18 watt TMB squeal & hum
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- MSVguitar
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Re: 18 watt TMB squeal & hum
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: 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: 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
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: 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: 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
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- Bieworm
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Re: 18 watt TMB squeal & hum
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
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
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"THIS should be played at high volume..preferably in a residential area"
- JMPGuitars
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Re: 18 watt TMB squeal & hum
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.
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Re: 18 watt TMB squeal & hum
The Speaker Jacks are grounded, I didn't identify them in the pic.JMPGuitars wrote: ↑Wed 02/01/23 9:16 amI 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.
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Re: 18 watt TMB squeal & hum
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.Bieworm wrote: ↑Wed 02/01/23 2:55 amIMHO 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
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Re: 18 watt TMB squeal & hum
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.
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- MSVguitar
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Re: 18 watt TMB squeal & hum
Thanks Josh, I will make the change to the OT and Speaker Jacks.JMPGuitars wrote: ↑Wed 02/01/23 7:20 pm
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.
Mark
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