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Hum in Ampeg Reverberocket, HELP!
Old 26th January 2016
  #1
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Hum in Ampeg Reverberocket, HELP!

OK, I have an early 90's Ampeg R12R Reverberocket Combo I just got off ebay. It had an issue where it would sound great, and then all of a sudden sound like crap. I found a wire jumper that was cold soldered and actually showed burning of the pcb from the heat of the bad connection. I fixed that and the amp sounded fabulous, as they do. Except it has too much hum. Just sitting there it hums to the point it bugs you unless you are playing.

It's 25 years old, so I figured time to recap, so I did. But the hum is still there. It is not affected by the clean channel Volume, but on the gain channel it can be made much lower by turning down the Master Volume. The clean Volume is ahead of the V2 tube stages and the Master Volume on the gain channel is after the V2 stages.

It seems like a bad filter cap on the preamp section which powers V1 and V2, but V1 is quiet. If the B+ on these tubes had too much ripple it should affect both V1 and V2 since they share the same voltage.

I have tried replacing the tube with the original tube, and the replacement tubes are all brand new as well. So I don't think it's a filament bleed over issue from a bad tube?

I used a scope and I can see the noise on the output of the V2 second stage tube, but the input looks clean. If I pull V2, the noise is much lower, at a level I could probably live with, and what I would consider to be normal for a tube amp.

I am kind of stuck.
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Old 26th January 2016
  #2
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temporarily lift the cathode bypass cap on V2 (C46 in the schematic) to see if it is a leaky cap. Also look at the wiring for the input jack. I had one once come to me after someone bought it off the bay and it was noisy like that and someone wired the input circuit different. Changed it back to stock and everything was fine.
Old 26th January 2016
  #3
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I replaced all those caps. C46 is on V1. The stage I am seeing the noise is V2 pin 7 looks quiet, but the output side of that tube R17 has the noise on it.

The input jacks are pcb mounted so can't be wrong.

Thanks for the reply.
Old 26th January 2016
  #4
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Pull out all the tubes except the output tubes. See if its quiet.
Then start putting them back in one at a time.
If its noisy with only the output tubes, it could be magnetic leakage from the power tranny to the output tranny.
You could take off the filament centre tap and try a 100 ohm pot across the filament lines, with wiper to ground.
Old 27th January 2016
  #5
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I plugged a shorting plug into the Return input and it was dead quiet. It's just this second half of the V2 tube?

I do have a pot for that. I just thought there must be something broken for it to be so loud? Perhaps it's because they run the center tap directly to GND?

I had the head version of this amp the R-50H, and it hummed a little, but nothing like this one.
Old 10th June 2019
  #6
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Ampeg Reverberocket R212R hum cancel

Hi CapnDenny1

I also have a Ampeg Reverbrocket R212R reissue from the 90's (EL34 version)
I also have the same issue

I want to cancel the hum I read all your messages, I looked at all your pictures but I don't understand everything (I am French :-/ )

Could you sum up what you did exactly to remove the hum? with details and pictures, it would be vrey great!
I would really appreciate

Youw wrote :
"Moving that heater wire next to the signal at the R14-R13 junction got rid of about 90% of my problem. I can hear the noise, but it's so low I can live with it."
"The biggest helps were the moving the heater wire, and the humdinger pot. The pot had to be a little off center for minimum hum. So the grounded center tap would not have been the minimum hum."

What/where is the heater wire? How to move it?
Where is the humdinger pot? How to modify it?

Thank you very very much

All the best,

Max
Old 3 weeks ago
  #7
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audiospecific's Avatar
 

I've been sitting here looking, and I think the guy had an older jet12 amp because according to the re-issue schematic there is no hum adjustment pot (aka humdinger).

If you have a scope, look across the heater secondary winding (probe on one side and ground clip on the other heater side, ac coupling) you should see a pretty sinusoidal wave. If this wave has a notch around center crossing or distorted in any way, the transformer is faulty. What it is the center tap has shorted slightly between its own secondary..


There is a way of temporarily patching this issue, but you still would need to change the power transformer eventually...
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