18 watt TMB squeal & hum

[jimgerrr]

Hello!

I built a Mojotone 18 watt TMB. Amp fired up first try but when I play at higher volumes, there is a lot of hum and squeal. With the amp dimed (which sounds great!) the gain above 12 o clock everything oscillates, and the pitch changes when adjusting the treble, mid, and low knobs. The normal channels are perfectly fine! I used shielded cable for the master volume and gain leads. For grounding I have the preamp, and inputs grounded together, output Jack grounded to the chassis, and the outputs all grounded together.

What I've tried so far. The chopstick test;
The squeal doesn't appear to change when moving wires or tapping caps, or tubes. However when I hover my hand over the input, and output I notice the hum greatly increases.

I also tried following JMPGuitars grounding scheme but there was no change, so I reverted it to how I had it initially.

I also added aluminum tape to shield the chassis. It helped the squeal a little bit but the hum is unbearable (it's basically as loud as a chord with the amp dimed)

I've also swapped all of the preamp tubes, in case one of them was microphonic.

Let me know if there's anything else I need to post, I can take better pictures if needed, or audio samples. Any assistance would be greatly appreciated as I'm losing my mind diagnosing this!

See below for my voltages.

V1
1 181.2
2 .8
3 1.656
4 6.5
5 6.5
6 134
7 0
8 3.1
9 6.5

V2
1 138.6
2
3 .88
4 6.48
6 221
7 1.2
8 2.4
9 6.48

V3
1 163.4
2 0
3 64.7
6 210
7 0
8 64.7
9 6.49

V4
3 11.1
7 344
9 319

V5
3 11.1
7 360
9 318

V6
1 299
3 352
4 25
5 4.9
7 298

[geoff 1965]

here's the mojotone schematic,

18-Watt-TMB-SCH.gif

first observations of your voltages show the plate voltages on your phase inverter should be fairly equal and yours are quite off at 163 & 210V, also the same for your el84's,the plate voltages are 344 & 360.
did you use a balanced tube for V3 and are the el84's a matched pair?

[JMPGuitars]

Use this to keep track of / compare your voltages: files/JMPGuitars_18_Watt_TMB_Voltage_Test_Chart.pdf
Note that the version you have has the PI as V3 and TMB as V2 per their schematic.

As geoff mentioned, your voltages are a little wacky. I would strongly suggest reimplementing the modern ground scheme, and then do a highlighter test of the schematic. Only highlight connections and components after verifying their value and connections are exactly what they're supposed to be.

An audio sample would be helpful.

Thanks,
Josh

[geoff 1965]

i like these mojotone kits and you've done a nice & neat build, however! one area is jumping out at me and that's all the lead wires to your pots & tube sockets run underneath the board.it's going to make it hard for you to check for bad connection/solder joints, i would prefer them to run over the board to the turret where you can see them.
note,the 470K resistor between the wiper of the gain pot and the grid of V2 looks suspect,it's laid against the turrets with no mechanical connection.

[Daviedawg]

Geoff has noted the voltages which point to an issue somewhere. In this case the effect of your proximity leads me to think you may have a poor solder joint somewhere leading to ground. My approach would be to take a magnifier and examine every solder joint looking for messy or dull solder. Reflow any that are dubious. I know it seems basic but even after many years of building I just recently had that very issue, probably from rushing work.

That takes you to Geoff's point about underside connections on the board which you cannot see without lifting the board. I am not a fan having experienced this sort of issue with a turret board. I prefer eyelets or tag strips.

Dd

[TexasTele]

Sorry you are having trouble. I hope I’m not leading you down a rat hole, but maybe this will help. Like other folks here say, it could be solder joints. I had a build years ago that hummed, squealed and oscillated horribly. I spent months trying to figure it out. I thought solder was just solder. I was wrong. The amp was my first build. I had been using a cheap soldering iron. I wound up buying a decent soldering iron, the kind that maintains tip temperature. It was a few hundred dollars. I desoldered everything and completely rebuilt the amp using the same layout, ground scheme and component types/values. Viola! The rebuild turned out awesome. You might try reheating the solder joints. Be sure to use a heat sink. For example put an alligator clip on the leads to protect the parts from overheat, and heat the joints long enough to completely melt the solder. I think soldering technique is critical. Another thing is to make sure all the tube sockets are secure and solid so that each pin of every tube has a very good connection in the socket. I use a cheap dentist pick for that. Best of luck.

