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I have a client’s Marshall JCM 800 Lead Amp head on the bench (2205 50W, Ch Switching) with VERY HIGH HUM present. Even with dead-patching the Effect Loop Return, there’s considerable hum.
In relative terms, 120Hz ripple component is about 2dB higher than 60Hz component. Ripple on the first filter is about 10V P-P (450VDC), while at the screens, past the choke, less than 200mV P-P (448VDC), and it drops on down from there.. So, I’d say the power supply filters look ok. The bias supply ripple on the first stage is about 120mV P-P ripple, and on the second stage where it’s feeding the grids of the power tubes, maybe 10mV P-P. I did replace both of the 10u/F/100V bias filter caps, while I had the board lifted up. Bias current reading is around 33mA per tube (using my bias probes).
During the hunt on the PCB for solder fractures, I did find a number of fractures…input jack, ground lead of some of the pots, but everything else looked fine Repaired what I found, put the board back down to see if anything changed. No difference.
Af first, having forgotten Channel Switching doesn’t work unless something is plugged into the Input jack. I had already gone backin and removed the IC….CA 3046 NPN Xstr Array. All five devices on it are stellar, so put it back in, found my error on switching, so the Channel switching works fine.
The DC power supply created off the heater winding, full wave rectifier with a 3300uF/16V cap looks just fine…no ripple present.
With the dead-patch removed, the preamp section appears to be the main source of hum. I went tube by tube, swapping each out to see if any were contributing to the problem. Swapping out V4 & V5 tubes did lower the hum a little bit….not the problem.
I read some of the previous notes posted by our faithful contributors without finding a solution to this.
I tried to revise the grounding without success (lifted the ground wire to the chassis off the Foot Switch jack, lifted the ground wire from the Insert Send/Rtn jacks while adding a Ground Wire right from the Input jack to chassis, so the pots, input jack, front end cathode circuits didn’t returned to the power supply ground where the HV C/T, Heater C/T & Filters tie in. I had verified in that process I had isolated the preamp stages grounds. But, as that only made it worse, I restored it as I found it.
I did note lowering the AC Mains with the variac dropped the hum level some, so I next removed the heater supply connections from the power transformer, and used the heater supply from my HP 712C HV Power Supply, whiile leaving the HT & bias supplies being sourced from the Marshall P/T. That didn’t change a thing….kinda hoped it would, having had solved this sort of problem on a couple Vox AC30 amps. Lowering the AC Mains this time didn’t change much at all, it now only changing the HT & bias, and NOT the heater voltage.
I haven’t yet powered the amp up off the HT & Bias supplies of the HP 712C. I thought I’d first report this as it stands, and see if there’s something obvious I’m missing.
In relative terms, 120Hz ripple component is about 2dB higher than 60Hz component. Ripple on the first filter is about 10V P-P (450VDC), while at the screens, past the choke, less than 200mV P-P (448VDC), and it drops on down from there.. So, I’d say the power supply filters look ok. The bias supply ripple on the first stage is about 120mV P-P ripple, and on the second stage where it’s feeding the grids of the power tubes, maybe 10mV P-P. I did replace both of the 10u/F/100V bias filter caps, while I had the board lifted up. Bias current reading is around 33mA per tube (using my bias probes).
During the hunt on the PCB for solder fractures, I did find a number of fractures…input jack, ground lead of some of the pots, but everything else looked fine Repaired what I found, put the board back down to see if anything changed. No difference.
Af first, having forgotten Channel Switching doesn’t work unless something is plugged into the Input jack. I had already gone backin and removed the IC….CA 3046 NPN Xstr Array. All five devices on it are stellar, so put it back in, found my error on switching, so the Channel switching works fine.
The DC power supply created off the heater winding, full wave rectifier with a 3300uF/16V cap looks just fine…no ripple present.
With the dead-patch removed, the preamp section appears to be the main source of hum. I went tube by tube, swapping each out to see if any were contributing to the problem. Swapping out V4 & V5 tubes did lower the hum a little bit….not the problem.
I read some of the previous notes posted by our faithful contributors without finding a solution to this.
I tried to revise the grounding without success (lifted the ground wire to the chassis off the Foot Switch jack, lifted the ground wire from the Insert Send/Rtn jacks while adding a Ground Wire right from the Input jack to chassis, so the pots, input jack, front end cathode circuits didn’t returned to the power supply ground where the HV C/T, Heater C/T & Filters tie in. I had verified in that process I had isolated the preamp stages grounds. But, as that only made it worse, I restored it as I found it.
I did note lowering the AC Mains with the variac dropped the hum level some, so I next removed the heater supply connections from the power transformer, and used the heater supply from my HP 712C HV Power Supply, whiile leaving the HT & bias supplies being sourced from the Marshall P/T. That didn’t change a thing….kinda hoped it would, having had solved this sort of problem on a couple Vox AC30 amps. Lowering the AC Mains this time didn’t change much at all, it now only changing the HT & bias, and NOT the heater voltage.
I haven’t yet powered the amp up off the HT & Bias supplies of the HP 712C. I thought I’d first report this as it stands, and see if there’s something obvious I’m missing.
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