18 watt TMB squeal & hum

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Daviedawg
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Re: 18 watt TMB squeal & hum

Post by Daviedawg »

Are you reading The EL84 pin 2 voltages correctly? There should be negligible voltage there. The other pins are ok which is very strange.

The heaters should be roughly the same voltage supplying the rectifier as on the other valves.

Dd
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

jimgerrr wrote:
Sun 12/22/19 5:46 pm
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?
From your photos and inconsistent amp behavior, I'm getting the impression that either your soldering iron is poor quality, or not producing enough heat. Either way, I highly recommend watching the videos I linked here: viewtopic.php?f=4&t=25396

Here's some examples of issues I see, and these issues are all over the place.


DSC_4066b.jpg
In the above image, the turret marked OK, is just okay. It's slightly solder deficient, but looks mostly smooth and shiny.

Bad 1 you can see the resistor lead is oxidized, and not at all smooth or shiny. This means that the solder joint is bad, and that there was not enough heat or solder to make a proper intermetallic bond.

Bad 2 has many of the issues of both Bad 1 and Bad 3. If you can see that much flux left over, that means that there was insufficient heat to melt away the flux, or the soldering technique didn't take care of it.

Bad 3 has a better connection, but way too much solder flux. Sometimes too much flux simply needs to be scraped off with a nylon brush, and then you can inspect the quality of the solder joint.


DSC_4062b.jpg
In the above image you can see that the wires need to be pre-tinned better, and that you need to be careful of dripped solder. A tiny bubble of dripped solder can cause major problems. You can also see in all the photos that there's excess flux all over the place. Get yourself a few nylon brushes/scrubbers, they can go a long way in producing a clean build. I've been using no-clean solder for a while, and it's much less effort.


DSC_4061b.jpg
All of your tube sockets still have issues. Solder lugs, and solder joints in general, need mechanical connections. That means that you should pre-tin the wire, and bend it like a hook in the solder lugs/tabs. Once the wire has a nice mechanical connection, then you solder it in carefully.

The next problem is that you need a specific gap between the wire insulation and the solder terminal. Pin 2 is okay, though it obviously lacks the aforementioned mechanical connection. On pin 3 and pin 9 you can see that there is no gap at all, and that the insulation is melted around the terminal. That negatively impacts the quality of the solder joint, and the mechanical reliability of the wire. Watch the videos, there's also such a thing as too much gap.


Regarding voltages:
I assume your power tube pin 2 voltages are 13mV and 12mV, which are good readings if that's the case. If you're reading 12V and 13V, then that's a problem.

A: on the chart is your B+. All heaters should be around 3.15V measured to ground per side, for a total of around 6.3V per tube heater set. This is true for all tubes in the amp.

Pin 3 of the power tubes should be 12V. Once you've worked out all the other issues, you will want to remeasure and correct this. The other power tube voltages aren't bad. The inconsistencies will likely be improved when cleaning up the soldering.

Thanks,
Josh
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Re: 18 watt TMB squeal & hum

Post by jimgerrr »

Josh. Thank you for such a detailed response. You are correct about those voltages being mV. The heaters are all correct except for the rectifier tube, which I can't get a proper reading off of for some reason. I'm going to crank up the heat on my iron and try and clean up these problems. I'll post results in a couple of days.
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

jimgerrr wrote:
Mon 12/23/19 12:15 pm
The heaters are all correct except for the rectifier tube, which I can't get a proper reading off of for some reason.
I don't know what transformer you have there, and I can't see how it's wired clearly enough.

You should have the 6V and 0V wire from the rectifier heater connected to the rectifier. The extra wire (5V) should not be connected to anything. Normally that gets cut or wrapped, with the end cut clean and then covered with heatshrink tubing.

I see a yellow wire in one of your photos going from the PT to a tag board. I don't know if that tag is earthed or not. It should not be. That heater connection is not center tapped like the normal heaters usually are.

Thanks,
Josh
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Re: 18 watt TMB squeal & hum

Post by Explorerman »

Hi mate, did you end up finding the problem? (I’ve just started a M’tone TMB build so i’m keen to explore potential issues to look out for).
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MikeyV
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Re: 18 watt TMB squeal & hum

Post by 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.
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Re: 18 watt TMB squeal & hum

Post by 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 :roll:
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Re: 18 watt TMB squeal & hum

Post by 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.
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Re: 18 watt TMB squeal & hum

Post by 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!
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

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

Read that thoroughly, and review the linked layout for grounding best practices.
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Re: 18 watt TMB squeal & hum

Post by 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!
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

MikeyV wrote:
Wed 01/15/20 1:50 pm
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
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Re: 18 watt TMB squeal & hum

Post by 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?
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Re: 18 watt TMB squeal & hum

Post by 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....
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Re: 18 watt TMB squeal & hum

Post by 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!
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Re: 18 watt TMB squeal & hum

Post by 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.
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Re: 18 watt TMB squeal & hum

Post by JMPGuitars »

MikeyV wrote:
Thu 01/16/20 9:06 pm
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.
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Re: 18 watt TMB squeal & hum

Post by 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.
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Re: 18 watt TMB squeal & hum

Post by 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.
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Re: 18 watt TMB squeal & hum

Post by 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.
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