Marshall JCM800 2203KK Hum issue

[ct88]

438.8 V and 431.5 V

[g1]

438.8 V and 431.5 V

I want to know AC volts. Is your meter set to AC not DC?

[ct88]

Oh sorry, i misread
3,295 VAC

[g1]

Oh sorry, i misread
3,295 VAC

Is that comma the European decimal point? So around 3 and a quarter volts AC?

[ct88]

Here in Italy we use the , instead of .
Basically it is used as yours .
Exactly, 3 and a quarter circa

[g1]

Ok, so that is less than 1% ripple at the HT Preamp node, so not likely to be bothering the elevation of the heaters through R132.
But why not check anyway. Measure DC volts on each side of R132, then repeat with meter set to AC volts.

[ct88]

Of course, gonna check it tomorrow and write them down
Just guessing, could it be a leaky capacitor somewhere to induce that noise?

[ct88]

R132 has 429.2 VDC 63 VDC to Ground and 0.110 VAC

[g1]

So far all those numbers seem good. A leaky capacitor should show somewhere in the readings and nothing seems suspicious as far as the supply filters.
A leaky coupling cap would show up in the preamp tube voltages.
Check DC volts at pins 1,3,6, & 8 of all three preamp tubes.

[ct88]

V1 1: 172.3 v 3: 1.594 v
6: 242.6 v
8: 2.21 v

v2 1: 151.8 v
3: 0.728 v
6: 239.3 v
7/8: 152.5 v

v3 1: 256.4 v
3/8: 39.88 v
6: 242.6 v

[g1]

V1 1: 172.3 v 3: 1.594 v
6: 242.6 v
8: 2.21 v

V1 pin6 is the only one that seems wrong. Can you double check both pins, 6 and 8 for V1.

[ct88]

V1 pin6 is the only one that seems wrong. Can you double check both pins, 6 and 8 for V1.

My bad, i wrote twice the v3 on pin 6

The correct reading on V1 is 206.5V for pin6 and 2.150V for pin8

[g1]

Those numbers are all good then.
You said you had resoldered lots of the preamp area, I would also check grounds. Also check that all pot nuts are tight and other chassis hardware that goes to ground.

[ct88]

Here i am again, checked all the grounds and probed them but still didn't found the problem
Any suggestion that i probably could miss?

[ct88]

Little update:
today i swapped all the ceramic caps in the preamp for silver micas and got a little better, but still there.
Bypassed V2 and 75% of the noise get away