1973 Marshall (where is the ground for the heaters tap?)

[bruno]

I have an old marshall here that is humming and I really don't recall if the heater tap has a ground of it's own, I certainly see no grounding across the chassis for it, but I can't tell for sure on the power transformer itself as there are no markings.

Any help?

[sluckey]

Need more info.

[bruno]

Ok this is a 1973 50w JMP Marshall PA (model nº 1985), it's basically like a JTM50, with solid state rectification, and higher filtering... no bright cap across any of the mixing resistors.

There's a low hum in the amp, and the amp itself sounds freaking amazing, except for that low hum. The filter caps, have been exchanged by the previous owner, and look right, as does the wiring, which hasn't been very messed with.

I can't tell if the power transformer has been rewinded, but if it has it was done in a pro-fashion,  as it looks neat and all the voltages appear to be ok.

I was looking at the heater wiring, and since I greatly improved the hum leves on my old ac30 by removing the heater grounding and adding a virtual center tap of 100ohms resistors, I thought maybe I should check this too on the marshall... looked around and I can't tell if the power transformer has a center tap for the heater wiring, but I also can't find the heater circuit going to the ground at any point.

Any other info that might help?

[sluckey]

I really don't recall if the heater tap has a ground of it's own, I certainly see no grounding across the chassis for it

If you can't tell from looking then try this... Measure the AC voltage between EL34 pin 2 (or pin 7) and chassis. It will be about 3.15VAC if there is a center tap on the filament winding. The schematic I'm looking at shows a center tap.

[bruno]

thanks sluckey I'm going to check it... could a poorly wound power transformer be the source of the hum! I can't seem to find any other problem.

[HotBluePlates]

could a poorly wound power transformer be the source of the hum! I can't seem to find any other problem.

Not likely. It's hard to imagine the amp malfunctioning for almost 40 years with no one fixing it.

You're certain the low hum sounds like line-frequency hum? If there's not a tube issue, or a heater wiring issue, you'll want to remember that a failing bias cap will leak line-frequency ripple to the grids of the output tubes. So if the bias cap looks original (it would probably be an Erie-brand cap), you might want to try tacking in a replacement.

[bruno]

Hi HotBluePlates,

good to read from you... Yes the bias cap does look original, I'll look into that one. The amp sounds so nice, even with the low hum in the background, that I feel obliged to really get this one working properly! The circuit of this PA resembles, old JTMs and 67 plexi a lot more, despite having much high filtering and a different power rail.

I doesn't sound so "out of control" like many of the metal panel era marshall seem to do.

[HotBluePlates]

I only mention the bias cap because I once fixed a 60's-era Princeton Reverb with a loud hum. The hum was present even with the phase inverter tube pulled. The only place it could be happening was at the output tubes.

The hum was smooth and low; filament hum (due to no ground reference) has always sounded buzzy to me. The only source of 60Hz hum (in the US) at the output tubes, while being smooth and low, was the bias circuit. Swapped the cap, and the amp was fixed.