Noisy/buzzing reverb tank

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Noisy/buzzing reverb tank

Postby cliftonhanger » Fri Mar 19, 2010 1:17 am

Hi diy-ers, This is my first post here and thanks in advance for your time.

I have a 1970 Dual Showan that I recently had blackfaced by a local tech. Everything was working fine until I removed the chassis to look at the mod. When I put the chassis back into the head I started having a problem with the reverb tank.

With the amp ON and standby Off and all dials set to 0, when I sweep up the reverb pot a loud hum occurs and gets louder as I turn up the reverb dial.

Troubleshooting:
I disconnected the rca plugs and unscrewed the reverb tank and completely removed it from the head (i placed the disconnected tank and cables 3 feet away from the head).
Result = problem still occurs when sweeping the reverb dial. (all other pots are still on "0")

Next > With the reverb tank sitting outside of the head (2 feet from the head), I plugged the rca connecters back into the reverb in/outs.
Result = the noise is reduced (by 80% or so) when i sweep the reverb pot. pretty much back to normal.

Next > I then simply set the reverb tank (in the leather case), back into the head without screwing the tank to the head.
Result = The noise is bad again when I turn up the reverb pot.

Next > I then tried a different reverb tank from my 1978 twin reverb, tried all three scenarios above and the results were identical. (so its not the tank).

Next > I swapped out all preamp and power tubes and the problem still persists as specified above. (so its not the tubes)

Notes:
This amp has a three prong cord installed; however, every time I switch the toggle all the way to the left, I get shocked when I touch the toggle switch to flip it back over to the right.. (is this normal? this might be a separate issue entirely)

With the reverb set to 0 and all other dials set to normal settings (between 3-5 for vol/treble/mid/bass pots) there are no noise issues. Its only with the reverb pot.

Any thoughts about why this is occurring?

Thanks again, i have been searching a bunch of threads and could not find anything that was a good match for this issue.

Peter
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Re: Noisy/buzzing reverb tank

Postby Writer Frog » Sat Mar 20, 2010 5:38 pm

Hi Peter,

Welcome to DIYTube!

First of all, I do not, and have never owned or used a guitar amplifier, so whatever I say is based on pure conjecture. I assume your amplifier is somewhat similar to this one:
Image
Does the reverb unit work at all besides the loud hum problem?
From your description of the problem and troubleshooting, it is possible that you have grounding issues with the circuit within the red rectangle. Make sure the the grounding points in green rectangles are indeed connected to the amplifier's chassis ground.

This amp has a three prong cord installed; however, every time I switch the toggle all the way to the left, I get shocked when I touch the toggle switch to flip it back over to the right.. (is this normal? this might be a separate issue entirely)

With 3-prong power cord, there should never be an exposed "hot" part.
What toggle switch is this? Have you measured the voltage level (AC or DC) at this toggle switch in reference to the chassis ground?
You may have amplifier-wide grounding issues.

Let's be careful out there...
/Matt
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Postby Geek » Sat Mar 20, 2010 8:38 pm

Hi,

If the switch is stock, it's likely dirty and there's voltage breakdown to the toggle.. That's where the shock comes from. Replace it.

As for the hum after reassembly, something came loose. Could be a broken shield solder joint on the cable or a bad plug.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby dhuebert » Sun Mar 21, 2010 9:01 am

Yes go through it and check all your continuity in the reverb circuit. Especially the grounds. The two most likely culprits here are a broken wire in the tank itself and a bad cable. BTW Does it still work despite the hum?

What toggle are you switching when you get the shock? If it is the ground switch on the back, your 3 prong plug is installed wrong. That switch should be removed with a grounded chassis amplifier. It is only there for floating chassis design (2 prong plug)
The hot black power cord wire should go to the power switch, the white neutral wire goes to the primary winding of the power transformer and the green ground wire should be bolted to the chassis and the ground switch removed and thrown away.

Image

Look at how this one is done. Fuse first then switch then pilot light then transformer.

Don
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