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Old 3rd August 2009, 06:54 AM   #1
rem280 is offline rem280  United States
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Join Date: May 2009
Unhappy Reverb tank hum

I have put reverb into a small tube combo and never bothered to actualy mount the reverb tank inside the amp while testing the drive and recovery circuits. Everything is great if the tank is sitting besid the amp but when it is mounted inside the hum is unbarable. there is no way to mount it in the amp that will reduce the hum to an acceptable level. can enyone sugest a good metheod for magnetic shielding. I dont think steel sheet metal will do because thats whate the reverb tank is already enclosed in. I looked at some design sugestions on accutronics website and they say that the recovery circuit should have a high pass filter with a critical frequency of around 100Hz to reduce mudiness from mechanical shock. this has not been a problem anyway but I figered if I dont need frequencys below 100 Hz anyway I could just filter out 60 Hz. not wanting to mess with my board I put a single pole filter with 100Hz Fc in the tank but thiss did not provide nearly enough attenuation at 60Hz to silence the hum. any sugestions I would prefer if this could be solved with shielding as apposed to a more complex filter
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Old 5th August 2009, 05:11 PM   #2
ChrisA is offline ChrisA  United States
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Default Re: Reverb tank hum

The best thing to do is to find the source of the hum and kill it there, shielding can never work as well. Have you tried the normal things yet, biasing the 6.3V heater circuit either around ground or higher. Going to DC (filtered and regulated) heater will kill any 60Hz for sure.

One other thing, the 60Hz may not be transmitted electrically. It could be mechanical. Is there a power transformer that is vibrating at 60Hz? I hate to suggest poking fingers into a powered up amp but you might check for mechanical vibrations at 60Hz.

"Everyone" does say to not send lower frequency through the reverb tank. I think that means to filter them out in the driver AND the recovery stages, all you need are some right-sized caps

Have you star-grounded to reverb tank itself or at least 100% checked there can't be a ground loop
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Old 6th August 2009, 01:22 AM   #3
Enzo is offline Enzo  United States
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The output end of your reverb pan is picking up the power transformer. That is why it doesn;t hum when outside the cab.

One must always mount the reverb pan with the output end as far as possible from the power transformer. You might get lucky and simply turn the pan end for end.

The pan itself shields the trnasducers from most noises, except it has the open side. Cut a piece of sheet steel to cover the open side of the pan. Actually a partial cover over the output end would likely suffice. SCREW the plate onto the pan for good contact. The steel plate will complete the shielding which will keep out most of the transformer noise.
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Old 6th August 2009, 08:44 AM   #4
rem280 is offline rem280  United States
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the noise is coming from the power transformer and is being picked up by the output coil. I tried moving the tank any way I could and did not get much improvement. it is a small enclosure and I don't have many opptions. I figured sheet steel would not do much for shielding because the tank is made of it and the open side is facing the bottom of the box. after doing more research and pricing special alloys like mu metal I decided to do more experimenting. I took the top of a project box which is a steel chanel and placed it over the power transformer with the open ends facing the front and back of the box and the hum became noticably quiter so I will make a sheet metal cover for the power transformer and see if it solves the problem. the amp is a cheap peavey and the transformer dosent have the usual steel cap over it the windings are covered in a plastic case with a copper strap around it.

P.S. My filter dident make a noticable difference because Its picking up mainly harmonics of 60 hz which I mesured with my scope. these frequencys are well above were I would want to filter
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Old 6th August 2009, 08:48 PM   #5
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I bought a Dean Markley guitar combo (solid state) that had the mains transformer sitting right against the circuitboard, precisely where the reverb circuitry was, hummed like crazy when you turned up reverb.
I have the transformer sitting in the bottom of the enclosure now, lengthened the cables and put the bundle of cables in a woven cable conduit.
All good now, I can have sounds swimming in reverb with no hum.
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Old 6th August 2009, 11:15 PM   #6
Enzo is offline Enzo  United States
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Rem, the fact that the open side of the pan faces down is irrelevant. You found that shielding the transformer worked, then so should shielding the pan. At least TRY a cover over the open side. It will only take a minute. SOme amps even come with a shield plate there.

