18 watt TMB squeal & hum

[MSVguitar]

I just started the build of the mojotone 18 watt TMB. So I don't have a completed amp. One thing that I noticed on the layout after reading this thread, is the wire lead coming from the 16uF filter cap for the preamp stage runs across the 470K resistor that connects the gain pot wiper to the grid of the first gain stage of the high gain section. This will cause hum. I will move the resistor to the tube socket and run a shielded wire from the gain pot. Unfortunately, I have already installed all of the components on the board, but are not soldered, so I need to take some sections apart. I should have checked this out before I started.

I am now reviewing the entire layout and circuit diagram to see if there are any other issues. Also, I noticed that a few people on this site are not happy with mojotone kits. All I can add at this time is that there are some issues with the build documentation and will let mojotone know. This is my first mojotone kit and so far I am disappointed with the layout and documentation. Hopefully I can make this a great sounding amp!

Mark

[JMPGuitars]

I just started the build of the mojotone 18 watt TMB. So I don't have a completed amp. One thing that I noticed on the layout after reading this thread, is the wire lead coming from the 16uF filter cap for the preamp stage runs across the 470K resistor that connects the gain pot wiper to the grid of the first gain stage of the high gain section. This will cause hum. I will move the resistor to the tube socket and run a shielded wire from the gain pot. Unfortunately, I have already installed all of the components on the board, but are not soldered, so I need to take some sections apart. I should have checked this out before I started.

I am now reviewing the entire layout and circuit diagram to see if there are any other issues. Also, I noticed that a few people on this site are not happy with mojotone kits. All I can add at this time is that there are some issues with the build documentation and will let mojotone know. This is my first mojotone kit and so far I am disappointed with the layout and documentation. Hopefully I can make this a great sounding amp!

Mark

I'm certain from your questions and diligence thus far that you should come out with a fine amplifier, once completed/troubleshooted.

There are always issues with the mojotone documentation. That and their weird layout is why there's so many issues with their kits. But don't worry, as I said, I'm confident you'll be able to work through all that, just take your time.

Thanks,
Josh

[crgfrench]

If you're building a Mojo kit, I recommend downloading the Stewmac documentation for it. Stewmac sells Mojo kits but has created their own documentation and IMHO the Stewmac docs are better. Disclosure: I have never built from a kit, always scratch built. But I have reviewed a lot of online instructions for kits.

[MSVguitar]

If you're building a Mojo kit, I recommend downloading the Stewmac documentation for it. Stewmac sells Mojo kits but has created their own documentation and IMHO the Stewmac docs are better. Disclosure: I have never built from a kit, always scratch built. But I have reviewed a lot of online instructions for kits.

Thanks for the info, I knew that Stewmac sold Mojo kits but never checked out their documentation. I looked for the 18 Watt TMB on the Stewmac site but didn't find it. I should good with the Mojo documents. I've built kits as well as a scratch build for a Silvertone 1483 circuit. I learned a lot on that one. So much more to learn.

Thanks,
Mark

[MSVguitar]

The amp chassis if finished. I made all of the changes for the grounds, added shielded wire on the inputs, gain, master volume and treble pots. I am getting a loud hum when I max the gain pot and set the master volume at noon. When I increase the master close to max, the hum is very loud and the amp starts to squeal. I used a scope to check the grids and plates of each tube, starting at V1B, to look for any noise with no signal at the input of the TMB channel. I set the Gain and Master at maximum and used an 8 ohm dummy load on the speaker output. Everything was clean until I checked the plate of V2B (pin 1) where I found a noise signal of around 200 mVpp. V2B (pin 1) is the node that attaches to the tone stack and then the master volume. Then I checked the grid and plate on the phase inverter. There was a noise signal on the phase inverter plates of 2 Vpp. Also, I checked the B+ for the preamp and phase inverter for noise and they were clean. I included a picture of the pots and a file that shows all the voltage checks.
Any suggestions on what to do next would be greatly appreciated.

Thanks,
Mark

[Daviedawg]

From a distance it seems like an oscillation is making its presence felt at higher gain levels. This can happen despite great care in the build.
Have you chopsticked the lead dress to see if the hum/squeal changes when leads are moved around? If not start in the areas where you detected a noise signal.
If you need guidance on the procedure just ask. As always safety matters.

