Twin Reverb (Pro Tube) Intermittent Hum

[fdesalvo]

Hi, gang.

I’ve got an incoming repair that I want to get ahead of, so I thought I’d tap into the ether here.

Customer has a Pro Tube Twin that’s going to land on my bench a second time. I originally fixed a reverb feedback issue, but it’s developed a problem in another area.

After warming up about 30 minutes or so, the amp will sporadically emit a low freq hum. Customer called me from a gig and I could hear the noise. I asked him to observe the power tubes while the noise was occurring and he said he saw the tube status indicator on one pair change from yellow back to green as the noise ceased. It happened again on the call; hum > green to yellow fault indicator > hum ends > yellow to green indicator.

Have any of you mutants seen this before? The obvious first place to look would be the tubes, which were recently replaced (kicking off this issue), but the intermittent nature of the problem is puzzling.

Cheers and happy new year!

frank

[Enzo]

My wife is in a bad mood, what do you think she will argue about?

Sorry, but from a brief description of tube failure, it is hard to predict what to look for. Has anyone seen tube failure show up along with both noise and bias indicators? I am sure so. Has anyone seen it caused by other than tube failure? Probably yes.

Take each power tube and hold it next to your ear. Rap the side of the tube with your knuckle. hear any sign of looseness inside it?

Any tube can fail at any time, new is no guarantee of good. Try a set of tubes you know works. I almost think the power tubes in my shop Twin Reverb spent more time in other amps for that purpose.

Your yellow/green is a clue to which tube is failing. If nothing else, swap tube positions and see if the problem travels with the tube or stays in the same socket.

[Justin Thomas]

Along with thetubes I'd check the bias supply thoroughly.

Justin

[g1]

I'm wondering if the bias lights may just be registering the 'signal' of the hum. (ie. if you play a note I don't think the light stays green)
I'm sure it could be a power tube fault, but I don't think the lights can be thought to be conclusively pointing to that yet.

edit: oops, I may be mistaken. Are these lights fault lights or are they bias indicatiors?

[fdesalvo]

Looking at the schem, the fault detection circuit is connected to the screens. Interesting amp. Can’t wait to dig in.

[nosaj]

Looking at the schem, the fault detection circuit is connected to the screens. Interesting amp. Can’t wait to dig in.

Lazy screens when heated touch another element giving a fault detection possibly.
nosaj

[g1]

Can you post the schematic Frank?
Got any new tune-age in the works?

[fdesalvo]

Thanks for the great breadcrumbs, gang. I’ll make sure to post up my findings once I have the beast on the table.

G1: with two kids in diapers wreaking havoc, there’s no time for music! The last thing I did before #2 came along was work with one of Vai’s lead engineers. Dude remixed and mastered one of my instrumentals and it came out great. He did in 6 hours what I couldn’t in hundreds lol.

[g1]

G1: with two kids in diapers wreaking havoc, there’s no time for music! The last thing I did before #2 came along was work with one of Vai’s lead engineers. Dude remixed and mastered one of my instrumentals and it came out great. He did in 6 hours what I couldn’t in hundreds lol.

I remembered that one which is why I asked.
https://music-electronics-forum.com/...l=1#post508030

[Chuck H]

I remembered that one which is why I asked.
https://music-electronics-forum.com/...l=1#post508030

I missed that first time around. Sounds good

[g1]

schematic:

[fdesalvo]

Thanks G1 & Chuck. Yah the consistent sleep dep really takes it out of a guy. Feel like my musical past never happened. Like it was all a dream lol.