T ECHVIEWS
The ’59-’ 63 Fender Concert Vibrato
Over the years, Fender has used two differ- through to V1B’s grid. The high and low sig-ent effects that are labeled “vibrato.” One nals are then amplified by V1A and V1B and
simply modulates the loudness of the signal. mixed together again by R6 and R7. What
The other is more interesting to me; it modu- makes this interesting is that the bias of both
lates the amplitude and also generates a V1A and V1B are changed by what I’ve called
swirling effect – almost like a phaser. I liked it low frequency oscillators – LFO1 and LFO2.
so much that I studied its principles and have
reproduced it with solid state technology.
tion I had was whether I could recreate the
Concert Vibrato sound with solid state parts,
so I designed two versions of Figure 2, one
with op-amps as variable gain elements and
one with JFETs. Both worked well, and both
tend to make you forget the words to the
song when listening to the guitar sound, one
of the signs of a successful effect.
Since I am not an expert in Fender history,
I’ve taken my historical data from the Fender
Amp Field Guide ( ampwares.com/ffg). From
that history, I see that Fender produced a
normal sort of tremolo in its earliest amps
LFO1 and LFO2 are the reverse of each
other; when LFO1 goes up, LFO2 goes
down. V1A and V1B make highs and lows
and have loudness variations, but at opposite
times. The equal-and-opposite LFO signals
then cancel out when added back together
at R6 and R7, but the treble and bass signals
Schematics for both of my versions are on
my personal web site, geofex.com, though
they are not for commercial use without prior
arrangements.
that they later referred to as vibrato. This
“vibrato” depended on the gain change
that happens in a tube when its bias point
is changed. This is the way most tube amps
produce their tremolo, but there is a problem with this. The change in bias voltage also
produces a change in the DC level on the
tube’s plate. This DC level change has to be
removed to avoid thumping sounds whenever tremolo is used. Different amps use various schemes to get rid of the thumps.
I imagine that this was what was on the
minds of the folks at Fender when the
Concert Vibrato was developed. In Figure 1,
the signal from the preamp tube is sent to
two filters; one is made up of R1, C1 and C2,
the other is R2, R3, and C3. The first is a low
pass filter, allowing the entire signal below a
set cutoff frequency to pass through to V1A’s
grid. The second is a high pass filter, letting
all the treble above a set frequency to pass
do not. This kills off almost all of the LFO DC
feed-through.
Simultaneously, the phase shifts from the two
filters cause the highs and lows to interact
with each other right where they cross over.
This causes a couple of interesting things;
the mixed signal sounds like it’s varying in
volume, but since different parts of the signal
are consistently at a high volume you don’t
get the apparent change in loudness like in
simple tremolos. The phases of the two signals cause a cancellation in the middle of the
sweep, causing a slightly moving frequency
notch that appears, sweeps a little and goes
away again. That moving notch is what
causes the phaser sound.
I’ve redrawn the Concert Vibrato in functional style in Figure 2. This shows a little more
clearly the two filters, the variable gain amps
and the equal-but-opposite LFOs. The ques-
The Concert Vibrato is responsible for something that has puzzled me about Fender.
Fender called tremolo by the correct term in
their early amps, so why did they later get
the terms vibrato and tremolo backwards?
The Concert Vibrato really does sound something like a vibrato, so calling it by the name
made sense. I think they then decided to go
back to the cheaper circuits, saving one and
a half tube sections, but liked the market
response to the name vibrato, so they never
changed it back to tremolo. It makes for
interesting speculation, anyway.
R.G. Keen
Chief Engineer
Visual Sound
visualsound.net
