Marshall VS65R hum on both channels

[x-pro]

check capacitance C14 C15

try doubling their capacitance rating.

[Osiris81]

try doubling their capacitance rating.

You mean C14/C15 on the sheet of the power stage I guess? So you think I should increase the values so that the input of the power stage is still grounded when the 'plop' happens?

[Osiris81]

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?

Unfortunately I do not have another tube. The anode voltages I measured seem to be out of spec (max 300V) for the ECC83 anyway... On another thread you've mentioned that Chinese tubes may work better in this amp https://music-electronics-forum.com/...782#post913782.
Maybe I'll order one if I get rid of the hum/hiss the amp has even without a tube...

I did some probing without the tube inserted and to me it looks like the hum is coming after the mix with the reverb (IC7). I just ordered some TL072 to replace IC7, maybe I'm lucky...

[g1]

I did some probing without the tube inserted and to me it looks like the hum is coming after the mix with the reverb (IC7).

x-pro had asked earlier if the reverb is turned all the way down.

[g1]

Unfortunately I do not have another tube. The anode voltages I measured seem to be out of spec (max 300V) for the ECC83 anyway...

The spec is measured from anode to cathode, not from anode to ground so I don't think that is a problem.
But the high cathode voltage you pointed out from the other thread is a good point which I had forgotten.
A different brand tube that can handle higher cathode voltage may be your solution.

[Osiris81]

Yes, the hum/hiss is there even if the reverb is turned all the way down

[g1]

A different brand tube that can handle higher cathode voltage may be your solution.

Alternatively, you could double the value of R21 to reduce the voltage to the tube. But I think the 2 tubes you have now are already damaged, so that would not help them.

[Helmholtz]

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

Your output signal doesn't look like typical power supply ripple.
Rather it's a mixture of frequencies (and a lot of noise).
I can see a repeating pattern at 50Hz.

Does the noise reduce when you pull the tube?

[Osiris81]

I guess lots of the noise is from the scope itself, I cannot really measure something useful in the +-20mV range...

I've now build a 100x amplifier with an opamp that I use to measure the signals. When using an additional low pass filter in the scope (20kHz) I can now clearly see a signal with a 50Hz base frequency at the input of the output stage (R1). The signal looks a bit different when there is no tube inserted and the amplitude is even a bit higher. Unfortunately I cannot upload the images here anymore because of a 1MB restriction (I guess this limit gets increased after some time...).

[The Dude]

FWIW: You can compress and resize photos with photoshop or similar. "Gimp" is freeware and costs nothing. I always compress and resize photos when uploading so that they load faster in browsers and don't waste valuable space on the site's server. There's just no reason to upload massive photos here. We're not making prints. It's purely informational. You should be able to get decent quality photos suitable for web use within the couple hundred kilobyte range.

[Osiris81]

I know, but I only have about 20kB left of the total 1MB and I cannot compress anything useful to 20k. I also cannot delete the older uploads...

[The Dude]

I misunderstood and thought you were trying to upload a single 1MB+ size photo.

[g1]

I know, but I only have about 20kB left of the total 1MB and I cannot compress anything useful to 20k. I also cannot delete the older uploads...

nosaj , what is the time frame for full upload privileges?

[Osiris81]

I can now clearly see a signal with a 50Hz base frequency at the input of the output stage (R1). The signal looks a bit different when there is no tube inserted and the amplitude is even a bit higher.

I've analyzed the two signals (with and without the tube) via FFT and the difference is, that the signal without the tube has no 100Hz in it, just 50Hz and some harmonics (but no 100Hz), while the signal with the tube has 50Hz and 100Hz and some harmonics.

Another thing I observed: When I unplug the speaker (running the amp without a tube), I hear a buzz from the electronics itself coming somewhere from where the filter caps of the tube are but I'm not sure if its coming directly from the caps, it does not come from the transformer however.

Another thing: Grounding the inputs of IC5 (the opamp right after the tube) does not help getting rid of the hum (should be grounded anyway when no tube is inserted). The next opamp in the signal path is IC7, however when I try to ground its negative input, I get a very loud hum/buzz. Grounding the output of IC7 (thats the input of the power stage) gets rid of most of the hum/buzz.

[g1]

Another thing: Grounding the inputs of IC5 (the opamp right after the tube) does not help getting rid of the hum (should be grounded anyway when no tube is inserted). The next opamp in the signal path is IC7, however when I try to ground its negative input, I get a very loud hum/buzz. Grounding the output of IC7 (thats the input of the power stage) gets rid of most of the hum/buzz.

These are multi-segment and switching op-amps. Please be specific about which exact pins you are grounding.