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grounding mod for standalone reverb unit

9/22/2005 7:05 PM
grounding mod for standalone reverb unit
A couple of months ago on this or another forum, someone suggested adding a series of resistors to the ground wire before grounding to the chassis to reduce hum when connected to an amp. They claimed the set up eliminated 99% of hum that was heard through the amp. They had already taken ground loops, etc. into account. Thanks for any info.. I should have noted what was done, but did not,and now Iwould like to give it a try on my own equipment if anyone is familiar with the above approach.
9/23/2005 12:45 AM
Dai Hirokawa

this? (from the Weber amps bbs I guess)  
[QUOTE]Date: June 21, 2005 at 13:00:55  
From: Bob Keefe  
Subject: Thread for mod to get rid of hum in Fender RI reverb tank?  
Does anyone have a copy of the thread that has the mod that  
gets rid of the hum in the Fender RI reverb tanks? I seem to  
remember something about a cap, a resistor, and two diodes  
all in parallel.  
Thanks in advance.  
Dr. Bob  
Date: June 21, 2005 at 14:18:03  
From: SG123  
Subject: From John Stokes..  
"to get around this ground loop problem without requiring the use of a lift adapter on the cord, is I built-in a pseudo partial ground lift between the reverb's chassis and the green wire on the line cord. It works perfectly, no hum, and yet the thing is still safe. The circuit for this is as follows:  
It uses 4 components, all wired in parallel with each other, and this parallel network is in series between chassis ground and the green ground wire on the line cord. The 4 components are a 10 ohm 1 Watt resistor, a .1 uF 400V cap, and TWO 400PIV, 3A rectifier diodes wired back to back (cathodes in opposite directions).  
The way it works is as follows: The 10 ohm resistor provides enough resistance in the ground lead that the effects of the ground loop are removed. The back to back diodes clamp any voltage difference between chassis and line ground to 0.7 volts. The cap provides high frequency AC bypass for noise, RF, etc. It works!"  
Date: June 21, 2005 at 14:50:54  
From: Bob Keefe  
Subject: Thanks, John(NT)  
Date: June 21, 2005 at 14:45:21  
From: Wally  
Subject: Re: From John Stokes..  
I am functioning in the 'dense' mode today, so just to be sure I have this correctly....  
Are the two diodes wired together and then placed in parallel with the cap and resistor?  
Date: June 21, 2005 at 15:34:08  
From: Charlie Hall  
Subject: Re: From John Stokes..  
He means everything is in parallel. The two diodes must be opposite polarity to each other but still in parallel with each other and also in parallel with the resistor and cap.  
That way, the voltage difference between chassis and ground connection will be limited to 0.7 volts regardless of the cycle of AC mains current.  
Date: June 21, 2005 at 16:10:35  
From: Wally  
Subject: Thanks  
I need all the help I can get today.  
Date: June 22, 2005 at 05:59:09  
From: Bp-Plickner  
Subject: Re: Thanks  
I guess I'm Low Tech. I just put a three prong adapter on the reverb tank AC plug and let the connector cable between the amp and reverb unit connect to ground  
Date: June 22, 2005 at 14:01:22  
From: PaulC  
Subject: Re: Thanks  
You can knock the hum way down if you just make sure the grid and cathode circuits of the verb return are grounded to the return RCA jack. that works most of the time for me...  
(looked useful so I happened to have saved it)
9/23/2005 7:57 AM

Thanks, Dai. Exactly the info I was searching for the last few weeks! Once I get the components together, I'll give it a try and report back.
9/23/2005 10:31 AM
Dai Hirokawa

no problem! FWIW, the reiss. Fender Rev.Unit has something like that where circuit ground is lifted from the chassis a bit w/a small value R and other parts as in the post IIRC. The input jack was also isolated from the chassis and there is some funky stuff on the PCB for the pots. Forget how exactly they were hooked up but there were metal brackets around the pots also connected to circuit ground via a 15ohm R??
9/24/2005 4:33 PM

This is great news - When I finished my G15 reverb unit it didnt take me long to realize that if I defeated the ground the humm went away, so I installed a switch to lift the ground but I never felt totaly compfortable with this for obvious reasons so I hardly ever use the unit. If this mod works Im sure I will use it a lot more often. Thanks for the info I will let everyone know how it works! , JOE
9/25/2005 1:25 AM
Simon May

Sorry to ask such a silly question but how / where do you plug in a standalone reverb unit ie front end of the amp or at the output .??
9/25/2005 2:43 AM
Dai Hirokawa

The Fender Reverb Unit is meant to be plugged in to the front end. Or at least that's how I think it was designed to be used and how it is normally used.

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