Marshall 50 JMP Hum - Heater Filament Pot Candidate?

[hazy]

1st off - I'm a newbie to the forum and just found you guys with a Google of my problem.

My Marsall 50 JMP (4-hole, 1987 schematic model) has a 60-cycle hum that won't die. I figure 60-cycle because I downloaded an audio of 60-cycle and that's it.

I read the thread here where the final solution was to put a 100 ohm pot between the heater filaments and it fixed the hum. Thread: http://ampgarage.com/forum/viewtopic.ph ... =cycle+hum

Here's what I've done:

Removing PI & V2 pre-amp tubes kills the hum
Replaced filter caps
Check lead dress
Repositioned heater wires
Cleaned/retensioned all tube sockets
Re-tubes all pre-amp and power tubes

The hum will not go away. It just showed up and I can't find it - and I've worked on Bassman, Bandmaster amd Marshall JMPs for years. Would the 100 ohm pot work here (possibly)? Help!

[Merlinb]

The hum will not go away. It just showed up and I can't find it - and I've worked on Bassman, Bandmaster amd Marshall JMPs for years. Would the 100 ohm pot work here (possibly)? Help!

Dude, it's a 100 ohm trim pot! It would be quicker to try the thing out than to wait for "maybe" answers on some internet forum.

Also, you can use any value from 100 ohms to 1k. (I prefer to use 500 ohms since some trim pots are only rated for 200mW or so). Make sure you disconnect the existing heater centre tap/ground reference first.

[hazy]

Dude, it's a 100 ohm trim pot! It would be quicker to try the thing out than to wait for "maybe" answers on some internet forum.

Also, you can use any value from 100 ohms to 1k. (I prefer to use 500 ohms since some trim pots are only rated for 200mW or so). Make sure you disconnect the existing heater centre tap/ground reference first.

Thanks. I've never done it so I wasn't sure if my issue was fixable with the pot.

Hazy

[hazy]

Last question: How do I install the pot on a Marshall 50w 1978 Drake PT? I can do the work; I'm just not sure where to make the connections.

I'm not a newbie to working on high voltage amps...I just can't get this fixed. So, if you are familiar with a Marshall PT from 1978 (50 watt) and where I need to make the connections, please let me know.

Thanks,

Hazy

[Merlinb]

Last question: How do I install the pot on a Marshall 50w 1978 Drake PT? I can do the work; I'm just not sure where to make the connections.

I'm not a newbie to working on high voltage amps...I just can't get this fixed. So, if you are familiar with a Marshall PT from 1978 (50 watt) and where I need to make the connections, please let me know.

Thanks,

Hazy

Find the existing centre tap on the transformer (heater supply) and disconnect it from ground. If it doesn't have one, then look for a pair of resistors soldered from each heater wire to ground and disconnect those instead. Basically you want to whole heater supply to be unconnected from anything except the valve sockets, before you do the mod.

Then connect each end of the pot to the heater wires. You can either do this right on the transformer, or on one of the valve sockets. Wherever is convenient really. Then connect the wiper to ground/chassis.

[hazy]

Find the existing centre tap on the transformer (heater supply) and disconnect it from ground. If it doesn't have one, then look for a pair of resistors soldered from each heater wire to ground and disconnect those instead. Basically you want to whole heater supply to be unconnected from anything except the valve sockets, before you do the mod.

Then connect each end of the pot to the heater wires. You can either do this right on the transformer, or on one of the valve sockets. Wherever is convenient really. Then connect the wiper to ground/chassis.

Thanks, Merlinb!

I haven't seen any resistors off of the PT or at the valves (power tubes) so I have the center tap running to ground - I'll disconnect that. I couldn't find a local 100 ohm pot; but, I did get (2) 100 ohm 1 watt resistors that I can use. Would the resistors at each leg of the heater filaments possibly help over just the center tap? I know that I can just try it and see; but, I would like to know if it has a chance for success.

Hazy

[Structo]

It depends on the transformer.
The center tap may work fine.
But usually two matched 100R resistors will be better balanced and the result will be less hum.

[Merlinb]

I couldn't find a local 100 ohm pot; but, I did get (2) 100 ohm 1 watt resistors that I can use. Would the resistors at each leg of the heater filaments possibly help over just the center tap?

Probably not- a pair of resistors is no better than the transformer's own centre tap. The whole point of a humdinger pot is that it allows you to create an 'off-centre' tap, as it were, and null out the hum in the valves.

You don't necessarily need a 100 ohm pot. Anything from 100 ohms to a 1k pot would be fine. Surely you can get hold of a trim pot somehow?

