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I have an 18 watt TMB that I built several months ago and the amp has always been very quiet until a few days ago. I am getting a pretty serious hum from the amp. I noticed that when I touched the strings, saddle, or anything metal on my guitar the hum would go away. I thought it may be the guitar so I tried another guitar and the same thing happened. I popped the hood on the amp and started looking for the culprit. I made sure all the grounds, switches, and knobs were secure. I swapped power and preamp tubes. I then started checking solder connections. I noticed that when I touched my stick to the lug where the 0.1 UF cap is connected to pin 7 on V3 of the preamp socket, the sound changed slightly. I tapped the 0.1 UF cap and then the .022 uF cap that is connected to pin 2 on V3 of the preamp socket with my stick and I could hear the tapping coming through the speakers from both caps. I re-flowed the solder on those connections and still hear the hum. I then used my voltmeter to check the current with all the tubes installed. All currents are accurate except pin 7 on V3 of the preamp. I am getting a reading of 54 VDC where I should be getting 62VDC(I think?) Am I on the right track? Could the two caps have gone bad? If anyone has any suggestions please help!!

Thanks,

Clay

If you haven't already done so, I would definitely try a different cable, and see if the hum continues. Have you changed anything else in your signal chain? New pedal or new power supply, perhaps? Moved your equipment to a new location? Brought some new piece of electronic equipment in proximity to your amp? The chances of two caps going bad simultaneously are very slight, so I would look for what else might have changed. Might also be a good idea to clean your input jacks with a spritz or two of DeoxIT or similar contact cleaner.

Yes, I have changed cables. I also tried all 4 input jacks, the normal and TMB channel. I noticed the hum in the studio and then brought the amp home hoping that there was a gremlin in the studio. But to no avail the hum remains. I have not gigged since November, so I know that it is not from movement. Really hoping for something simple here. Thanks for the input though.

Clay

If it doesn't happen with the cables unplugged from the amp, it is not the amp. Do that test & let us know.

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Stephen
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It still hums without the guitar. I rerouted some of the wires on the power tube sockets and output transformer yesterday. I also used a power conditioner. It helped a lot, but there is still some hum in the amp. The amp is quieter, but not like it was when I first built it.

Clay

I think I fixed the problem tonight. During rehearsal, the amp dropped in volume dramatically. I thought to myself this has to be a tube issue. I had my sound man bring his tube checker over and we checked all the tubes. Power tubes were fine, but ALL 3 Preamp tubes were bad!!! WTF So I put 3 more 12AX7s in from another amp and PRESTO!!! No hum, buzz, or any of the like. Any ideas what would have caused all 3 to go bad? The tubes were Mullard 12AX7s. All were purchased new and installed at the same time.

Clay