18Watt.com

Hello all! My first post, so a short introduction.

I've been working in electronics for ~40 years, and have access to laboratory-grade test equipment at work (oscilloscopes, signal generators, spectrum analyzers, etc.).

I've built a few effects pedals, and so far 4 amp kits (Tube Depot JTM45, Mojotone Studio One (two of them) and just finished a Mojotone 18W TMB.

They all work perfectly, except--the TMB has a loud 120 Hz hum which I cannot resolve. I've added shielded cable, which helped a little. I've checked all connections, as well as the wiring diagram and schematics.

I changed the ground points to only three (electric, power amp, and preamp).

With the preamp tubes pulled, the amp is silent. When I install V3 (phase Inverter) the noise returns. With V1 & V2 installed, without V3, the amp is silent. If I move the wires connected to V3 towards the circuit board, the noise decreases.

When I tap on the V3 coupling capacitor, I hear a loud pop. I ordered some replacement capacitors; possibly the coupling capacitor is bad.

Any other ideas?

Hi, welcome to the site!

Unfortunately the Mojotone kits (at least for 18W) are not great, and their docs are terrible.

You can read through the threads in my signature for tips, especially regarding grounding, which it sounds like you may have already done to some extent.

You mentioned moving the wires- we call that "chopsticking." Using a wooden chopstick, we'll move wires around to reduce noises and cure possible issues caused by lead placement. It's good to do this in your case too.

Considering the signal path, V1 and V2 are disconnected if V3 (PI) is disconnected.

It would be helpful to hear the noise to give more specific feedback besides what you're already doing.

It's also a good idea to share detailed photos of your amp build in its current state.

Thanks,
Josh

To clarify, this is what I did to try to eliminate the noise:


1. Swapped preamp tubes with each other; no change.
2. Installed a different set of tubes from another amplifier; no change.
3. Installed shielded cables to both inputs; no change.
4. Installed a shielded cable to the Gain control; hum was reduced by about 20%.
5. Removed the bus bar from the back of the pots, and installed it directly on the turret board; no change.
6. Transferred the Power grounds from the bus bar to the star ground; no change.
7. Removed the last B+ filter cap from the turret board, and moved it by the star ground; no change.
8. Removed the 5 VAC wire from the rectifier (thought it might be bleeding through); no change.
9. Re-flowed all solder joints on the tube sockets; no change.
10. Re-flowed all solder joints on the turret board; no change.

After all that, I decided to move the wires around to hear if there was any difference. Moving the wires from the PI reduces the hum a little if moved away from the tube. So, I started tapping on the turret board components. When tapping the coupling cap on the PI input stage (the third yellow cap from the left), I hear a loud pop. I thought the cap might be bad, but it checks good on my meter (10 nF).

I ordered new caps and input jacks, plan on replacing the 1M carbon input resistors with metal film, and finally install shielded cable on all the connections (except filaments) to the PI.

If all these don't work, then I don't know what I'll do with it.

It sounds like you've read a lot on the subject, as that's generally good ideas. However, don't go crazy second guessing what you've done.

You don't need to replace all the leads with shielded cable (but you can if you really want to). Input jacks, long runs from gain knobs, and sometimes plate wires are good to be shielded.

I don't agree with moving the preamp filter cap to the star ground, I keep preamp and power amp separated. But I'd be more concerned in this case with the long wire than the ground itself. I really hate the mojo layouts. A lot.

Take a look at this layout and compare logically with what you've got going on: files/JMPGuitars_18W_TMB_Modern_Classic_Layout.pdf
Here is the related schematic: files/JMPGuitars_18W_TMB_Modern_Classic_Schematic.pdf

Metal film resistors are much better for the inputs, and that should help to some degree.

Can you share a sample of how it sounds? Play guitar also and then mute the guitar strings so we can hear the difference between signal and noise.

Thanks,
Josh

It wasn't a preamp filter cap I moved; rather, it was the last cap in the rectifier circuit.

After playing through it, I guess the S/N ratio isn't as bad as I thought. Even my phone had a hard time picking up the hum.