[Firestorm]

If neither the center tap, nor the two 100Rs kill the hum, do try the pot (as Merlin says, it doesn't have to be 100R). This lets you precisely compensate for any imbalance in the winding. That said, the other issue that occurs in some Marshalls is that the .68 cathode bypass cap on the "bright" channel is too small to bypass heater and power hum to ground. These often benefit from floating the AC heater supply on top of a DC voltage. This requires a couple of resistors and a cap, so it's a bit more intrusive.

[Rick]

The cause of the hum should be investigated and addressed first. I'm surprised everyone overlooked that simple fact. In my experience, most 60 Hz hum cases come from a ground loop on the first preamp tube. If using metal input jacks, remove all grounds to the jack from any ground bus and then ground the cathode resistor(s) directly to the ground of the input jack. If your using a bus rod for the preamp, keep the input/cathode grounds separate from all those on the bus often helps. If the jacks are cliff plastic jacks, try grounding the cathode resistors to the jack ground, then run a ground wire from the jack ground to a star ground on the chassis or a ground away from all the other preamp and filter grounds. Use 20AWG minimum wire for grounds, you want low resistance in all ground paths so hum signals don't develop a voltage over the resistance.

[hazy]

I already tested all of the input jacks and they are not the source of the hum. I forgot to mention that in the original "what I've already done" section.

I had to replace V5 power tube socket a couple of years ago...forgot why, but it was bad. Is it possible that the center tap in no longer the balanced ground that it originally was and that's causing the hum?

I'll check Radio Shack locally for a low value trim pot. My local electronics dealer only had higher values.

Thanks again,

Hazy

[Gaz]

The cause of the hum should be investigated and addressed first. I'm surprised everyone overlooked that simple fact. In my experience, most 60 Hz hum cases come from a ground loop on the first preamp tube. If using metal input jacks, remove all grounds to the jack from any ground bus and then ground the cathode resistor(s) directly to the ground of the input jack. If your using a bus rod for the preamp, keep the input/cathode grounds separate from all those on the bus often helps. If the jacks are cliff plastic jacks, try grounding the cathode resistors to the jack ground, then run a ground wire from the jack ground to a star ground on the chassis or a ground away from all the other preamp and filter grounds. Use 20AWG minimum wire for grounds, you want low resistance in all ground paths so hum signals don't develop a voltage over the resistance.

It's not like the hum balance pot is big operation, he can try it with jumper cables to see if it works before tearing the whole thing apart. Am I right or am I right?

[solderstain]

I come from the "a picture is worth a thousand words" school...

Since the hum goes away when you pull preamp tubes, that pretty-much localizes the problem. I doubt it's anything to do with the input jacks, since they're not metal, or shouldn't be - those Marshalls originally came with the plastic Cliff jacks, and unless one of them is just flat damaged, it's probably not them.

What you didn't say is if you tried leaving V2 and the PI in and removing V1. When I trouble shoot something like that, I start by pulling V1 and working my way forward, one tube at time. Put all the tubes back in, and pull V1 first. If it goes away there, then that REALLY localizes the problem.

I've worked on MANY Marshalls, and owned a few (still have my metal-panel SL), and I've NEVER seen a Marshall that needs a hum balance pot. Never. Especially if it just showed up. There's something else going on that just hasn't been found.

First, localize the problem. Just pulling V2 and the PI only went part-way. Reduce the possible trouble-zone by pulling V1. Then, post some pictures - good quality macro pictures of the circuit board and the tube sockets - and perhaps a different set of eyes here may see something that got over-looked. I've been building amps since the late-1970s, and every once-in-a-while, I miss something that usually gets seen when I send some pictures to other amp-building buddies. It happens to the best of us.

[hazy]

Thanks, solderstain.

I've pulled V1 and the hum is still there. I've replaced nearly everything that touches V2 - since I think that's the spot - but with no luck.

Yesterday, I added the 100ohm pot and it made no difference (so I removed it). BTW, the 4 input jacks are metal as I changed them years ago because I like that look better than the plastic ones. I checked all of the jacks and they are not the problem. Below are 3 pictures I took this AM. I've pretty much given up on this one for a while.

[IMG:800:600]http://i123.photobucket.com/albums/o285 ... /Full2.jpg[/img]

[IMG:799:599]http://i123.photobucket.com/albums/o285 ... rside1.jpg[/img]

[Jana]

Lift those green grid wires off the heater wires! Stand them up, get them away from the other wires, shorten them up a bit (if possible).