18 watt TMB squeal & hum

[MikeyV]

I just finished a Mojo TMB build, and have the same exact problem.

Normal channel works great, TMB channel squeals and feeds back if I get the master and the gain knob up. If the gain is at 10, I can only get the master up to like 2 and it goes crazy. If I keep the volumes low, it actually sounds OK.

I did a voltage check last night, and found that everything looked just like the values given here by a member, except for pins 7 and 8 on V2A (which is the second stage of the drive channel) I get 0 volts. In the standard 18Watt.com voltage chart,it is listed as V3 TMB. Which is V2 in the Mojo layout. The PI is good (voltage are right, plus channel 1 works as it should.)

Having a hard time figuring out the problem. I worry about the bus bar, but really it should work, right? Maybe I need to move the jack ground on the drive channel) to it's own ground and off of the bus bar.

I'm wired exactly like the Mojo layout at the moment. Wiring is clean, heater wire is neat, stuff's twisted and tucked, etc.
I can try to post a photo tonight.

[colossal]

Yeah, please post a photo. My suspicion is that the amp is oscillating and it may be a lead dress or component placement issue.

This place is turning into the Mojotone Free Support group. They make the money and we fix their problems. Hmmm, maybe we should ask for a donation from them to keep the site running

[MikeyV]

20200114_173015.jpg

Here are some photos.

First, an overall. Then a closeup of the problem area.

20200114_173113.jpg

20200114_173028.jpg

I had a lot of noise chip-sticking the area around the blue filter cap and the 470 ohm resistor that goes between the ch 2 volume pot and pin 7 on V2 (the green wire on the right side of V2.

I re-flowed some joints, and that noise seems to be gone.
I also had noise when touching that same green wire with a chopstick, and the input wire from the gain channel jack to pin 2 on Valve 1. Those seem less sensitive after reflowing some joints.

I tried playing it again, and with the gain knob at 10, I can't get the master over like 4 or all hell breaks loose. Feedback and squealing.

Am I expecting too much to be able to dime both the gain and master knobs? or get close?

Overall impression is that that drive channel is WAY loud, like louder than my '73 JMP50 thru a full stack. It's crazy. and this is with the gain on 10 and the master on like 3.

Does anyone see anything whack? I've checked so many times.
Thanks for the help!

PS! to be clear, this is the Mojo kit with the JCM800 style channel 2, single input jack and master volume. And Ch 1 works great/sounds great.

[MikeyV]

20200114_173105.jpg

Here's a close-up of the gain channel input jack.

Some thoughts I had...

-is it bad to ground that input jack onto the bus bar at that location? (I've heard bus bars can cause issues)
-could I run that green input wire atop the board and make it short as possible?

-could mounting that 470 ohm resistor right onto the tube help?
-how about mounting the 68K grid resistor right onto the tube?

I'd like to get it working with minimal re-work, (obviously, lol!) but I guess these things would cut down on some of these issues.

Same goes for using shielded wire to the input.
Which I have, but I'd have to harvest it from another un-used build. I have that sweet teflon stuff.

Cheers!

[JMPGuitars]

Modern Ground Schemes: viewtopic.php?f=4&t=25372

that thoroughly, and review the linked layout for grounding best practices.

[MikeyV]

Thanks for that link. Seems pretty straightforward.
Lose the bus bar on the back of the pots, and run the grounds individually over to a bolt near the ch1 input jacks.

Looks like I can leave my PT/AC grounding as it is.

I bet this and some shielded cable would shut it up.

Question: Would this poor grounding scheme result in the intense loudness that I'm seeing? i.e. going from nothing to full blast between 0 and 3 on the master knob? In fact, with both the gain and master all the way down, I can still hear a bit of signal come out of the speakers.

What it seems like is that the gain channel is overpowered, or like there's nothing holding it back or in control. I know, not technical terms.

It sounds awesome in the short range that it works, from off(just a little sound) to awesome Marshall drive, but this all happens between 0 and 3 or 4 on the master (with the gain at 10). And it's ear splittingly loud by that level on the master. After that level, it's all feedback, squealing and honking.

If all this happened smoothly across the full range of the master pot, then I would say it's perfect.

Thanks!

