Fix Reverb Ground Hum
http://www.webervst.com/wwwboard/amps/messages/11903.html
any easy fix? you bet…and here’s the answer…
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Posted by mark n on April 25, 2000 at 08:16:40:
In Reply to: Re: DIY 6G15 reverb hum – any easy fix? posted by David Allen
on April 25, 2000 at 06:49:46:
: : Try replacing the 100-ohm heater resistors with a 250K pot, and play
around. Likely you have a conflict with the heater circuit ground.
: : I had this problem with my Marshall 18W clone, damned near drove me
nuts. Turned out that it wanted one side of the heaters grounded, and the
other isolated by a 220K. Weird.
: Did you use a shorting type input jack? Make sure it is good or it will
hum. I like to establish an AC ground at one of the PT mounting bolts for
the HV center tap, AC power cord ground, heater grounding resistors and so
on. The DC star ground is for all the other signal grounds including the
ground side of the filter caps. Make sure the 6K6 is connected to the choke
properly or it will hum. I’d try to figure it out because this circuit is
bad about humming when you have an amp involved. Plug both the amp and the
reverb unit into the same grounded outlet.
: Good luck. David
———> you, sir, have a ground loop. if this is a nasty, intense buzz,
then chances are the problem is not *in* the reverb unit, but is associated
with the interaction of your amp.
you properly note that the original units were not grounded, and you
speculate that lifting the green wire will fix the hum. it will, but don’t
do it.
for safety reasons, keep all audio equipment “3 prong”…..hard grounded
chassis. the solution is to make a special inter-connection cable. make a
1/4″ – 1/4″ “guitar cord”, however, be careful to only connect the shield on
one end. if this were to be used on a guitar, it wouldn’t work. but since
both pieces of audio gear are grounded [i.e. frames connected] then the
shield need not be joined at both ends.
use this cable to connect your reverb’s output to the amp’s input.
conventional wisdom says to plug the 1/4′ plug with the ground connected to
the amp; use the ungrounded plug for the reverb side. this should 100% cure
your loop.
a word to the wise…especially if you gig out: you now need a special
cable…make 3 or 4 of them, just incase you suffer a “stage failure”.
furthermore, when i make special ‘lifted’ cables like this, i ALWAYS use
bright shring wrap on the non-grounded end for identification purposes.
good luck!!
————>mn
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