Twin Reverb 120Hz hum

[jada3909]

Hello,
I'm working on a late 70's twin reverb...with the master volume pull out knob. I replaced the screen resistors on the power tubes today. Now the amp seems to have excessive 120Hz hum as soon as you flip the standby switch. I changed the filter caps about 10 years ago and visually they look fine. I'm kind of down hearted because I thought the amp just needed the pots cleaned. Any Ideas on where to start? Thank you for reading this!!

[pdf64]

What anode or cathode current is each output valve idling at?

[jada3909]

Hello, I just received my Hoffman bias checker kit today. As soon as I assemble it I'll take a reading. Thank you for your respones!

[pdf64]

It may be useful to identify whether removing the 12AT7 valve in the LTP phase splitter V6 socket affects the hum.

[jada3909]

Hello again,
This is going to sound foolish but do you mean running the amp with V6 removed?
Thank you,

[pdf64]

Yes, that’s it.
No guitar signal will be able to pass, but it’s useful to have a clue whereabouts the hum is entering the signal path.

[jada3909]

Alright, today I build the bias checker...very nice! great instructions! I after fumbling around looking for batteries for my meter I measured around 35ma. I noticed on the back of the amp there is a small pot specifically for hum adjustment...I figured I'd give it a try and it DID reduce the hum! I noticed that some hum is present when you first turn on the amp but after 20 seconds or so it quiets down. I tried removing the V6 tube, (before I tweaked the hum adjust pot),  and that didn't seem to add to or diminish the hum. So, all in all a good day....however, now I have absolutely zero  reverb and tremelo! My son said the reverb was having problems but we were able to bring it back by wiggling a tube. Have a great night all!

[pdf64]

So is this the 100W or 135W version?
It’s noted above the speaker output jacks.

[jada3909]

Yes, Its funny you ask because I always wondered if it was 100W or 135W and the answer was staring me in the face but I never noticed! It's the 135W model. This makes sense because years ago I cranked it, hit a power chord and the roar out of those speakers almost knocked me over...

[jada3909]

Hello,
I had some time to work on the twin today  and I think I have the hum under control...Now it's the reverb! The vibrato/reverb channel amplifies but the reverb itself is totally down. I pulled out the tank and visually it looks fine with no broken wires. I measured the DC resistance of the input and output transducers of the tank and they seem fine as well. I took a small speaker and plugged it into the reverb output and talked into the speaker. It amplified my voice and I could hear it through the amp speakers...I'm not sure what to make of that...Any suggestions would be greatly appreciated! Have a great day!

[Willabe]

I took a small speaker and plugged it into the reverb output and talked into the speaker. It amplified my voice and I could hear it through the amp speakers...I'm not sure what to make of that...

The speaker was acting like a mic.

[PRR]

Put the wires back. Turn the reverb way up. Is there ANY extra hum/hiss when you do this? Kick the amp. Does it thunder?

Take the main power tubes out for a moment. Find a way to connect a speaker to the tank input wire, from the reverb transformer. Play a lick. Does it come out the temporary speaker? Small and thin but not sick or dead?

[jada3909]

Gentlemen,
Just got back from vacation! I was excited to see your responses!! Thank you. I didn't even empty the car and I started tearing into the amp...I removed the output tubes and connected a speaker to the tank input jack and YES!! I could hear the guitar through the speaker!! Does that mean my reverb driver tube and transformer are ok? Could it be the tank after all? Thank you for your help!


P.S> as a side note...I next connected the reverb tank input to the reverb input and connected the test speaker to the reverb tank output,,,Hoping to hear a little reverb through my test speaker...No joy I couldn't hear anything.
Thanks for reading this!

[PRR]

> speaker to the reverb tank output

The output of a tank is VERY weak. Weaker even than an e-guitar. You won't hear anything in the direct-connect speaker. That's why Fleming and AT&T gave us amplifiers.

[jada3909]

Ok, and thank you for the response!  Here's an update on today's progress... ....I think the no reverb and no tremolo problem are related because I can CRASH the reverb tank by jostling it and it comes through the amp speaker, I can see the optical sensor blinking for the tremolo, and I can adjust the speed of the blink with the tremolo speed control, but I don't hear either reverb or tremolo coming through the amp speaker... ....I am really at a loss here. Has anyone run into this kind of thing before?
Thank you!

[jada3909]

Good evening!...Here's another update...I purchased a new reverb tank and now I have reverb!!! My original tank seemed good but I guess it wasn't. Anyway, I was gassed!!!! The tremolo is still not working however, but as I said above the neon light blinks and the resistance on the variable resistance terminals of the optoisolator causes my analog meter to ping pong back and forth!!...Good news!!! I just ordered a new intensity and speed pot because I don't know what else to check???, any advice would be appreciated...and always, thanks for reading this!