[JMPGuitars]

Question: Would this poor grounding scheme result in the intense loudness that I'm seeing? i.e. going from nothing to full blast between 0 and 3 on the master knob? In fact, with both the gain and master all the way down, I can still hear a bit of signal come out of the speakers.

Two(ish) issues.

A bad ground scheme, especially one relying too much on a star ground can squeal when you turn the master and pre up too high.

Lead dress can lead to oscillations which can also cause that problem. Chances are, you've got both happening.

Shielded input wire is good, but I would try it after the ground.

You'll want to fix up the ground scheme, and then see where you're at. You'll likely need to chopstick some wires around and see how that affects your noise. Or you could be lucky and the ground scheme will fix it all.

Thanks,
Josh

[MikeyV]

I found this layout here (I think) and it looks like the pre-amp grounding scheme is about what I want.
It's like the CeiraTone grounding scheme that I see.

Does this look ok? Will running the ch2 pots together as shown, and then to the ch2 gain pot, before going to the common ground by the jacks be OK?

[Explorerman]

Yeah I did this one up (posted in an earlier thread regarding the M’tone 18w TMB), I modified the M’tone layout to how I was planning to wire it. Do don’t take this for gospel, I wasn’t real sure on where to place the gain pot shield earths either, hopefully someone can chime in if this is good or not....

[MikeyV]

Well, I'll find out.
I'm gonna re-do the ground scheme first. Maybe also a shielded wire for drive channel.

This layout seems to make sense to me. I've read that the CeiraTone TMB layout grounding works well, but that one doesn't have the high gain CH2. In that layout, they just bus the grounds together from left to right over to the jacks of CH1. Which you probably know.

The way you have the TMB pots of the gain channel ganged, then going to their volume pot and then running to the ground point alone seems to me like a good idea.

I'll work up the courage, and find some wire, and report back.

I'd love to hear anyone's opinions on that ground layout before I dig in!

Thanks!

[Explorerman]

Yes please, let us know how you go! I’ll be very interested as my parts have only just arrived, so I plan to put this together over the next couple of days.

[JMPGuitars]

I'd love to hear anyone's opinions on that ground layout before I dig in!

It's not bad, much better than the original. I hate that layout in general though, it's very sloppy.

If you use shielded wire, make sure to only connect one end of the shielding, and heatshrink the other side so it doesn't contact anything.

[MikeyV]

OK, so I re-did the grounding scheme as shown in the diagram I posted.

Good news first. The Normal channel still works fantastically.

Sorta good news, with the gain on CH2 turned to 10, I can turn up the master to 6-7 now before all hell breaks loose. Now that I can get it higher, I'm hearing new noises.

-Lots of hum, like when you touch the tip of your guitar cable with your finger.
-wha-wah-wah sound only at higher volumes, motorboating? It comes on and then gets stronger until you kill it with the volume knob.
-Less squealing and more moaning/feedback.
-The TMB knobs affect these noises, but not much.

Something more. If I dime the master with the gain all the way down, I get what I'd call a normal hiss of a dimed tube amp. As I bring in the gain knob, then I start getting the loud hum, feedback, oscillation squeal, etc.

Doing the opposite, If I dime the gain knob, then bring in the master, it just gets super loud really quick.
It seems like the gain knob and what it's connected to bring in the trouble.

Now read this: I think I may have a bad component.
If I chopstick the whole thing, it's all very quiet, until I get to the 470K resistor that comes from the gain pot and feeds the grid of the second stage of CH2. See the schematic I posted with highlight. The wire coming off the gain pot, going to the 470K reistor mounted on the board, and the wire going from that to the grid of the tube are all very noisy. If I tap the actual body of the resistor, I get a loud "ploink!" sound thru the speakers. Only this resistor does this. Is that puppy bad?

I was going to re-flow the solder joints there last night...but Football..then the baby was asleep.

I plan on cleaning out the old solder, checking the component, re-soldering. If it's still bad, I'll order a new one.

[JMPGuitars]

Chopsticking isn't just hitting things. You can also use the chopstick to reposition wires and see if that effects your parasitic oscillation.

It could be the resistor, but I would check the gain pot wires first. You might need shielded wire from that pot. But it could also be adjacent wires causing trouble. Good luck and be safe.

[MikeyV]

I've also moved wires around with no real difference. I focused on the CH2 stuff mostly.

