18 watt TMB squeal & hum

[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

[Bieworm]

Your video is private… can’t open it

[MSVguitar]

Your video is private… can’t open it

I just changed it to unlisted. Sorry about that.

[MSVguitar]

I am working thru the 'Amp Builder's Troubleshooting Guide' from the Trinity Amp support page. I've done the following:
1. Checked the resistance for all the grounds back to the chassis at each ground lug. The resistance for all is in the range of 0.2 to 0.4 ohms
2. The hum is present on both channels when the volume control is turned up (gain at 0 on gain channel). The hum is affected by the tone controls.
3. The plate current in the output tubes are within 0.5 mA. I used the voltage drop across the plate resistor and the OT resistance to calculate the current.
4. All electrolytic capacitors have a resistance to ground of at least 4.5 Meg
5. I have all signal leads routed away from any AC wires. See picture in previous post.
6. The 68K grid stopper for the input of the gain channel is mounted on the tube socket. I did not do that for the normal channel.
7. The shielded grid wires for the gain channel run under the board. The troubleshooting guide recommends running them on top.
8. Reversed the OT leads on pin 7 of the output tubes. No change in hum or squeal.
9. The heater wires (twisted pair) from the PT run parallel to the Mains (twisted pair) to the PT. The guide recommends keeping them separate.

I appreciate your feedback,

Mark

[JMPGuitars]

You're chasing other things instead of looking at your circuit clearly.

I don't like anything about that floating cap can. You have your PI, and it looks like part of your tone stack connected to it.

Take a look at my layout here: files/JMPGuitars_18_Watt_Tremolo_TMB_Layout.pdf

Your cap can is taking the place of the two 22uF filter caps I have shown in my layout, but it's doing it in one of the worst ways possible. You've got giant leads running across the board, from the preamp to over the power amp. You have preamp filters being grounded at the power amp. That's bad. Running a long ground wire from the cap can to the preamp isn't a good idea either, as that now becomes a large antenna.

I would kill that cap can, and add a couple radial filter caps properly placed. Compare the locations of my filter caps to your circuit, and show where you think they should go.

Thanks,
Josh

[MSVguitar]

You're chasing other things instead of looking at your circuit clearly.

I don't like anything about that floating cap can. You have your PI, and it looks like part of your tone stack connected to it.

Take a look at my layout here: files/JMPGuitars_18_Watt_Tremolo_TMB_Layout.pdf

Your cap can is taking the place of the two 22uF filter caps I have shown in my layout, but it's doing it in one of the worst ways possible. You've got giant leads running across the board, from the preamp to over the power amp. You have preamp filters being grounded at the power amp. That's bad. Running a long ground wire from the cap can to the preamp isn't a good idea either, as that now becomes a large antenna.

I would kill that cap can, and add a couple radial filter caps properly placed. Compare the locations of my filter caps to your circuit, and show where you think they should go.

Thanks,
Josh

I agree with you 100%. I didn't like the mojotone layout that was provided with this kit, it's a POS. I did the troubleshooting to make sure I didn't cause any of the problems.

So, to kill the cap can and add the radial caps to the board, I will have to move the existing components to make room. I would rather make a new board and start fresh with a new circuit and layout, one that is proven. I have been studying the Trinity 18 Watt TMB and I can reuse all of the other components that came with the kit.

FYI. I am building this for my nephew, he wanted a tube amp with the Marshall overdrive tone. We saw mojotone 18 Watt TMB kit and he ordered it. I'm not happy with the kit and the fact that I didn't do more research. I wish I would have found this forum first.

I will do what I need to do to make it right. No matter how old we get, there is always more to learn.

Thanks,

Mark

[JMPGuitars]

I agree with you 100%. I didn't like the mojotone layout that was provided with this kit, it's a POS. I did the troubleshooting to make sure I didn't cause any of the problems.

So, to kill the cap can and add the radial caps to the board, I will have to move the existing components to make room. I would rather make a new board and start fresh with a new circuit and layout, one that is proven. I have been studying the Trinity 18 Watt TMB and I can reuse all of the other components that came with the kit.

FYI. I am building this for my nephew, he wanted a tube amp with the Marshall overdrive tone. We saw mojotone 18 Watt TMB kit and he ordered it. I'm not happy with the kit and the fact that I didn't do more research. I wish I would have found this forum first.

I will do what I need to do to make it right. No matter how old we get, there is always more to learn.

Thanks,

Mark

Yeah, I hate their kits, I might have mentioned that.

Before you go that route though, you can try adding radial filter caps.

Experiment 1:
Cut the red wire on both ends coming from the cap can.

Place a radial cap here:

radial1.jpg

These are caps I use: https://www.mouser.com/ProductDetail/647-URS2G220MHD1TN

You might even have room for that to lay horizontally.

Thanks,
Josh

[MSVguitar]

I see what you are saying, here is what I think I would have to do for Experiment 1:

Experiment 1.JPG

To move both capacitors to the board, here is what I think I would have to do:

Experiment 2.JPG

So my thoughts on this is I will have to get some dimensions for the caps and see if I can make them fit. Order the parts and install them. There is still a chance that I will have issues with the circuit.

To go the route of building a new board for a Trinity 18 Watt TMB, I already made a parts list. I have material to make the board, I have all of the electrolytic capacitors and some of the other caps. Just need to order a few capacitors and resistors and make a template for the board layout. I have heard good things about Trinity amps. I feel like cutting my losses and move on from the mojotone kit circuit and layout.

I really appreciate your feedback and time to help me out.

Thank you,

Mark

[MSVguitar]

So, I thought about how to try experiment 1 with parts that I have available. I came up with a perf board set up to try it. Not pretty but I was able to check it out.

Experiment 1 actual.JPG

No change. I am ready to move on to the new board! I'll post pictures and results as I make progress.

Thanks,

Mark

[MSVguitar]

I removed the mojotone board and started a new build using the Trinity 18 Watt TMB circuit and layout. I was able to reuse all of the mojotone parts less one cap can.
This is the chassis build showing the ground scheme. There is one difference in this build compared to the Trinity layout. The OT and speaker grounds are located at the PT ground point. The layout that I used shows the OT transformer grounded at the speaker jack using a switchcraft jack.

Chassis Grounds New Build.JPG

This is the new circuit board, I built the turret board last night and placed all of the components today.

Turret Board New Build.JPG

This is a much cleaner build! Looking forward to finishing it up.
Also, this looks like the same circuit as the 18watt TMB Designed by Richie that's in the download section with a few different component values.

I Look forward to your feedback,

Thanks,
Mark

[JMPGuitars]

I removed the mojotone board and started a new build using the Trinity 18 Watt TMB circuit and layout. I was able to reuse all of the mojotone parts less one cap can.
This is the chassis build showing the ground scheme. There is one difference in this build compared to the Trinity layout. The OT and speaker grounds are located at the PT ground point. The layout that I used shows the OT transformer grounded at the speaker jack using a switchcraft jack.
Chassis Grounds New Build.JPG

This is the new circuit board, I built the turret board last night and placed all of the components today.
Turret Board New Build.JPG

This is a much cleaner build! Looking forward to finishing it up.
Also, this looks like the same circuit as the 18watt TMB Designed by Richie that's in the download section with a few different component values.

I Look forward to your feedback,

Thanks,
Mark

Excellent choices. Not following their speaker jack was also a great choice. Output speaker jacks need to be chassis isolated to reduce noise issues. Also, relying on a jack for grounding like that is bad because the jack could get loose over time, resulting in the ground being lifted.

Lookin' good so far!