From Dstor–(at)–eathstar.cris.com Sat Aug 6 10:19:24 CDT 1994
Article: 21888 of alt.guitar
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From: Dstor–(at)–eathstar.cris.com (Dr.Distortion)
Newsgroups: alt.guitar
Subject: Re: Fender Vibro King
Date: 6 Aug 1994 09:12:55 -0400
Organization: Concentric Research Corporation
Lines: 46
Message-ID: <32028n$c8--(at)--eathstar.cris.com>
NNTP-Posting-Host: deathstar.cris.com
collins_ji–(at)–andem.com (Jim Collins)
wrote:
CO>fat. I love it. Anyway, the biggest knock on the amp when it first came
CO>out was that the FAT feature was not footswitchable. Imagine my surprise
CO>when I unboxed the beast to find that our friends at Fender have been
CO>reading their mail. The FAT feature is now footswitchable. The amp comes
CO>with the same dual footswitch that the reissues come with, only the two
CO>features that are footswitchable on the Vibro King are the tremolo and the
CO>FAT. Reverb is not footswitchable.
I have studied the schematic for the Vibro-King at length, and it might
be of interest to the more technically inclined readers of this group
that the famous FAT switch is nothing more than a switch that grounds
a 22 uF bypass capacitor across the cathode of the first gain stage.
This is an OLD trick, although I’m not implying that that makes it any
less useful. A good tech could convert the #2 input jack into a foot-
switch jack for the FAT function in minutes… something you owners of
Vibro-Kings might want to consider, once your warranty expires.
It’s also easy to make the reverb footswitchable without altering
the amp in any way. All you need to do is rig up a pedal to ground
the hot lead of the return from the spring tank. You could use a
“Y” adaptor to hook up the switch in parallel with the reverb
return cable.
Here are some more comments on the Vibro-King’s circuit:
1) In a break with Fender tradition (at least Fender tradition since
the late ’50s), there is NO negative feedback loop. This might explain
why the amp is gaining a reputation for volume and harmonic richness.
2) The reverb section is NOT an exact copy of the old standalone Fender
reverb unit, although it’s close enough. It uses a 6BQ5 for the reverb
driver and one triode preamp before the driver, unlike the original
with two triodes before the 6K6 driver. The recovery circuit is also
slightly different. But like I said, it’s close enough; it embodies
the overall “spirit” of the original.
3) The tone stack is an inspired variation on the old “Blackface”
circuit. It uses a 150 pF treble cap instead of 250 pF (something I’ve
been doing to my stock Fenders for a while, since I think the 250 pF
emphasizes the high mids too much) and a larger-value midrange pot
(25K, I believe, although I don’t have the schematic handy). The
volume control comes BEFORE the tone stack, and is separated from it
by the second gain stage. The treble pot wiper is connected (via
a coupling cap) to the grid of the phase inverter.
Overall, it’s a surprisingly simple circuit.
—