It's interesting to me that the first stage input wire is only mildly sensitive to mechanical rustling, but the input wire to the second stage is super sensitive.

It's my first "high gain" amp so I've never really run into this issue with the Tweed and Blackface stuff I've built.

Kinda lame that the Mojo layout doesn't really work. But like I said, re-grounding it didn't help that much. There's still something very wrong with this channel.

Using my brain hurts, but thinking about it, if that grid resistor was bad, or not providing the resistance it's supposed to, I guess it COULD give the problems I'm seeing.

I dunno. I'll change that that resistor out and fiddle more with the gain pot wires and see what's up.

[MikeyV]

Made some progress.

Fiddled around with a few things, put most back. I wired the input jack differently, like shown down at the bottom .

I also added a shielded input wire from the jack to the grid, and put the 68K resistor right on the tube.
This helped a lot. I can turn it up much more, but there's still feedback and squealing as the volumes go higher.

Next step is to shield the wire from the gain pot to the 470K resistor on the input to stage 2 grid, and move that same 470K resistor to the tube socket and off of the board. The *ploink* sound is gone, must have been a bad joint.

A few questions:
1) I can only measure 500K resistance across the Master Volume Pot. I de-soldered it and conformed it's 1 Meg, but in the circuit, it only shows 500K max, if the other TMB controls are in a certain position (on/off). Is this normal?
I'm thinking it the Master is only providing half the resistance, it may account for the jumpy action of the master know ( nothing to face melt in a short range, then squealing beyond that). I've checked the wiring here a lot, but not I don't trust the damn Mojo schematic or layout.

2) Is there something wonky in the wiring of the Bass pot to the treble and mid pots? It matches the CeiraTone TMB layout, but it doesn't seem to jive with the schematic provided by Mojo. There's an error in the bass pot wiper wiring on the schematic and it's throwing me for a loop. Again, checked many times, don't trust the Mojo drawings.

3) Why is this thing so loud? The normal channel seems OK to me on full blast, loud but not sounding like it's gonna blow out the windows. The TMB channel is as loud as a full stack with 50W JMP50 on top.

I tried a lower gain tube (5751, with an Au = 70, I think) in the second gain stage and not much change there.

Thanks for bearing with me, I'll keep going. I think that shielding that volume pot wire will help.

Mike

[MikeyV]

OK, looks like I fixed it.

Like I said before, changing the grounding helped a little, then adding a shielded wire from the input jack to the first gain stage on CH2 (along with moving the 86K resistor off of the cliff jack and right on to the tube) helped a lot.

What really fixed it was adding a second shielded wire from the gain pot to the second stage of CH2, along with moving that 470K ohm resistor right onto the tube.

Only real issue left is a good amount of hiss, but I realize that's mostly normal with everything dimed. I reckon I'll work on that a bit, probably can't do much.

Thanks for the help. I hope this helps some others that get the Mojo kit.


Cheers, Mike

PS:I'll post a few current photos soon.

[Explorerman]

Great to hear (gives me a little more hope with my current build)! Yes please list some pics when you get a chance.

[MikeyV]

I forgot to add.

It's still LOUD as HELL. If you dime the gain, it's pretty loud at 3-4 on the master, and then it seems to get a bit louder, and adds more power section distortion as you crank the master volume up.

Pretty rippin' for 18 watts. Guess I'll look into an attenuator. lol.

[JMPGuitars]

I forgot to add.

It's still LOUD as HELL. If you dime the gain, it's pretty loud at 3-4 on the master, and then it seems to get a bit louder, and adds more power section distortion as you crank the master volume up.

Pretty rippin' for 18 watts. Guess I'll look into an attenuator. lol.

Congrats on making all that progress! Yes, these amps are LOUD. People think wattage = volume, but it doesn't. a 10W amp is 50% the volume of a 100W amp. Then you figure in other things like speaker efficiency and cabinet, and it's crazy.

I'm assuming your kit had carbon comp resistors. If so, they hiss. The only place that theoretically benefits from carbon comps is the plates, and usually only on the PI. Carbon Film is much quieter, and Metal Film quieter than that. Some argue that Metal Film is sterile by comparison, but nobody has really proven that scientifically.

One easy place that makes a difference, and I would use MF (metal film) for is the input grid resistors. Those are the most prone to noise.