Hum in a Gibson GA8T

[acorkos]

This is the Gibson Discoverer Tremolo version with the 6BM8 power tubes. It hums when you get the volume control up past halfway...hum gets progressively louder as you turn up the volume. At the halfway point, the guitar volume is pretty loud, with no hum, but after that, the hum increases and the guitar volume gets louder too. I swapped in all new tubes...no change. Filter caps are newer....swapped in some new caps...no change. Moved the filter cap grounds...no change. Everything appears to be wired correctly, grounds are all connected as they should be....and 95% of the components are original. Any ideas as to what could be going on, or where I might find a problem? Thanks for your help!

[The Dude]

Does the 95% include the "multi/can" cap?

[Chuck H]

Well, that WOULD be the filter caps. Which are said to have been replaced. But when, by whom and with what???

As usual, it would be helpful to know the frequency of the hum.

Here's a link to the schematic:

Prowess Amplifiers - Gibson - Schematics - Ga 8t

[The Dude]

That was my point. ALL of the electrolytics, or just the ones easily recognized by a novice? No dig on the OP at all, but not everyone knows what "that big silver thing with solder lugs" is.

[acorkos]

all electrolytics have been replaced, including cathode bypass cap on V1 and the cathode cap
I replaced the other 3 caps involved in the V1 circuitry, but no change...still hums when the vol pot gets past midway

[Jazz P Bass]

Simply because you replaced the capacitors does not mean that you shouldn't 'check' there functionality.

Get your meter out and run a few voltage checks on the power supply nodes.
Vdc & Vac.

[acorkos]

I did check the voltages...all were consistently about 10% higher than those listed on the chart, and no VAC on the power supply nodes.
There is no hum until the volume control is up past halfway, and the guitar sound is perfectly normal all the way up...leads me to believe the problem is before the volume pot

[Jazz P Bass]

Have you checked V1B pin 6 coupling cap (.02) for Vdc leakage?

[acorkos]

Have you checked V1B pin 6 coupling cap (.02) for Vdc leakage?

I replaced it, and zero VDC on the meter

[Jazz P Bass]

Have you verified that both input jacks 'close' properly?

[acorkos]

Have you verified that both input jacks 'close' properly?

yes

[Jazz P Bass]

You got me.

Grounding V1 pin 2 after the volume control should kill it.

Ground pin 7 for grins & giggles.

[52 Bill]

Did you change/check the tone control caps? Value of the tone and volume pots?

The fact that it is quiet until you get it up halfway makes me think some sort of ground problem/loop. Are both V1 cathode grounds (pins 3&8) at the same place?