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Scooter Takes The Juice

From detritu–(at)–x.netcom.com Fri Jul 14 14:09:04 CDT 2000
From: Lord Valve
Newsgroups: alt.guitar.amps
Subject: SCOOTER TAKES THE JUICE!
Date: Thu, 13 Jul 2000 03:12:41 -0600
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X-Server-Date: 13 Jul 2000 09:08:02 GMT
Xref: geraldo.cc.utexas.edu alt.guitar.amps:263263

Lord Valve Speaketh:
Ho HO, a *shocking* tale indeed…

I decided to give my fearless assistant Scooter Barnes
a short respite from the sandbox shit he usually has
to mess with on his bench. After all, one’s fingers
do tend to get all cramped up from handling those itty-
bitty three-legged fuses all day, and I figured he could
benefit from stretching his hands a bit pulling genuine
thermionic amplification devices from their sockets and
whatnot. So…we had a Gibson GA-20 that a dude brought
in; he musta got it at a garage sale or something, because
it was beat to hell…no back, the handle was rotted off,
and parts were hanging loose inside the chassis. We didn’t
have a schematic, but luckily a friend of mine who owns a
shop on the other side of town had a print for this amp,
and after Scooter talked to him on the phone for a few
minutes, he was able to determine where everything went,
and he soldered it all back together and fired it up.
At first, no signal would come through, but it made nice
crunching noises when any of the controls on the signal
generator were operated. This was found to be due to
the fact that the range selector switch on the sig generator
was set to X100K (oops) and the old Jensen in the GA-20 just
wasn’t up to that kind of frequency response. That little
problem dealt with, the amp was found to be operating
normally, so Scooter hooked his git-fiddle up and proceeded
to give the amp a workout. Mike Marchelya happened to be
standing around (he comes in once a month to get slobbered
on by my coonhound Blue, who, for some unfathomable reason,
loves his ass with an unbridled passion) (hi Mike) and
we both remarked that it sounded really good…funky and
bluesy. (Scooter’s playing didn’t hurt it any, either…
the boy can *pick* some gee-tah, I tell ya.) Scooter
thought he was hearing some speaker-fuzz, and he reached
into the back of the amp to push on the cone a bit to
see if the VC was rubbing. He shot right back out of
the amp, and proceeded to call upon the Name of one of
the Three Most Famous Black People (Aunt Jemimah, Uncle Ben,
and Mother Fucker, you guess which one… 😉 and it was
obvious that he had tasted a little B+. Now, techs *do*
get shocked from time to time, it’s true, and it usually
happens because we aren’t paying attention to what we’re
doing; getting zapped isn’t remarkable in that respect.
In this case, however, the mechanism for Scooter’s trip
to Arcy-Sparky Land was something neither one of us had
ever seen before. This is an old amp, with the output
tranny mounted on the speaker frame. Somehow, the speaker
frame was energized with the full B+, around 300 volts.
Scooter was holding his guitar, with his hand on the strings
when he reached in to push the cone, and that’s how he got
zapped. We suspected the old cloth-covered wires had rotted
insulation, and that one of the OPT primary wires had rubbed
through and touched the frame. Nope…they all checked
fine. I had him pull the OPT, and I took it over to my bench
where I could test it with my Fluke 8050A. It turns out that
there was a high-resistance short from the primary to the
laminations. Since this circuit floats the output (no ground
reference for the speaker voice-coil) and since there’s no
frame-ground wire for the speaker, the B+ appeared on the
frame (and on Scooter’s arms, when he grabbed it) without
blowing the fuse. My Fluke said there was ~800K between
the primary and the laminations, but it was a jittery reading
and I suspect that it may have been much lower when the
windings were energized. Scooter, BTW, being a strapping
young fellow, did *not* require a change of skivvies after
the incident, although he was kinda nervous for around an
hour afterwards. So now, whenever we get one of those
old clunkers in with a speaker-mounted OPT, the *first*
thing we’re gonna do is check for juice on the speaker frame.
Ya never can tell when a *surprise* is gonna show up…
So Sayeth the Lord.

Lord Valve

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