[crgfrench]

Did you happen to add negative feedback (I know it's not shown in the schematic)?

Also you are using grounded jacks? There's no ground indicated on the schematic bus other than the jacks.

It looks like if you use both grounded input and output jacks there is a ground loop per the schematic, with the bus grounded at both ends?

[JMPGuitars]

I just looked at your photos. this thread and watch the videos on soldering technique: viewtopic.php?f=4&t=25396

Soldering technique is more important than a good ground scheme, and the good ground scheme is very important.

Personally, I prefer turrets over anything else. But, like anything else, they need to be used properly. That means 1. a good mechanical connection, and 2. good soldering technique. Unfortunately your build has issues with both.

Solder joints need to be free of contaminants and other debris. I see tube sockets with the wire insulation sticking through the solder lug.

To play the highlighter game I mentioned previously, you need to determine that every component is the correct value, but each component and connection must also have a good/clean/shiny solder joint before highlighting.

I agree with the others about running the wires over instead of under the board, but the quality of the solder joints is the first thing you need to work on.

Don't feel bad about any of that- I've been soldering for over 25 years, I still review those videos, and I STILL learn and improve my skills.

Thanks,
Josh

[crgfrench]

If nothing else works, one last thing I would try is rewire your filament leads so they run over the top of the sockets and come down vertically to the pins, instead of running them along the chassis like that.

[jimgerrr]

Thanks for the replies everyone! I've redone the grounding to JMPs, I touched up a few solder joints
while using my macro lens and verified the schematic. I'm still getting squealing and hum. I also noticed that when I have it plugged into my pedalboard, it oscillates wayyyy more. Plugging directly into it cuts down some squeal, but it's still not acceptable. I do have a silly question though. When taking voltages, does it matter where the master volume is? I noticed the V3 voltages are within 10 volts when the master is around 12 o clock. When I dime it, pin1 significantly drops to the 160s from 200v.

Edit: another side note. When plugging into the high gain channel, there is a huge squeal when the input cable touches the sleeve of the jack. Is this normal? It doesn't happen on the regular channels.

[geoff 1965]

have you tried swapping V3 tube for one of the tubes in V1 or V2 to see if you still get the voltage drop?
i wonder if you are getting any DC voltage leaking into the phase inverter grid? set your multimeter to DC millivolts and carefully check pin2 of V3 on both half and full volume, just an idea.

[jimgerrr]

I just swapped the tubes and it does the same thing.

MV on 0
V3
P1 - 206v
P2 - 44v
P6 - 208v

MV on 10
V3
P1 - 170v
P2 - 44v
P6 - 208v

[JMPGuitars]

I'm still getting squealing and hum. I also noticed that when I have it plugged into my pedalboard, it oscillates wayyyy more.

Please post new photos, and new voltages using the chart I linked before. Make sure we can see the turrets from an angle, and all the tube socket connections.

Before that, I would recommend reversing your cap can so it faces the other way. The ground lead should be much shorter and more direct than it is. I'd also recommend using a 5W resistor between the two sections. The resistor there looks like it's been overheated.

Thanks,
Josh

[jimgerrr]

See attached for the latest pictures.

[jimgerrr]

See below for my latest voltages. These were taken with nothing in the input. I noticed when I had a guitar plugged in with gain and volume maxed, voltages would drop like in my first post. I have replaced the overheated resistor on the can cap. V2 (TMB) i couldn't get a proper read on pin 7 and 8. I checked all the connections and still no luck.

Also I get strange readings on the heaters of the EZ81 tube. Are they supposed to be consistent with V1-v6?