Marshall Mercury hum

[MarkusBass]

I've got very old Marshall Mercury amp with several problems. The most painful is hum. This is 5W amp with power amp based on single EL84 and solid state preamp. The PC board does not look nice - it was seriously overheated. Also the power transformer was replaced and the heater voltage is 5.8V only. When the amp was designed they didn't care much about ground loops. The ground track is a track around the PC board and it creates several ground loops. I already corrected this slightly and the hum is much lower but I still can hear it. Do you think that the hum can be caused by overheated PC board (at EL84 socket)? Or, the hum is a feature of this amp?
I checked (measured) the power supply capacitors and they seem to be OK.

[nosaj]

I've got very old Marshall Mercury amp with several problems. The most painful is hum. This is 5W amp with power amp based on single EL84 and solid state preamp. The PC board does not look nice - it was seriously overheated. Also the power transformer was replaced and the heater voltage is 5.8V only. When the amp was designed they didn't care much about ground loops. The ground track is a track around the PC board and it creates several ground loops. I already corrected this slightly and the hum is much lower but I still can hear it. Do you think that the hum can be caused by overheated PC board (at EL84 socket)? Or, the hum is a feature of this amp?
I checked (measured) the power supply capacitors and they seem to be OK.

[ATTACH=CONFIG]41092[/ATTACH]

Have you checked for continuity between pins of the el84 socket? Is it possible some of the pc board carbonized?

nosaj

[MarkusBass]

Have you checked for continuity between pins of the el84 socket? Is it possible some of the pc board carbonized?

I checked it. The board is discolored but it's not carbonized.
It seems that I solved all the problems with this amp. I think that the hum is by design :-). There is half-wave rectifier used and a small capacitor. I increased the capacitor and changed the ground tracks to star grounding. I cannot hear any hum now. The amp will be tested next week.

Mark

[nickb]

I think it would be very inexpensive and worthwhile to swap out the single rectifier for a bridge. This will halve the hum again and be significantly kinder to the old power transformer.

PS: ...and increase the cathode resistor a little to try to extend the life of the PCB as much as possible.

[MarkusBass]

Thank Nick for the suggestion. But the owner of the amp wanted to keep the circuit as close as possible to the original. With increased capacitor in the power supply and changed grounding the amp sounds quite good. But it's very difficult to overdrive it which is unusual for Marshall. I will give it back to the owner tomorrow.

Mark