Dd

[JMPGuitars]

Could you post some large photos of your build overall? To confirm, you did separate the grounds per my ground scheme, correct?

I would tend to agree with Dd, it seems likely your lead dress may be causing your issue.

Thanks,
Josh

[MSVguitar]

Here are some full view pics of the chassis. I also included a pic of the scope trace from V2B pin 1. I'm using a 10X probe on the scope. It looks to me that there is an oscillation with a higher frequency noise signal. Josh, as for the ground scheme, I did implement your ground scheme. There are three ground points, one for the preamp close to the high gain input, one for the power amp and one for the AC Mains. On the preamp ground point I separately grounded each input jack, the normal channel pots, the TMB pots, phase inverter and the last filter capacitor which is mounted on the board. I also routed the OT ground so that it was as short as possible.
Dd, I will adjust the lead dress using a chop stick. I wanted to start that last night but ran out of time. Any suggestions on how to do that would be greatly appreciated.

Thank you,
Mark

[JMPGuitars]

On the preamp ground point I separately grounded each input jack, the normal channel pots, the TMB pots, phase inverter and the last filter capacitor which is mounted on the board.

All at 1 single ground point, correct? Ideally, all of this should be connected to the higher gain input jack ground, and that single ground wire running to the chassis, close to the jack.

That said, your lead dress looks sus (as them kids say). Your wiring looks like you tried to make the runs from the tube sockets prettier than they should be. You don't want your plates and grids running parallel, and you want to create distance too. Those running parallel will create a squeal.

[MSVguitar]

Yes, all at one ground point. My thought about separating the grounds and running to a single point was to eliminate ground currents from the TMB controls passing through the input jack, shorter path to ground. As for the tube sockets, I will redo the lead dress to create more space and if I have to cross over grids and plate leads I will cross at right angles.

[MSVguitar]

I revised the lead dress on the tube sockets as shown in the attached pictures. I can set the gain and master volume higher, but it still has a squeal when I approach max on the master with the gain at max. I checked V2B pin 1 with the gain and master at max and the noise is half, about 25 mVpp. So it definitely made an improvement. I also moved the leads around with a chop stick and didn't have any change.

Thanks again for your continued feedback,
Mark

[JMPGuitars]

It looks like you raised your plate wires if I'm not counting wrong. I do the opposite. I usually place the plates tight against the chassis, then raise the grids away. YMMV, but that's what works for me.

Here's an example of one of my builds:

_JMP3283.jpg

In mine it's red = plate, black = cathode, blue = grids.

[Bieworm]

It looks like you raised your plate wires if I'm not counting wrong. I do the opposite. I usually place the plates tight against the chassis, then raise the grids away. YMMV, but that's what works for me.

Here's an example of one of my builds:
_JMP3283.jpg


In mine it's red = plate, black = cathode, blue = grids.

lol.. I'm in the flying plate wire camp. Works for me.. but he should listen to you though

[JMPGuitars]

lol.. I'm in the flying plate wire camp. Works for me.. but he should listen to you though

Ha, floating any of them might work, but IME, the plates are a bigger PITA when floated. I've tried every order from chassis to air, and my best results were (bottom to top) plates, cathodes, grids. Previously I had cathodes, plates, grids, but those results weren't as consistent.

[MSVguitar]

lol.. I'm in the flying plate wire camp. Works for me.. but he should listen to you though

Ha, floating any of them might work, but IME, the plates are a bigger PITA when floated. I've tried every order from chassis to air, and my best results were (bottom to top) plates, cathodes, grids. Previously I had cathodes, plates, grids, but those results weren't as consistent.

I revised the lead dress to float the grids and put the plates and cathodes against the chassis. I also re-flowed the grounds and some of the grid and plate connections at the board and tube sockets. It is better but I still get hum and a high pitched squeal. I set the master at 10 and the gain at 8 and recorded a video while chopsticking the leads. https://youtu.be/-JeYGpllyS8
Maybe this is normal for a high gain amplifier/mojotone 18 watt TMB.

One of the 18 Watt TMB layouts in the download section (Richie's schematic) has a tweaker's Table that shows changing the master volume from a 1M pot to a 500K. This would decrease the signal going into the PI and provide a more 'usable' range on the master volume. Your thoughts on this and any other tweaks would be appreciated.

Thanks,

Mark