Hum with global negative feedback

[sbirkenstock]

Hi Everybody,

just got my latest 6L6 amp up and running.
I do build in a switch for global negative feedback in my amps.
Now surprise: with gnf I get a bad hum.
I measured 200 Hz (my power line is 50 Hz). Only that frequenzy, no 50 or 100 Hz.
I made sure it´s not positive feedback by exchanging the outputs of the phase inverter.

My build before has the same layout, and there is no problem like this.
I could not find any difference in the build, everything in the power amp is wired the same.
And it is like a Fender Blackface amp, except for the gnf switch.

I checked with my oszilloscope.
So after the main filter caps both amps have like a 10 volt sawtooth "signal".
It´s both Vibrolux style power transformers, and the voltage flexuates between about 460 and 470 volts.
That´s before the choke, after the choke it´s very "quiet".

Then I looked at the signal from the speaker out.
The new amp has the same 10 volt sawtooth signal on the speaker out (with no global negative feedback attached),
where the old one only has noise.
And obviously that is introduced back in the long tail phase inverter creating the hum when ngf is attached.

I check the output transformer if maybe the leads are exchanged, but I get the correct ohm readings.
It´s a Hammond 1760L. Push-Pull, and I guess the "signal" from B+ should be cancelled out.


Other than that the amp sounds really good with no gnf (at low volumes at least, did not risk turning it up).

Any hints please?

Stephan

[cdemike]

What does the B+ string look like? It sounds to me like:

Rectification > Reservoir (to OT CT) > Choke > Screens filtering (screens) > Resistor > Preamp, etc.

If that's right, does everything at the reservoir check out? Everything connects as it should? ESR and capacitance within tolerance?

[sluckey]

Ensure the OT secondary is properly grounded.

[pdf64]

Are both 6L6 working properly? eg scope the cathode current at 1R sensing resistors.
Have you checked that the OT primary wiring is correct, eg maybe hammond got a CT and leg swapped over?

[sbirkenstock]

omg.....

I recently switched to a ground bus.
I made sure that the chassis would only connect at one single point to ground.
Therefore I also switched to isolated speaker jacks....
In other words, the OT secondary was not grounded at all......
Just forgot the wire. The older amp has is, but in black and kind of hidden.

I did not even put a simple thing like this into my schematics lol
Now it is in bright red.
So completely my fault.
I am not sure when I would have found it without your help.
Thank you so very much!