Jon   (z-man1@worldnet.att.net)
    Date:
         2/12/2001 4:37 AM
 Subject:
         Re: Magnatone trem and Brown face trem
Bruce,
 
I did that a while back after scoring a shi.load of
varistors that worked well after changing the bias
voltage on them. Posted a query here if anyone would
be interested in a kit to either build up a free-standing
pedal, rackmount, or incorporate the circuit (stereo or
mono) into a preamp. I've also modded the circuit to be
very Univibe-like, with 4-5 stages and foot pedal
control of the oscillator speed, and a pull up pot to kick
in feedback for a resonance boost. Got very limited
response, so scrapped the idea of promoting it. 
 
Matthew, for a good description of the circuit, see RG's
blurb in The Technology of Phasers and Flangers.
Although there are some similiarities to the brownface
Fender trem circuit, the varistors (which are silicon
carbide) actually do a vibrato via an oscillator driven
variable R in a phase opposed R-C circuit. Fender's
brown tremelo wiggled the bass and treble with the
phase split. MOV's (metal oxide varistors commonly
found in surge suppressors and the like) don't have the
same "knee" in the curve and don't work in these
circuits...I wasted a lot of time chasing down blind
alleys.....
 
BTW, a nice tweak is to add amplitude modulation
(tremelo) to the Magnatone vibrato circuit for more
pronounced effect, especially with the feedback loop.
cya,
Jon