Moving the pan is often hard for space reasons. ABout the only thing that helps this complaint it turning it end for end.
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Old 11th August 2009, 03:35 PM   #7
rem280 is offline rem280  United States
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I tried covering the bottom of the reverb tank with a steel plate and it did not make much difference so I covered the transformer with a sheetmetal box. The noise did go away but now I am geting feedback from the tank when I turn it up very loud. I was wondering if a reverb bag would likly fix this problem or if someone has any other sugestions. Also what reverb bag is the best
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Old 11th August 2009, 04:05 PM   #8
ChrisA is offline ChrisA  United States
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Quote:
Originally posted by rem280
I tried covering the bottom of the reverb tank with a steel plate and it did not make much difference so I covered the transformer with a sheetmetal box. The noise did go away but now I am geting feedback from the tank when I turn it up very loud. I was wondering if a reverb bag would likly fix this problem or if someone has any other sugestions. Also what reverb bag is the best
Looks like what I said above worked, "find the source of the hum and kill it there." that always works better than shielding the part that is picking up the noise.

I think the same solution would work here to. Remove the speaker from the cab and put in inside a sound proof box - 100% sure way to end feedback. But maybe not what you want to do.

Reading on the accutronics web page it says the #1 thing to do is to drive the reverb tank "hard" (nearly to saturation). They say this is especially important in applications such as guitar reverb where the tank may be in the same box as the speaker. The idea is to improve the signal to noise ratio by making the signal bigger. They say that all spring reverbs are microphonic. That is just their nature, so you drive them hard to drown out the microphonic signal with "good" signal.

I think I'd also try and figure out if the sound is getting into the tank from the air or if the tank is getting vibrations transmitted from its mechanical contact with the wooden cabinet. Try setting in on top of a folded towel or strips of packing material to isolate it from the cabinet.
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Old 13th August 2009, 12:01 AM   #9
Enzo is offline Enzo  United States
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Reverb pan feedback is a common problem, yes that is what the vinyl reverb bag is for. Keeeping the acoustic energy of the speaker away from the springs is key.

A reverb pan can feed back even when not being driven at all. The speaker vibration drives it.

Get a reverb bag and put the pan in it. it helps to cover the open side of the pan with something, A piece of cardboard will do, as fender has done for years. Tape that to the pan v\before it goes in the bag.

make sure the mounting screws are not holding the pan down tight. Tight to the floor transmits sound energy through the floor into the pan. Without a bag, there should be four soft rubber grommets. Screws through the grommets should be only tight enough to keep the pan from flopping around. In the bag, a screw through the end flap of the bag on each end is enough to keep it in place.

The broad flat top surface of the pan can resonate. Go to Home Depot or someplace and get some rubber foam weather stripping. Size not critical, but maybe 3/4" wide by 3/8" thick. The stuff comes in a roll and has peel off adhesive strip on the back. Cut a piece of this and stick it down the length of that flat top surface. That will damp out its vibration. Car manuafacturers do this inside large body panels in their cars to prevent sheet metal noises. You will find some amp makers already do this.

And if there is room, sometimes just moving the pan a couple inches makes a difference. Standing waves can form in the cab, and moving the pan away from them can be all it takes.
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Old 20th August 2009, 06:03 PM   #10
ChrisA is offline ChrisA  United States
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Dick Dale has got to be the "king of reverb" and one of the original surf music players who is still going strong. I think Dale suspends is Fender reverb unit from the ceiling on a rope. This is kind of extreme but his sound depends on big time use of spring reverb and I guess a suspended unit would not pick up vibrations from the floor.

EDIT.

Funny that the automatic censer system would censer a common first name. (Starts with "D" and ends in "k" with four stars.) This is the link to the real person http://en.wikipedia.org/wiki/Dick_Dale if you can't figure it out.

EDIT again. Wow it even censers links.

Last edited by ChrisA; 20th August 2009 at 06:10 PM.
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