Marshall VS65R hum on both channels

[Osiris81]

I've tested the amp in this configuration and it is dead silent. The output however is very quite and distorted in clean channel, maybe because my supply is only +-30VDC? The +-15VDC for the preamp seemed to be fine.

I will scope the voltage rails when the transformer is hooked up again later.

[Osiris81]

-LT:
-41.62V (multimeter)
Ripple: +-160mV 100Hz (scope in AC coupling)

+LT:
41.64V (multimeter)
Ripple: +-192mV 100Hz (scope in AC coupling)

-15V:
-15.38V (multimeter, keeps slowly rising -> more negative)
No ripple (scope in AC coupling)

+15V:
15.74V (multimeter, keeps slowly rising)
No ripple (scope in AC coupling)

[g1]

How much hum do you scope at the output? Is it 50Hz or 100Hz?

[Osiris81]

I'm not able to measure the output with the scope (A-B), the signal is too low...
I've tested it with the multimeter and the output is about 6mV AC and 8mV DC. When I ground the input of the output stage (the amp is nearly quite in this mode) I measure about 1mV AC and 8mV DC as well.

[Osiris81]

Ok, I was able to scope the output now. Seems the same as I've measured with the multimeter. Base frequency is 100Hz.

[x-pro]

I also noticed that it now makes a plop some seconds after power-up

check capacitance C14 C15

[x-pro]

Turn on the native power supply. Set the master level control and reverb control to the minimum position and check the background level again.
If it is quiet, stop breaking the unit.

[Osiris81]

check capacitance C14 C15

They measured fine but I've replaced them anyway. Unfortunately no change in hum/hiss and the "plop" is still there.

[Osiris81]

I've also replaced the filter caps for the 15V rails now (C12/C13/C25/C26). Still no change...

[g1]

They measured fine but I've replaced them anyway. Unfortunately no change in hum/hiss and the "plop" is still there.

You said the 'plop' is not there with the old tube?

[Osiris81]

Yes, that is true:

original tube (yugosi avia ECC83):
hum/hiss at startup, additional strong hum about 15s later when the tube is warm, tube is microphonic

new tube (JJ ECC83S):
hum/hiss at startup, 'plop' about 5s later, additional hum about 15s after startup when the tube is warm, tube is not microphonic

note: the 'plop' and the additional hum was not there when I initially inserted the new tube, that just developed during the last week although the new tube was running only for about 1 hour max...

[x-pro]

Check the supply quality of anode circuit V1. Check the diodes in this circuit.

[Osiris81]

I've just realized that I've made a big mistake!

When I powered the amp with my bench supply and the internal supply for the tube, I've connected my bench supply directly to +LT and -LT. I just saw that there is a whole circuit that grounds the input of the power stage for some time during startup. The circuit is connected to CON11 to R41 and finally leads to T16 which grounds the input. In my case, I've left CON11 unconnected and T16 was switched on all the time (almost completely), that is why the volume was so low when I've powered the amp with my bench supply...

I've now connected +LT and - LT to my bench supply again, but also connected C15 to +LT to disable T16 and the hum/hiss is here even with the bench supply...

Long story short, the error seems to be somewhere else, not in the power supply. I guess I'll have to check the supply of the tube now.

[Osiris81]

Check the supply quality of anode circuit V1. Check the diodes in this circuit.

I've checked D1 to D4, they all seem fine. I've replaced C6 and C7 (but they were fine anyway).
I've measured the voltage across C7 with the multimeter (DC):
After power up, the voltage rises to about 370VDC, then the plop happens and the voltage falls to about 365VDC.
Across C6 it rises to 354VDC and after the plop falls to about 338VDC.

AC is about 20-30mV.

So one anode voltage is about 338VDC, the other one (I've measured after R20) about 180VDC

[g1]

new tube (JJ ECC83S):
hum/hiss at startup, 'plop' about 5s later, additional hum about 15s after startup when the tube is warm, tube is not microphonic

note: the 'plop' and the additional hum was not there when I initially inserted the new tube, that just developed during the last week although the new tube was running only for about 1 hour max...

New tubes are not necessarily good, I've had them bad right out of the box, or within days or hours of operation. Do you have a third tube you can try, maybe from some other amp?