Low Voltages on 78 Twin Reverb 133 (UL)

[caesparza]

Replaced a couple of 12AX7 from my Princeton Reverb which is working properly. When turned on the amp made a loud noise (kind of white noise). Again got microphonic tubes on the same spots specially V2 V4.

Then I plugged the Princeton to check for the same tubes and it seems they are microphonic. Everything points to the tubes I guess. The funny thing is that the princeton seems to work ok with that tubes, no issues of any kind.

[Enzo]

The input stage tube on amps like this will always be somewhat microphonic.

[Chuck H]

Maybe there are different problems popping up because there's grime and oxide in the tube socket pin holes. On older amps these should be cleaned as part of the regular maintenance package for older amps. Same goes for jacks.

If you ground the signal in the preamp anywhere and the hum is reduced it would indicate that the source of that amount of hum (possibly a noisy tube) was behind that grounded point and therefor not in the power amp.

[caesparza]

I cleaned the sockets and tightened the power amp ones.

But, if I ground the signal, the amp is going to go muted, Am I right?

[Chuck H]

Right. And that's the point. If it only hums with a guitar plugged in then the hum is coming from the guitar and not the amp. The idea is that if the AMP is humming then it'll be doing it whether there's a guitar plugged in or not. So muted isn't an issue. When you mute a point ahead of a hum source the hum is muted and (BING BING BING BING) you've isolated the hum to a specific area of the circuit. And now it will be easier to troubleshoot.

[caesparza]

Today I decided to compare the Twin 78 135W with the 77 100W.

The 77 is dead quiet, no hum of any kind compared to the 78. I decided to swap the preamp tubes from the 77 to the 78, all the little strange noises that came from time to time disappeared, but the hum is still present. Then I replaced the power tubes, same situation. The amplifier hums even with nothing connected to the input.

Chuck, I'll proceed with the test you mentioned to try to isolate the problem, I've got a couple of guitars for setup so I'll come back once I got the results.

[caesparza]

Something got me thinking about the removal of the master volume. Basically I disconnected the cable that goes to the junction of the two 220K resistors and the one that goes to the .001 cap and put a jumper between the two points. Then I used shrink tubing to isolate the removed cables. Could they be generating the hum?. Should I remove completely those cables?

I can not upload the images, don't know what is wrong with the upload process.

[Chuck H]

If you disconnected the board ends of those leads and just left the cables connected to the pot then those leads shouldn't cause any trouble. They're out of the signal path. If you disconnected them from the pot and left them hanging off the board then they could pick up noise and should be removed.

Since you've eliminated tubes as the cause of the hum you can actually REMOVE tubes to isolate the hum if it's in the preamp. Since V1 is already out, pull V2. Is the hum still there? If yea pull V3, etc. Keep going until the hum stops. If the hum doesn't stop before you get to the power tubes then the hum is in the power amp.

[caesparza]

Hi Chuck,

Double checked the MV removal, I left a shield that goes to one of the vipers of the volume pot to the board, removed as well to avoid any problems, hum still present. Then proceeded as requested, removing one tube at a time and the hum went away after removing V4, that is the microphonic position.

Does that means that the hum is generated before V4? Right now I don't have the reverb tank connected.

[Chuck H]

Well the good news is that you've isolated the hum to between V3 and V4. That means there's nothing more to gain adjusting the bias balance and hum pots or swapping power tubes. The hum is in the preamp behind V4, but after the previous tubes and their related circuits.

It may be significant that you have the reverb tank disconnected IF you have the reverb knob turned up at all. There may be more to it also but I'd need to be looking into the amp myself to see it. You should at least try plugging in the reverb pan in case the ground scheme on the jacks is affecting a shield or something with the tank out.

[caesparza]

Connected the reverb tank. Hum still there up to the 4th tube. The hums goes up and down with the reverb level control.

So I need to check the hum issue on the circuit from the input to the 4th tube output. I got an scope that have never used, could it be handy for diagnosing the hum?

Any pointers on where to look to address the issue?

Thanks a lot.

[Chuck H]

Does it hum with the reverb level at zero?

[caesparza]

Yes.

[Chuck H]

First try a different tube in V4. I know you're already using tubes from a known quiet amp, but do it anyway just to be sure. Otherwise...

In the schematic I'm looking at there's a switch on the master volume pot that also makes changes to the reverb circuit. I might suspect a ground loop in the design because of how it's arranged. Does your amp have the switch?

[Shawnobi]

Not sure if I'm pointing out the obvious. But that's a UL Twin. There should be a pot accessible (on the back?) to tweak the hum balance. There is also Output Tube Matching.

These should both be adjusted when installing fresh power tubes.

1 - turn the Master Volume to 0 and set the Output Tube Matching for minimum hum (this is balancing the Push and Pull idle currents in the output transformer and therefore cancelling any residual power supply ripple).

2 - turn Master Volume to 10 and set the Hum Balance for minimum hum