trinityamps.com

I have an 18 Watt TMB that has been very well-behaved for six years. Recently it developed an intermittent hum that today became more of a chronic buzz.
I have isolated it as in the amp, and in the TMB channel only. The "normal" channel works fine and is dead quiet in terms of hum. Actually, even yesterday, the whole amp played for about an hour dead quiet, and then today, the hum became chronic. The volume knob of the TMB channel controls the hum. If it is zeroed, no hum. The hum is there with nothing plugged in, so this is entirely independent of guitars, cables, etc.

I swapped out the V2 tube, and for the heck of it, V1 & V3, each/ all with no effect on the hum. The hum is frankly a loud buzz of the sort that un-grounded turntables have, so a lost ground is my first guess. (I note that I posted a similar problem shortly after the 2013 build, and the suggestion was to change V2--and that "fixed" it--it seems possible now that I moved the wire in investigating that problem, then, and improved, but did not fix, the issue--or maybe V2 really was bad). Suggestions as to how to track this down, beyond my short list below, or any guesses as to a likely culprit for this buzz?

I am about to go in and I will chopstick-test for loose connections. Will test V2 readings and proceed to re-soldering anything suspect if those readings are correct.
---
update:
I fixed it after finding a culprit wire with a "chopstick" test: it was a wire coming off of V1, actually. It is the wire with braided shielding, and the weird connection seemed to be on the jack side--tapping on the wire on the jack side produced the symptom. I re-soldered all connections relating to this wire and the TMB side is clean again.

Yes, it sounds like bad ground or a loose connection in the TMB channel someplace between the input jacks and the volume control. Probably a bad solder joint that finally has worked its way loose. You are on the right track. Check all the connections and resolder anything that is the least bit questionable. Cold solder joints don't always look bad to the eye.

update:
I fixed it after finding a culprit wire with a "chopstick" test: it was a wire coming off of V1, actually. It is the pre-amp ground wire with braided shielding, and the weird connection seemed to be (as you suggested) on the jack side--tapping on the wire on the jack side produced the symptom. I re-soldered all connections relating to this wire and the TMB side is clean again. Funny that it didn't affect the normal channel. Does the "normal" channel have a different ground?

Glad you got it fixed.

According to the TMB layout drawing I'm looking at, the grounds for the Normal and TMB input jacks connect to different points on the board. So it's quite possible that one channel would work and the other would hum like crazy.

The braiding on the input wires carries no signal. It's only there to shield the inner signal wires until they get to the first preamp tube. That's why the braid is only connected to ground at one end. If it was grounded at both ends it could cause a ground loop and induce hum rather than prevent it. The actual ground connection for the jacks is a solid wire that runs from the jacks to the circuit board.

Your problem was most likely just a bad connection at one of the ground terminals on the input jacks.

Good work finding the issue. When a amp has worked well for all the years, solder joints and tubes are are always suspect.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps