Marshall VS65R hum on both channels

[g1]

The schematic shows a connection from the guitar plug JS1 to the input of the tube (connection R9/R10/tube socket). This connection is ground when no guitar is plugged in. My preamp has no such connection, instead, the plug is connected to the output of IC7c (connection between C10/R16). So when no guitar cable is connected, the input of the power amp is grounded and that's why the amp is silent.

Are you following the traces, or are you measuring resistance? Can you post some photos of the circuit board.

[Osiris81]

I followed the traces.

Top:
https://freeimage.host/i/JRJ3gTJ

Bottom (mirrored):
https://freeimage.host/i/JRJFMFI

Mirrored bottom overlayed on top:
https://freeimage.host/i/JRJFPwb

Marked traces: red where the connection should be according to the schematic, blue where the trace actually is:
https://freeimage.host/i/JRJKHNa

[g1]

That's amazing work.
Is that bare wire jumper yours? It appears it may be a ground loop or someone trying to deal with a ground loop?
Is the item between the two 330R5W resistors a jumper link?

[Osiris81]

Thanks

The bare wire jumper is mine, I should have mentioned it. I thought there is a grounding issue so I installed that. It did not help but didn't hurt either, so I just left it there...

Yes, the item between the resistors is just a jumper link (LK19).

I've rewired the connection from the guitar plug yesterday so it matches the schematic. It did not help other than now the amp has that hum even if no cable cable is plugged in (as expected).

I started to wonder if this hum maybe is just normal for this amp, but I've watched several videos on Youtube about this amp and they all seemed very quite between playing. Anyway, I just ordered two new Chinese tubes, maybe they help at least a bit.

[Osiris81]

Since I'm now able to upload images again, here the three photos of the preamp again.
Top Layer:


Bottom Layer:


Overlay:

[Osiris81]

I've also received the two new tubes from china:

The first tube is similarly to the JJ ECC83S I've bought before: I makes a plop right after power up (and then two additional quiet plops afterwards unlike the JJ). After that, hum/hiss is about the same.
The second new tube has no plop at startup, but a very loud hum afterwards, quite the same as the original tube, but this one is not microphonic.

[x-pro]

The bare wire jumper is mine

I don't think jumpers belong there. A "Common" circuit can cause excitation (hum). The "Common" connection should be on a different board. On the power supply board, away from the connectors on the main board.

[Osiris81]

As I mentioned, the jumper had no influence on the problem.

But anyway, I've removed the jumper again and restored everything as it originally was. I just left the better of the Chinese tubes in it for now and reassembled the amp. I guess I'll have to live with it how it is...

Just one final note for unexperienced people that work on this amp in the future: When the tube is inserted, the big capacitors (C6 + C7) discharge quite quickly to safe voltages once the amp is powered down. When no tube is inserted however, they do not discharge at all for hours. When you have your mains plug still plugged in (or the ground lead of your oscilloscope still attached to the chassis) you'll risk getting zapped quite severely when poking around (even when only using one hand, don't ask me why I know that). So, always discharge the capacitors (and measure the voltage) before poking around. Inserting the tube after the amp is powered down seems not to work to discharge the capacitors, you'll have to use a resistor across the rectifier diodes.