1972 Twin Reverb Hum

[manolo]

1972 Twin Reverb master volume NO push pull!

New TAD tubes (all) , new electr caps ,new diodes and blackface convertion (left the master volume)

I get loud humm (400Hz) when reverb pot on 10.

New reverb pan+cable . new reverb jacks

450v and each tube runs with 23mA

Filaments( 100+100ohm) 3vac and 3.4vac to ground

When v3 (12at7) is out then no hum

What else should i look for?

Thanks

[g1]

How does the reverb sound when set around "4" ? Is there proper reverb sound? Much hum?
Most people do not set them much higher than that, they all hum at 10, the only question is how much is normal.

[manolo]

The reverb sound is ok but the hum is loud enough
My 64 super reverb is dead quiet on reverb on 10!
I play surf..

[g1]

Ok, so it's more than the typical hum, and re-reading, I see you mentioned 400Hz. If that is the case, I wouldn't call it hum, more like a steady tone.
Two common causes are a microphonic tube or the tank itself feeding back. Try another tube in V3 position (or swap it with the phase splitter tube).
With the reverb running at that high a setting, the tank location may be a problem. Try removing it from the cab and away from the speaker magnets to see if it makes a difference. Does it have the stock speakers?

[Tom Phillips]

...My 64 super reverb is dead quiet on reverb on 10! ...

Since you have two amps try this...
Put the TR and the SR back to back and connect the SR tank to the TR. Just swap the cables at the chassis. The SR is left off and unplugged, of course, and the TR tank wires are just left hanging. There will be some slack so you can offset the cabinets and move them around if there is still hum. You want to keep the end of the SR tank under the SR preamp away from the power transformer end of the TR. This probably won't matter much because of the tall cabinets but that's the general rule. If the hum is still there then you know it is not caused by the tank or cables. If the hum is still there then swap V3 & V4 from the SR into the TR. Let us know what happens and we will go from there.
Tom

[manolo]

Ok
I swapped v3 & v4 & reverb tank from my super reverb(which is sounding great) and again the same hum..

[Tom Phillips]

Ok
I swapped v3 & v4 & reverb tank from my super reverb(which is sounding great) and again the same hum..

Bummer. But at least a few things have been ruled out. It's time to look inside the chassis and troubleshoot for bad components, loose ground connections, misplaced / broken wires etc. Your first post mentioned "new reverb jacks." It is unusual that the jacks would need replacement. What's up with that? Perhaps there was a wiring mistake introduced or a bad ground connection. Note that you can compare the wiring dress of the two amps. The 64SR will be neater than the 72TR but the connections are very similar.

[g1]

New TAD tubes (all) , new electr caps ,new diodes and blackface convertion (left the master volume)

Was the reverb hum there before all these parts (and reverb jacks) were replaced?

[manolo]

Tom, i changed the rev jacks because of the hum
g-one, not sure about this.
This amp was dead because of cold solder joints on all of the rectf diodes.(one leg of the bias diode was in the air!)
Thanks guys..

[Tom Phillips]

...This amp was dead because of cold solder joints on all of the rectf diodes.(one leg of the bias diode was in the air!) ...

Work like that indicates that an inexperienced person tried to service the amp. This makes everything suspect I suggest a lot of careful checking of all the wiring connections and lead dress. Melted notches in the wiring insulation will be one indication of areas where a hack messed with the amp.

...i changed the rev jacks because of the hum...

Do your best to resist the urge to change things unless the troubleshooting points to a specific problem with the part.

Cheers,
Tom

[manolo]

this thing is driving me crazy....
tested with another reverb pot-with the reverb transformer of my SR-change cathode res on v3 section-new tank-new tubes-new caps
and still have this f*&^% reverb hummm
im thinking of to replace the filament wiring from my SR (preamp section) to this....dont know what else...

[g1]

Considering this is a silver to blackface conversion, it is quite possible you have a lead dress issue. If that's the case you can replace every component in the amp and the hum will still be there.
Are the wires routed the same as the super?

[Chuck H]

Know this, it's probably not a bad component. Replacing anymore parts isn't likely to fix the problem. And if it's 400Hz it's not filament hum.

Let's try to isolate the problem. Unplug the reverb cables from the amp and clip or solder a 2.2k resistor across the reverb return jack +/- tabs. Now turn the reverb knob to ten (all other knobs except the reverb and master on 0). Does it still hum? If yes then the problem is in the return circuit and NOT any of the reverb parts.

400Hz never sounded like a hum to me. How did you measure this?

[Enzo]

And if we did this, I didn't see it:

Someone suggested you pull the pan out of the cab and get it away from the amp speakers. Does it act different? That will verify it is acoustic.

Do ANY of the controls, other than the reverb knob, affect this noise in ANY way?

400Hz is feedback,not hum, as was mentioned above. Especially when you remove the drive tube and it stops. That means the pan is being driven to this.

It is VERY important that reverb pans float. ANy chance you screwed the pan down tight to the floor? Pans should have the four soft rubber grommets under them as a cushion, and the screws through those ONLY tight enough to prevent the pan from moving around the bottom. It should never be screwed down solid.

Since you "blackfaced" it, that means you made changes. You might have swapped places on two parts on the board. Lead dress was mentioned and is important.

I completely agree with CHuck, this is not likely a bad part. Reverb feedback almost never is.

The reverb is usually in a vinyl reverb bag to help isolate it from sound energy. Is yours? If it is in a bag, remove the bag for now. Make the amp do it, then place your fingers on the top of the pan to stop any vibration. ANy result? Flip the pan on its back to expose the innards, any change? Now ball up a rag or something and put that on the springs to prevent them from resonating or vibrating. Stop the noise?

[manolo]

Chuck , With a 2.2k the hum is gone,only some hiss

measure 400hz with my fluke meter hooked to the speakers

Enzo , pull the pan out of the cab and get it away from the amp speakers does not help

Do ANY of the controls, other than the reverb knob, affect this noise in ANY way?
all pots to 0 = silent
only master vol to 10 = some hum
master + reverb on 10 = more hum


ball up a rag or something and put that on the springs to prevent them from resonating or vibrating. Stop the noise?

Yeap!