switched the primaries & all was well.
Glad you got it going. I Love mine , although at higher volumes, above 1/3, I have trouble controling the Bass, may make some changes there.

Joe

[Guy77]

I've that exact thing happen. Because there was no "squeal" as one normally would experience, it took some time to figure it out.

Same here, no squeal just loud humm. I dis connected the negative feed back first just to verify, switched the primaries & all was well.
Glad you got it going. I Love mine , although at higher volumes, above 1/3, I have trouble controling the Bass, may make some changes there.

Joe

Thanks Joe , yes happy its working fine now. Going to jam with a group of friends in the garage next weekend and happy it will be ready. I have a 2x12 on the way (made by traynor) that I will load with a G1265 and a Celestion Gold . Can't wait to crank up the volume!
In regards to the issue controlling the bass at higher volumes what I do is turn off the Deep switch and the Mid Boost and finally adjust the Contour more to the right. Another thing that may help is to change your negative feed back resistor. The schematic shows it as 12K I believe (100 watt version). I will be changing mine to a pot or 3 way switch to have more tonal control. Another thing I did was change power tube coupling caps down from .1uf to .05uf.
The one I just built is a 50 watt version.
I added another pic , this one showing the rear of the amp with the new thermal sublimation faceplates I am using, they are made from aluminium and reflect light.




Cheers

Guy