Marshall VS100R hum problem

[avoverdu]

So the DC supply helps quite a lot. You can
(a) Fit a DC supply
(b) Try elevating the heater DC voltage [cheap to do] or
(c) keep trying different tubes until you find one you are happy with
(d) Live with it.

What's it to be?

I didnt give up!!!! I´ve been busy at work this week but today im gonna get some parts. Im going to try all your suggestions until I find the cure LOL. By the way I did some tests that I will ppost later in detail.

Thanks guys!!

[avoverdu]

250mV rms is a lot of hum. I don't recall these being especially bad in that respect. Am I mistaken?

For certain HK leakage is a component but how is it getting into the ground? Unless of course the test in post 19 is wrong. I wonder if there has been a misunderstanding? I took it that with R48 and R77 shorted and with the tube in the hum was there. On a re-read maybe that is not the what was meant. Perhaps the OP could clarify?

A cheap alternative to a DC supply would be to elevate the heater supply up to 100V or so with a divider* off the plate supply and isolating pin 9 from ground.

*PS
i.e. isolate pin 9 from ground by cutting track. Add 100K 0.5W from ground to pin, add 390K 1W from pin 9 to to pin 1, add 2.2uF 100V ( bigger uF and bigger volts would be OK). Check voltage across 100K after rework. Aim for 75V to 100V.

Hi Nick. Im ready to perform what you just suggested. What Im not sure is where to put the 2.2uF cap? In parallel with the 390k resistor between pin 1 and 9?

[nickb]

Hi Nick. Im ready to perform what you just suggested. What Im not sure is where to put the 2.2uF cap? In parallel with the 390k resistor between pin 1 and 9?

Better (updated) instructions:

Isolate pin 9 from ground by the cutting track ( or whatever it take)
Add 100K 0.5W from ground to pin 9
Add 390K 1W from pin 9 to to pin 1
Add 2.2uF 100V across the 100K (bigger uF and bigger volts would be OK).
Check voltage across 100K after rework. Aim for 75V to 100V by tweaking the 100K if necessary