18 watt TMB squeal & hum

[Bieworm]

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[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!

[MSVguitar]

The amp is done! The first time I powered it up I still had some intermittent squeal and a fairly loud hum with the gain and master volume at 10. Better than the mojotone build, but not what I wanted. I made several 'tests' adjusting the lead dress, shortening tube socket lead wires where possible, tried a 5751 tube in V1....

The final version does not squeal and the hum is reduced. Here are the things that I did that I think helped:
1. Replaced the mojotone cap can with a F&T cap can (32uF/32uF @ 500VDC). The specs for the F&T show a lower leakage current at 500VDC than the mojotone. I checked the leakage current with my old Sprague T-05 and confirmed that it was lower.
2. Moved the EL84 8.2K grid resistors from the circuit board to the tube sockets.
3. Moved the TMB input and gain pot ground connections from the phase inverter section to the preamp section located at the cathode resistor ground for the first triode.
4. Changed my soldering iron tip from 700 deg to 800 deg and re-flowed all the solder joints.
5. Replaced V3 (PI) EH 12AX7 with a Sovtek 12AX7LPS. I mentioned above that I tried a 5751 in V1 but I did not keep it there. V1, V2 and V3 are 12AX7's.

The TMB channel sounds great with the master volume and gain set at 5. There is just a little hiss and no hum at these settings. When the master volume and gain are set at 10, there is a lot of hiss but no squeal and and a little hum. I checked the output across an 8 ohm dummy load and it's 14 watts RMS with the MV and Gain set at 5, which happens to be at the point where the output starts to clip. It's pretty loud. I can't envision myself or my nephew using this amp with the MV and Gain at 10.

Here is a picture of the completed chassis:

Chassis complete 31Jan23.JPG

Some of the voltages are low but I think it's related to the TMB cathode follower bias which is -0.6 VDC grid to cathode. The cathode follower bias was + 0.9VDC on the initial test.
Here is the voltage chart:

Trinity TMB TEST VOLTAGES COMPLETED BUILD.pdf

I'm very happy with the amp, the Normal channel and TMB channel sound great. I will never know if I could have gotten to this point with the mojotone design. I didn't like the layout of the mojotone kit (see my previous pics). The build docs had errors. On the other hand, they make great cabinets (I have 2 other amps with mojotone cabinets). I have bought other parts from them and will continue to do so.

As always, I appreciate your feedback.

Mark

[Bieworm]

IMHO any tube amp will hiss when volume and gain controls are at 100%. One has to ask himself how often an amp will be played at full volume, and when one does, is that hiss a problem in that particular situation?
Do realise that the hiss is often a consequence/side effect of the really great tones you can get from the amp… tones that wouldn’t be there in a completely quiet and sterile amp.
You ‘could’ eliminate a lot of hiss when you add a NFB loop, but I doubt you will keep the tone sucking amp taming loop

[JMPGuitars]

I don't see a ground wire on your output jacks. It's not 100% necessary on an 18W without NFB, but it's still usually a good idea to ground the output jacks. You know, to avoid damaging the OT or yourself.

[MSVguitar]

I don't see a ground wire on your output jacks. It's not 100% necessary on an 18W without NFB, but it's still usually a good idea to ground the output jacks. You know, to avoid damaging the OT or yourself.

The Speaker Jacks are grounded, I didn't identify them in the pic.

Chassis complete rev1 31Jan23.JPG

[MSVguitar]

IMHO any tube amp will hiss when volume and gain controls are at 100%. One has to ask himself how often an amp will be played at full volume, and when one does, is that hiss a problem in that particular situation?
Do realise that the hiss is often a consequence/side effect of the really great tones you can get from the amp… tones that wouldn’t be there in a completely quiet and sterile amp.
You ‘could’ eliminate a lot of hiss when you add a NFB loop, but I doubt you will keep the tone sucking amp taming loop

I agree. I was just stating a fact, didn't want someone thinking that this was a completely quiet amp in all settings. This amp sounds great. It has many usable settings and the hiss not an issue. I wouldn't change anything.

[JMPGuitars]

The Speaker Jacks are grounded, I didn't identify them in the pic.

Normal practice is to connect the OT negative lead directly to the output jack, and then run a ground wire back to the star ground. The black lead from the OT is not ground (it's the negative lead going to the speaker negative terminal), but the jacks should be grounded.

[MSVguitar]

Normal practice is to connect the OT negative lead directly to the output jack, and then run a ground wire back to the star ground. The black lead from the OT is not ground (it's the negative lead going to the speaker negative terminal), but the jacks should be grounded.

Thanks Josh, I will make the change to the OT and Speaker Jacks.
Mark