AC30 Normal channel ground loop fix

[Big Bear]

It is not the same model number on the back of the amp, but actually these schematics share the same model number on the pcb.

http://www.voxamps.com/downloads/circuits/ac3093pa.jpg

http://www.voxamps.com/downloads/circuits/ac3093pr.jpg

[Big Bear]

I believe that I am looking at this issue all wrong. In a long tail pair, shouldn't I expect high DC voltage on the grids of the valve?

Please correct me if I am wrong. I am seeing 60V on the cathode across 48K that is about 1.2mA, which seems low for both triodes but this shouldn't be the problem.

So the fact that this large hum on the output when V6 is in, is caused by another issue and I should not be concerned with the high grid voltage?

[pdf64]

The actual grid voltage will be a volt or two below the cathode voltage. Try to measure it and the meter will drag it down and screw up the dc conditions.
If the plate and cathode voltages indicate that things are ok, then that's fine.
Most likely the hum is coming from earlier in the signal path.
First step is to identify which side of the LTP it's coming from; vib/trem or Normal/bright?
Pete

[Big Bear]

Thanks for the response Pete,

With just V6 in the hum is present, and I am reading the same albeit small signal on each side. Is there a way to determine which side it is?

Would this suggest a power supply or grounding problem?

[pdf64]

Is C35, the B+ bypass cap for the LTP grounded, ie solid connection between bottom of 47k tail resistor and the cap -ve? Does that have a solid connection to the other main power amp / power supply ground?
If you ac ground each LTP grid via 0.1uF caps, does the hum stop?
Pete

[Big Bear]

Connects are solid as far as I can tell.

When grounding the grids through the 0.1uF caps the hum actually gets louder. That is grounding them to the star ground point.

Cheers

[pdf64]

How about grounding them directly to the ground end of the tail resistor?

[Big Bear]

Hum is about 40mV at the OT secondary.

With both LTP grids AC grounded through the 0.1uF caps the hum goings down slightly to 25-30mV.

Cheers