The first few seconds of the clip is me turning the Master volume control up & down. The Master volume control is a 1M log pot, but it seems the volume jumps from almost nothing to very loud with just a minor turn of the control. I cleaned it with Deoxit, but it didn't make a difference. Would a different value or taper fix this issue?

The last cap in the rectifier circuit - I assume you mean the B+ filter cap. I also assume it's filtering is taking place in the preamp.

Anyway, from your recording, it doesn't sound like the hiss is that bad. Replacing the input resistors might help a little. Better coupling caps (especially in the spot you suspect) might help too. Overall, carbon film is noisier than metal film, and perhaps that's also contributing to what I think sounds like maybe an acceptable amount of idle noise.

Remember, this is a tube guitar amp, and its intent is to make tasty noise. In hi-fi you might want the noise floor lower, but you're going from fairly quiet in your sample to well distorted guitar. Turn on any distortion pedal and you'll probably hear similar (or worse) noise ratios.

The volume knob sounds normal from your description. There's a very brief low volume, then a short range with clean headroom, and then the rest of the volume pot range cranks further into overdrive.

The pot is scratchy for the first 5-15 degrees of turn, so I'm going to replace it with a better quality pot (CTS). I'm also going to move the PI wires further away from the tube, and possibly replace the carbon resistor with metal film.

Thanx for the help!

Pot scratchiness can also mean there's DC leaking into the pot(s). Make sure your caps are doing their jobs.

Just checked--no DC voltage on the potentiometer or after the coupling caps.

About scratchy pots..
there are some band aid ways to address that, but I for one wouldn't bother. I mean, how often will you turn that knob in a playing situation? I've read about people putting diodes or caps from lug 3 to ground and such. This will always have a downside somehow.
As for the CC to MF conversion. Go ahead if you want to quiet it down some, but there as well, I wouldn't bother. Occasionally I use CC on purpose, that is for their sweeter sound in some applications. Nowadays I tend to use 2 watt CF at the plates and tone slope resistor.
It generally is beneficial to use higher wattage resistors, or bigger physically sized ones. They tend to be more quiet. It's all nitpicking though. If you want to take it even further you could also check the foil side of the caps and put them in the correct way. ( cathode bypass caps --> foil side to ground / coupling caps --> foil side to the plate resistor / tone caps --> foil side to the slope resistor)
I'm with Josh on the noise subject.. tube amps should make some noise, but only related to the dynamics of the amp.. not something's wrong noise
A fine example of wanted noise is the NFB in some amps. When I build an amp with NFB I always put it on an on-off switch. and it's generally off for the same reason: dynamics!!!

I decided not to make any changes, and finally gave it a proper test run. It rocks! I can't believe how loud this amp is. It sounded a lot better than I thought it would.

The only issue I still have with it is the overly sensitive Master Volume control. At about 8 o'clock, it's a reasonable volume. However, when I barely turn the control just a bit more clockwise, it gets insanely loud. I tried putting a 470k resistor on the input, output, and then finally the wiper terminal, and the problem still exists. I put the same 470k resistor in parallel with the pot; no change. Finally, I installed a 10-turn 50k resistor in the 1M pot's place; still the same. I just don't get it.

Hanover Fiste wrote: ↑

Thu 02/15/24 12:44 am

I decided not to make any changes, and finally gave it a proper test run. It rocks! I can't believe how loud this amp is. It sounded a lot better than I thought it would.

The only issue I still have with it is the overly sensitive Master Volume control. At about 8 o'clock, it's a reasonable volume. However, when I barely turn the control just a bit more clockwise, it gets insanely loud. I tried putting a 470k resistor on the input, output, and then finally the wiper terminal, and the problem still exists. I put the same 470k resistor in parallel with the pot; no change. Finally, I installed a 10-turn 50k resistor in the 1M pot's place; still the same. I just don't get it.

There ain't no cure for that thing. If there was, this would be all over the internet. the next logical step would be a PPIMV type Lar Mar IMHO. I've had good results with that one, but never implemented one on an 18 watt amp though

I've done some more research on this, and it seems to be the way they normally operate. I'll just have to be more careful setting the volume--when the wife is home, anyway!