’64 Vibroverb™ Custom
(This is the model name for warranty claims)
5
CIRCUIT DESCRIPTION
PRE-AMPLIFIER
The Normal pre-amplifier is comprised of V1-A and
V1-B and associated circuitry. The Vibrato pre-
amplifier is comprised of V2-A and V2-B and asso-
ciated circuitry. V1-A and V2-A present a high
impedance input and provide gain before the tone
controls. V1-B and V2-B recover gain lost in the
tone controls so as to fully drive the power amp.
POWER AMPLIFIER
V6 receives the single ended signal from the pre-
amps and generates two out-of-phase signals
which are necessary to drive the push-pull output
stage comprised of V7, V8 and T3.
POWER SUPPLY
T2 steps up the AC line voltage to the high voltage
necessary to power the tube circuitry. D6-7 are in
series with V9, the tube rectifier and rectify the sec-
ondary AC voltage which is then filtered by a series
of capacitors and dropping resistors to achieve the
proper voltages necessary to power the different
stages. The rectifier switch bypasses V9 in DIODE
Rectifier mode and automatically adjusts the bias
for V7-8.
REVERB CIRCUITRY
The signal from V2 is fed to V3 which drives T1 and
matches the impedance of V3 to the reverb tank.
V4-A recovers the reverberated signal which is
mixed with the original at V4-B.
TREMOLO (VIBRATO)
CIRCUITRY
The output of the Vibrato channel is amplitude
modulated by the combination of R48 and OP1, an
assembly made up of a neon bulb and a light de-
pendent resistor. A variable low frequency oscillator
comprised of V5 and associated circuitry drives the
bulb which modulates the resistance in parallel with
R48.
The MOD SWITCH
The MOD switch changes the bias points of V2-A/B
and disconnects the Normal channel and R48 from
the output of the Vibrato pre-amp. This results in
the disabling of the Normal channel and the Vibrato
function. This also gives the Vibrato channel more
gain and distortion.
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