Humming Plexi

[rf7]

I've got a Plexi Supertremelo on the bench that hums when it's on Standby (what does that mean?), and it hums when it's on and the hum gets nasty when you crank the presence control. It looks like 120Hz on my scope, so that's a power supply problem, correct?

I've jumpered some of the filter caps with newer ones, but it's not changing the symptoms. Should I just recap it and get it over with? Could anything else cause that hum?

[deluxe reverb92]

whenever there's hum, i say recap it. how old is the amp? it has a three prong plug right?

[Enzo]

Hums in standby? Out the speaker? or mechanically? Just which model is that?

120Hz means it is power supply RELATED, not that there is necessarily a power supply problem. A lost ground return could let ripple noise in just as easily as a weak cap.

In standby there is no B+ - verify that - so nothing should be amplifying. That means anything coming out the speaker would be either induced into the OT by magnetic field, or some actual current is flowing through the OT primary.

The field around the power tranny can couple into the laminations of the OT and cause a low level hum. You could insert a sheet of steel between the two trannys and see if it helps. I doubt that is the case. That usually results in a low level background hum and no effect on signal. Besides, that would be 60Hz.

So I am left with current through the OT primary. First thing to mind is a bad tube. In fact the first several things are a bad tube. An internal short could connect either the bias to the OT or the heaters. Pull all the power tubes and see if the hum goes away. If it does, put them back in one at a time and see who causes the hum. Yes, do all this in standby. And certainly a bad tube would sound crappy when in play mode as well. Or just try a different set of known good tubes - don't have to be new, just good.

Scope the B+ when it is on. Pull the power tubes. Is the B+ pretty clean? SOme ripple is OK, a few volts, on the power tube plate node, but the screens and everything after should be really just DC. If it is nasty, then maybe a leaky cap is there.

I vote against the recap at this point unless there is direct evidence of a bad one. I fear what will happen is you will throw a lot of caps at the amp and then have an amp full of nice new caps that still hums. Make the amp work, THEN recap if you like.

[rf7]

Hums in standby?

Yes.

Out the speaker?

Yes.

or mechanically?

No.

Just which model is that?

Marshall 1959T - A late 60's 100 watt JMP Super Lead with tremelo. The nearest schematic that I can find is on Page 204 of Doyle's History of Marshall book. And it does not match that schematic as well as I'd like.

It hums in Standby with the power tubes removed. There is no B+ on the tubes in Standby. When it's on, with no power tubes, the B+ looks fine, 1V peak to peak on the plate, and just DC elsewhere, except on the 1Kohm (suppressor screen?) feeds to each tube, where I see a weird high frequency trace that my scope won't trigger on, that jumps a volt or two sometimes. That comes off of the choke, before the B+ hits a dropping resistor (10K in the amp, 8.2K on the schematic). I don't know why that looks so weird.

I fear what will happen is you will throw a lot of caps at the amp and then have an amp full of nice new caps that still hums.

I had that exact thought also. Especially since jumpering new ones in did not help.

[Chuck H]

If you have hum in the speakers with no power tubes in place and the amp in standby that may mean that you have voltage on the chassis that is not properly earthed making it's way to the speaker jack somehow. Check all ground connections for continuity and check for voltage on the chassis with your black probe on a seperate ground.

Chuck

[Enzo]

I have the 1959T.

Is the standby switch properly wired and both sides working? It should break both sides of the PT HV secondary. No one wired around one side or something I hope.

Hmmm.


IN standdby there is no B+ on the OT, is there any AC?

If the output tube socket shorted pin 2 and 3, some heater AC could flow through the primary, but as soon as the sandby was flipped it would pop the HV fuse. SO much for that.

An experiment would be to disconnect the CT wire of the OT. WHat happens?

In keeping with Chucks theory, disconnect the ground from the speaker jacks, in other words connect the speaker directly to the OT without one side going to ground. It will be independent of the chassis then. ANy news?

This is crude brute force troubleshooting, but disconnect enough things and the OT isn't even there anymore. Either we are fooling ourselves or there is current from somewhere flowing through that tranny. By disconnecting the wires we can try to learn where any such current is coming from.

Unbolt the OT and rotate it 90 degrees. That would affect any magnetic coupling to the PT. You sure it is 120Hz?

[Steve Conner]

I've had the magnetic coupling problem between PT and OT myself, it made the amp hum at 50Hz through the speaker even in standby. But in my case, when the amp was taken out of standby, the hum more or less vanished.

I think when the power stage is powered up and running, the negative feedback rejects anything that gets magnetically coupled into the O.T.

[rf7]

Check all ground connections for continuity and check for voltage on the chassis with your black probe on a seperate ground.

Chuck

Did not measure any appreciable voltage on the chassis. Noticed that the Bulgin AC cable was missing the ground socket, so I have a 2 prong cable, in effect. I jumpered the chassis to AC ground and it still hums.

Is the standby switch properly wired and both sides working? It should break both sides of the PT HV secondary. No one wired around one side or something I hope.

No, the standby only breaks one side of the PT HV secondary. It's the old school original style Plexi circuit.

IN standdby there is no B+ on the OT, is there any AC?

No AC voltage, but -.62Vdc shows up there.

An experiment would be to disconnect the CT wire of the OT. WHat happens?

Still hums.

In keeping with Chucks theory, disconnect the ground from the speaker jacks, in other words connect the speaker directly to the OT without one side going to ground. It will be independent of the chassis then. ANy news?

It's quiet, hum is gone!

Unbolt the OT and rotate it 90 degrees. That would affect any magnetic coupling to the PT. You sure it is 120Hz?

I can't rotate it, the leads are too short to move it much.

It looked like 120Hz when I cranked everything. I'll have to double check that.

[Chuck H]

When you wired the speaker directly to the OT did you still have the speaker - terminal connected to the OT - lead but lifted from chassis ground???

Chuck

[rf7]

When you wired the speaker directly to the OT did you still have the speaker - terminal connected to the OT - lead but lifted from chassis ground???

Chuck

Damn, I did it wrong! I just pulled the OT bottom wire from the output jacks, so the jacks were still connected to ground.

[Enzo]

Its a Marshall, aren't the jacks plastic?

Screw the jacks, the experiment was to connect the speaker direct to the OT. That was the point, to eliminate any connections to ground, chassis, whatever.

My old 1959T drawing shows the DPST switch in the HV.

If disconnecting any part of the speakre jack wiring eliminates the hum, then explore the ground wiring around it. Find any wires that share speaker return and power supply return currents.

Since this hum also distorts the output when in play mode, we need to find where the interference is coming from.

[guitaraid]

Hi,

If you still have hum on standby with no power check a PCB around power valves if there is no visible marks then unsolder all power valves sockets and check again if there is still a hum.
I have had a few humming amps on standby mode and in most cases replacing valve socket(s) helped.

Cheers.

[rf7]

Unfortunately, the guy who brought me the amp wanted it back, even with the hum, so I did not get to go any further into it.

Thanks everyone for your help! I really appreciate it. This hum stuff always causes me problems, but I'm learning some, thanks to you guys.