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Reverb Feedback Issues SOLVED
#1
I've been having problem with new build. It's essentially a Deluxe Reverb Combo but with some added decoupling and improved grounding. The issue I'm having is with the reverb tank.  Outside of the combo the reverb is quiet and operates as it should. When placed inside the combo the reverb starts to feedback building into loud howling.  Moving the tank around, flipping is orientation,  and padding the reverb bag really hasn't helped at all. I've tried a few different tanks and am at my wits end about what I'm doing wrong.  Huh  Any ideas?  Thanks for everyone's help!
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#2
Hi makinrose

We are assuming that with the reverb control at zero there is no howl? How about with the preamp volume down? Circuit-wise, this is like a DR reverb channel only, no normal channel. Hopefully each cathode has its own Rk and Ck as that is one step to true improvement of grounding.

It is nonintuitive, but you might try a rubber band on the recovery tube.Leave it in place until you resolve the problem.

You might also want to go backwards through your ground improvements to see if they really are.
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#3
(03-12-2019, 01:26 AM)K O\Connor Wrote: Hi makinrose

We are assuming that with the reverb control at zero there is no howl? How about with the preamp volume down? Circuit-wise, this is like a DR reverb channel only, no normal channel. Hopefully each cathode has its own Rk and Ck as that is one step to true improvement of grounding.

It is nonintuitive, but you might try a rubber band on the recovery tube.Leave it in place until you resolve the problem.

You might also want to go backwards through your ground improvements to see if they really are.

Thanks for the reply. Correct---it's basically a Deluxe Reverb with just the Reverb/Vibrato Channel.  Correct a zero there no howl.  The feedback problem happens regardless of the pre-amp volume---in fact I can remove that tube and still get it.  The problem appears to be in the recovery stage for the verb since I can remove the 12AT7 and still generate feedback.  

The filtering is a compromise since there was not enough room for a filter cap for each triode. Instead I've got a filter cap for every two triodes in the pre-amp, a cap for the reverb. a cap for the tremolo, a cap for the driver and caps for the Va and Vs. It's not a perfect setup but I was hoping it would be better than stock. Going backward through is a good idea.
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#4
Hi makinrose

Modern radial-lead caps take soooo little space how can there not be room for full decoupling? A typical 22uF-450V is 16mm diameter, about 0.7". I would revisit that for sure.
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#5
Perhaps that should be revisited in the future but right now the reverb issue is most pressing to solve and understand.
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#6
Hi

I don't know if this makes any difference but some fenders have a 220pF cap right across the input to the verb return.

What kind of wood is the cab made from? Maybe the cabinet needs damping.
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#7
(03-12-2019, 04:06 PM)nauta Wrote: Hi

I don't know if this makes any difference but some fenders have a 220pF cap right across the input to the verb return.

What kind of wood is the cab made from? Maybe the cabinet needs damping.

Thanks for the help.  I did try adding that cap but that didn't help. The volume of amp really does not matter so I don't think it's the cab or matter of dampening.  The cab is pine though....
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#8
Hi

Is the output stage hum-balanced? If the cabinet is not stiff enough it will transmit the vibration of the PT and OT to the reverb tank.

A simple yet awkward test to do is to get the tank to howl then lift the tank away from the speaker and hear if it quiets down (after the crashing). If it does, it suggests that there is a hum field within the cabinet - a sympathetic vibration due to shae or size. If the cabinet is square or close to it standing waves are easy to set up.
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#9
(03-12-2019, 07:09 PM)K O\Connor Wrote: Hi

Is the output stage hum-balanced? If the cabinet is not stiff enough it will transmit the vibration of the PT and OT to the reverb tank.

A simple yet awkward test to do is to get the tank to howl then lift the tank away from the speaker and hear if it quiets down (after the crashing). If it does, it suggests that there is a hum field within the cabinet - a sympathetic vibration due to shae or size. If the cabinet is square or close to it standing waves are easy to set up.

The output stage is hum balanced so that is covered. 

The cabinet is stock Deluxe Reverb repro cab.  I'd imagine it's stiff enough---it's professionally built but it could be a problematic factor. The feedback issue is directly related to the proximity of the speaker to the tank.  If I move the tank outside of the cab I don't get any feedback and the reverb is very quiet.  Inside the cab after the reverb is turned about half-ways up the feedback begins to build. 

I have tried a few positions for the tanks and padding the inside of the reverb bag without success. Somewhere along the line I'm sure this is some small error on my part. Prior to this I've built a number of 60's Fender type amps without issue.  Those amps were randomly grounded and used a traditional layout.  This was meant to be an improvement over those builds and in all other respects it sounds better and has less noise.
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#10
I finally got around to fixing it. The problem seems to stem from the cab and the not padding the reverb tank enough in the bag. I learned that if it was feeding back all I needed to do was touch the tank for it to stop. I added foam in inside the reverb bag to help dampen it and changed the feet out on the cab so that the floor might take up some of the vibration. Right now I'm not having any other problems. This one of the reason I prefer heads! Thanks for the help guys!
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