1972 Twin Reverb Hum

[Chuck H]

It's acoustic feedback then. Normally with this situation the sound rises in intensity if left to continue. Sometimes though, the sound just rises softly to a fixed, lower volume.

Try removing the pan from the amp again. DO NOT SET THE PAN ON TOP OF THE CABINET. Try to locate it away from the back of the amp. If this still doesn't stop the feedback you have an especially resonant frequency issue of some kind. I've managed to fix this by EQing the recovery circuit. Another fix would be to damp one spring at a time, locate the spring that's resonating and change the length of that spring. I don't recommend this because there's a very real risk of ruining the pan (but desperation will drive you to all sorts of ideas).

Other things to consider. If you're using a larger than normal cap value to couple the reverb driver, change to the normal 500pf value. If you're using the large, three spring pans know that they are almost impossible to silence in a 2x12 cabinet with an amp over 40W. Change to a short pan or a two spring pan.

Wait!.. I'm a little puzzled now about something. You tried the pan from your other amp and it had the same problem?!? Normally all reverb pans are created differently enough that this shouldn't happen. At the very least the frequency should been obviously different. I think you need to try some of the above experiments again and pay extra close attention to what might actually be happening with the consideration that it's acoustic feedback.

[manolo]

http://music-electronics-forum.com/a...everbab763.gif

the B+ of power supply goes to reverb transformer and to tremolo section
i fed the reverb transformer from C (after the 1k/1w) and no more hum!but little reverb..
can anyone explain this? Maybe the new diodes (1n4007)?

oh and the hum stops when i put a rag on the springs to prevent them from resonating as Enzo said....

[52 Bill]

the B+ of power supply goes to reverb transformer and to tremolo section
i fed the reverb transformer from C (after the 1k/1w) and no more hum!but little reverb..
can anyone explain this? Maybe the new diodes (1n4007)?

oh and the hum stops when i put a rag on the springs to prevent them from resonating as Enzo said....

If you reduce the voltage to the driver transformer, you will reduce the signal that is driving the reverb tank. So you will have less reverb and less "hum".

If the "hum" goes away when you cover the reverb tank, then the problem is a mechanical one and not something wrong with the circuit. Enzo asked about how the tank was mounted. So how is it mounted?

[manolo]

52 Bill, the hum goes away when i put a rag on the springs not the tank.The springs self-vibrating-resonating produce the hum .
Side by side have a Super reverb which sounds ok with the same tank.

[52 Bill]

52 Bill, the hum goes away when i put a rag on the springs not the tank.The springs self-vibrating-resonating produce the hum .
Side by side have a Super reverb which sounds ok with the same tank.

Oh, so the amp is producing a 400hz signal that is being sent into the reverb tank and then is being amplified by the recovery circuit.

Can you find this 400hz signal anywhere else in the preamp, like at the input of the reverb driver?

[Enzo]

And the screws are good and loose?

[manolo]

Screws are good and loose..

[g1]

In post #5, Tom asked you to connect the twin to the super's reverb cables leaving the super tank in the super cab.
Did you do this? Swapping the tanks is not good enough, it was to test the tanks location.
You mentioned pulling the tank out of the cab, but try what Tom had suggested with the super cab.
If it still howls, then it is likely a 400hz oscillation going into the tank like 52Bill said, rather than tank feedback.

[Chuck H]

Indeed! If the cabinet is resonant at "400Hz" then it vibrates the reverb at 400Hz.

But the OP reports that the problem persists with the tank removed from the amp!?!

Manolo, it's not the diodes. (why are the OP's in the threads I'm posting on suspecting diodes for no apparent reason!?!)

Manolo needs to perform all the tests again, as described and suggested and carefully observe what's happening and if there are any changes.

[manolo]

Thanks guys
I made all the above tests and still the same
The amp is producing a 400hz signal that is being sent into the reverb tank and then is being amplified by the recovery circuit as 52 Bill said
A 400hz oscillation going into the tank as g-one said
The question is where this frequency came from...
The same hum appears when master volume turned to 10 and then the reverb circuit amplified this...

[Chuck H]

400Hz isn't produced spontaneously anywhere is the amp and I can't think of any failure mode that could cause it other than microphonic tubes, which you have swapped out for known working tubes.

Please do this, Unplug the driver tube and unplug the cable from the input side of the pan but leave the output of the pan plugged into the recovery circuit. Does the hum stop?

[Enzo]

Yes^^^

I am having a hard time wrapping my head around the amp having ability to generate a 400Hz signal anywhere other than reversed plate wires.

If it sent 400Hz into the springs, then it would come out the other end and also could be dampened by muting the springs, but in that case, the drive signal should still be there at the pan input jack for all with a scope to see.

[manolo]

Please do this, Unplug the driver tube and unplug the cable from the input side of the pan but leave the output of the pan plugged into the recovery circuit. Does the hum stop?

yes it stop

[Chuck H]

Ok... Now plug in the input side of the pan, but leave the driver tube out. What happens?

[manolo]

Now plug in the input side of the pan, but leave the driver tube out. What happens?

nice quiet..