60 cycle hum in tremolo channel after shorting pin 7 to ground

[bobtait]

I have a mojotone 18 watt british that when you use the tremolo foot switch to turn of the tremolo, the tremolo channel hums. Right now pin 7 on V3 shorts to ground to disable tremolo. Is there a better way to wire it, maybe just use the switch to break the connection from pin 7 to the tremolo intensity pot? This is the last bug I have to chase down on this amp, any help is appreciated.

[JMPGuitars]

Hi Bob,

Grounding out the tremolo is the correct way to mute it. However, that can be implemented poorly. If it's creating a ground loop, or if there is bad lead dress, that can certainly cause issues.

If you know which schematic was used, please post that along with high res photos of the build.

Thanks,
Josh

[bobtait]

I will post photos when I can open it up again. But it was built I believe on the standard mojotone wiring diagram. However that diagram shows the footswith right next to the speaker outputs and the footswitch in my amp is way across the build right next to the main capicitor and grounded to it. It had a wire running all the way across the chassis to the tag where pin 7 was soldered by a short wire and hooked up there. I moved that wire back to pin 7 and tried to cross any looms at 90 degrees. This build was pretty messy, I had to reflow all solder joints and place grid stopper resistors on V1 and V3. That stopped the RF I was getting and the amp is, over all, very quiet now. I didn't really need resistors on pin 2 of both but I misread the diagram from looking at it 180 degrees off from my view of the amp. So, now there are grid stoppers on both pin 7 and 2 which is probably ok. It stopped the RF. Anyway it was a long run from the footswitch jack, across the chassis from the positive of the foot switch jack to pin 7. I will get more info in as soon as I can open it up again. But for all intents and purposes, I could gig with it right now. I don't need the tremolo channel.

[Bieworm]

The footswitch jack is preferably near the tremolo tube or the part of the circuit where you're grounding the tremolo. The ground wire should theoretically be wired to the negative side of the filter cap of that stage.

but not all is science and theories aren't always the single way to go. For example I have amps where the reverb RCA jacks need to be grounded at the cathode ground of those stages or otherwise they will induce noise. On other similar builds those RCA jacks are grounded directly to the chassis and are noise free.

BUT!! when you experience trouble you should always go to the theories of good practice and see if that helps with the problem.
In case you overlooked, have you tried rolling tubes for that spot? Tremolo's are finicky on tubes.

[JMPGuitars]

Long wires, especially long ground wires, can become antennas. Try making that ground wire as short as possible. I usually ground the mute jake to jack grounds for the output speakers, which goes back to the star ground in the power amp anyway.

The white jack in this image mutes the reverb or tremolo:

_JMP3283.jpg

The jack is switched, so when a cable is inserted, it disconnects and overrides the front panel mute switches.

Thanks,
Josh

[bobtait]

Here are some photos of the amp wiring. The only things I have done so far is: re-flow every solder joint, clean all the pots and lube them, put grid stop resistors on V1 and V3 and moved the blue wire that goes from pin 7 to the switch back to pin 7. Before it was attached to the lead from pin 7 that went to the tag board. The radio stations are gone but I still have the hum in the Tremolo channel if I use a footswitch and turn the reverb off. Some of the wiring, to me, looks problematic, but right now it is quiet and sounds good. I am really new to going inside a tube circuit but I have many years experience in sound engineering, building and repairing guitars and of course playing in bands.

[Bieworm]

Reverb?

[bobtait]

No reverb. It is a Mojotone 18watt British. Copy of a Marshall 1974X. Pretty much they stuck to the Mojo wiring diagram but they were kind of ham handed on the dress. That is why I wanted you guys to see it and tell me what you think. And to everyone, thanks for the replies from everyone so far!

[JMPGuitars]

I know you didn't do it, so I'll be blunt...the lead dress in there is a nightmare. You're lucky that it's as quiet as it is! The heater wiring is scary, on top of the fact that I don't generally like cloth wire in amplifiers.

There are other safety issues all over the place too. This looks like a fire hazard.

