Help with Marshall amplifier

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Fuzzer
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Post by Fuzzer »

Greetings people!

It's my first post aroud here, and it's embarrasing that it can't be a contribution in some way but I will greatly appreciate if someone could give me a hand with this;

I have a Marshall 50 watt 'plexi' (I think the right Model is 1987X) that has some serious hum and ghost notes. I changed the big filter can-caps, and the little electrolytic ones inside the amp; no improvement. I checked the tubes, they're balanced, there's no big difference between the two. I replaced every single resistor in the bias circuit. Nothing.

I don't know what else to try. Again, I'd really apprecciate any help, thanks in advance.


By the way, sorry for my english, is not my main language.

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paulc
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Post by paulc »

What kind of test equipment do you have? Do you have a scope? That would be a big help in trouble shooting the problem.

Without one there's a couple of things you can do to try and isolate the problem. 1st off when you checked the pwr tubes you measured them for current draw - right? A current imbalance can cause a good bit of hum. If they're good pull the inverter tube and see if the hum is still there. If so it's a pwr amp/pwr supply issue. If not then its in the inverter or preamp section.

If you're getting hum in the pwr amp section there's a couple of things you can check. A bad bias supply can cause hum/ghosting if it's putting AC into the grids of the pwr tubes. Check the AC ripple on the grids of the pwr tubes for that. On the plates of the pwr tubes you'll have a couple of volts of ripple which is normal. On the screen side you should see nothing more than a few ACmv. A bad heater supply can induce hum. Check the AC voltage on pins 2 & 7 of the pwr tubes referenced to ground. You should see around 3vac or so on each leg. If one leg reads a good bit more than the other then you could have a problem with the center tap of the heaters.

Check all chassis grounds. A poor ground can cause your problems.

Layout of the wires can also induce a good bit of hum. Make sure none of the wires from the preamp tubes are laying on the heater wires. Move the grid wires around, and you'll be able to hear this.

Hope this helps some.

Later, PaulC
Tim & timmy pedals/Heritage amps

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