Remove Ground Switch

From ingin–(at)– Thu Sep 14 11:16:40 CDT 1995
From: ingin–(at)– (Mark Ingino)
Newsgroups: alt.guitar.amps
Subject: Re: Converting Super Reverb AC Power Cord
Date: 14 Sep 1995 15:13:04 GMT

In article <4377bm$19q--(at)>,
Jim Patrick wrote:
>I’m in the middle of finally changing the power cord on my AB763 Silverface
>Super Reverb from a 2 prong to a 3 prong (I got tired of electrocuting myself
>=:–(at)–). Anyway it appears that I need to disconnect the so called ground switch
>and matching cap entirely out of the circuit, reconnect the 2 AC leads, and then
>hook up the green wire ground to the chasis. It looked like the bolts of the
>output transformer have terminals in use already for ground connections going
>to the chasis. I thought that this would be the appropriate place for my cord ground
>as well.
>Can someone who has done this mod confirm that what I have planned is correct?
>Also does anyone know of a drop in replacement for the round AC outlet on
>the back of the chasis to also convert this to a 3-prong type?
>I’m still trying to figure out how UL ever approved this wiring in the first place.
>Anyone know the history of old Fender ground switch?

Jim- There have been a couple of threads on this so I’ll just pass on the
information I’ve gathered here and in various publications. The ground switch
is from the days when plugs were neither grounded or polarized and UL
approval was not a requirement. It allowed the .01uF capacitor to be
switched from either the hot or neutral leg to chassis ground, depending
on which way the AC plug was inserted, and was intended to reduce AC line
noise. The problem is that the capacitor creates an AC path to the chassis
that can cause shocks if you touch another chassis at another potential.

It sounds like what you have planned is fine with the following precautions
in mind: You want the AC hot lead to go to the fuse before the switch,
making sure the lead goes to the *bottom* of the fuse container first.
Ground can go where you saw the other leads. Neutral can go to the trans-
former lead. You won’t need the ground switch anymore so you can free up
any wiring to the switch and use it for something else or put a line out
or bias measurement jack there instead. Make sure that you discharge all
the caps in the amp before you start working on it. Be careful and
good luck.

Mark Ingino


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