[bobtait]

OK, so what is the best wire to order, in how many colors. And since the build is from mojotone should I just follow their wiring diagram, or, is there a better diagram using the same parts I should download? I can straighten this thing out.

[bobtait]

Here is a photo of the TMB clone I just bought that I was told was put together on a JMP design. How does this one look to you? Anything in this one for me to worry about?

[JMPGuitars]

OK, so what is the best wire to order, in how many colors. And since the build is from mojotone should I just follow their wiring diagram, or, is there a better diagram using the same parts I should download? I can straighten this thing out.

My suggestion would be to keep the transformers and chassis, and pick whatever interests you from our download section.

I use 20AWG PTFE wire. TubeDepot is a good source for the wire.

[JMPGuitars]

Here is a photo of the TMB clone I just bought that I was told was put together on a JMP design. How does this one look to you? Anything in this one for me to worry about?

This one is lightyears ahead of the other one. I don't like the excess wire wraps; they should have been trimmed. The AC wires including OT wires should be twisted and secured.

That said, if you love the amp, and it's not noisy. S'all good. I'll look again a little closer, but nothing jumped out at me yet as being scary.

Look at the Tremolo TMB build photo with my PCB in it I posted above. That will give you an idea of the cleanliness I expect to see as a minimum for professional work.

Thanks,
Josh

[JMPGuitars]

Go through the build, and look very carefully for pins that appear to be near anything they could short out on, like the socket mounting hardware:

Screenshot 2025-06-03 164907.jpg

That might be an illusion, but I can't tell from the photo. Either way, the pins can probably be pushed away from the nut.

Also make sure that no components are touching each other. Then make sure if they have leads connected, they have good contact, only once. No wobbly things that can bounce around.

You should look for bare wires that are exposed more than they should be. I hate cloth wire in amps, but in this case, you can probably push the cloth to cover the wire better.

Review the soldering videos in my signature. They include standards for exposed wire near terminals and such.


See if you can find out whether that cloth wire is actually rated for this, and if it's fire resistant.

Thanks,
Josh

[bobtait]

I ordered some 22 awg Solid Core Hookup Wire PVC insulation. And I was going to order some 18 gauge too. This is stuff just off Amazon. Tell me if this is a mistake, I will find something else. Thank you for all the help so far. I am just going to completely redo the tremolo amp. And, I will check over those things you mentioned in the TMB one.

[bobtait]

I checked tube depot and most of their wire was stranded cloth covered. They did have some aerospace grade but it was stranded too. Does it matter if it is solid or stranded. I can see where solid would be better for a well planned, measured and executed loom, but what do I know, I'm just getting started on this stuff.

[JMPGuitars]

I checked tube depot and most of their wire was stranded cloth covered. They did have some aerospace grade but it was stranded too. Does it matter if it is solid or stranded. I can see where solid would be better for a well planned, measured and executed loom, but what do I know, I'm just getting started on this stuff.

I only use stranded. I use the 20AWG PTFE stranded wire they sell. It holds its form very well, as you can see from my photo. Those bastages owe me a referral lol: https://www.tubedepot.com/products/20-g ... -ptfe-wire

Note: you need a good quality wire stripper to work with PTFE.

[bobtait]

Well I found some of that wire in solid, 20 ga like you use, I will let you know if I find I shot myself in the foot, but I think I can wrangle that wire. I am going to plot exactly where I want it going, and make the entire section and install it. I just have to plot my wiring harness with care. Once again thanks for all the input. It is greatly appreciated.

[bobtait]

This is turning into a great learning experience LOL. So far I have determined that the power transformer is mounted backwards, parts are missing and it must be an early version of that model because the tremolo footswitch is mounted, per the back plate labelling, in the wrong position. I don't know yet if I can run some shielded cable to it or if I have to just drill another hole and move the jack right next to V3. Mojotone will only sell the new revised back plate in sets and they want 50 bucks for a front and back face plate.

[bobtait]

On second thought, I could sacrifice one of the speaker output jacks. The second jack is right where the tremolo jack is on the new revision. Just plug the old tremolo jack hole and relabel the second speaker jack as Footswitch.