Fender Twin Amp 1994 Service Manual - Theory of Operation
’94 TWIN AMP
THEORY OF OPERATION
INPUTS
J1 and j2 are high and low sensitivity inputs that feed the first preamp stage (V1A). V1A provides a gain of about 38
for channel 1, and about 49 for channel 2. The gain difference is due to different loading of the plate of V1A. The
signal then couples to relay K1B, which routes it to the channel one or channel two circuitry.
CHANNEL ONE
The ’94 Twin Amp is a direct descendent of classic tube amps like the Twin Reverb and the 410 Bassman. Channel
one is the traditional Fender design which offers the vintage sound as well as contemporary sounds with the use of
switch-selectable Gain and Master Volume controls. It’s like two channels in one. The first stage of Channel One
contains the Treble (R17), Bass (R18) and Mid (R19) tone controls. The signal from the Treble control feeds through
Relay K2B, which routes the signal to either the Clean or Vintage Overdrive section of channel one.
CLEAN
The signal from pin 11 of Relay K2B feeds through the Clean Volume control (R11) to V1B. R12 and C4 provide the
Bright boost feature via a pull switch on the Clean Volume control. V1B provides a gain of about 45. The Clean
channel circuitry terminates at pin 6 of Relay K2A. K2A selects between the output of the Clean or Vintage Drive
circuit.
VINTAGE DRIVE
The signal from Pin 9 of Relay K2B feeds through the Gain control (R8) to the first stage (V2A) of the Vintage Drive
circuitry. V2A provides a gain of about 20, then drives the second gain stage (V2B). V2B provides a gain of about
48; then couples the signal to the vintage drive Volume control (R23), which also contains the select switch for the
Clean/Drive feature. When in the Channel 2 mode, Relay K1A connects the wiper of the Gain control (R8) to ground,
muting the input to the Vintage Drive stage. The Vintage Drive circuit terminates at pin 8 of Relay K2A.
CHANNEL TWO
Channel Two is like owning a second, modern hot-rodded amp with more soaring gain than previously found in a
Fender tube amp. The signal from pin 9 of Relay K1B feeds through the Channel Two Gain control (R25) to the first
(V3A) and then second (V3B) stage of the overdrive circuitry. From V3B, the signal is fed to through the Treble
(R35), Bass (R36), and Mid (R37) tone controls. Finally the signal travels through the channel two Volume control
(R39), which also contains the Channel Select switch. Channel two terminates at pin 9 of Relay K4B. K4B selects
between channels one and two.
EFFECTS LOOP
The selected (Ch-1 or Ch-2) signal from Relay K4B is fed to a Split-Load Phase Inverter (V4A). The output from Pin 1
of V4A follows two paths to Relay K3A. One path travels through one half of the Mix control. The second path
connects directly to K3A. The signal from Pin 3 of V4A drives the Effects Send Jack (J3), and is normalled to the
Effects Return jack (J4). The Effects Level switch provides three signal levels for matching a wide variety of external
signal processing devices. The switch also maintains unity gain between the Send and Return jacks. The signal from
the Effects Return jack feeds V4B, then travels through the other half of the Mix control (R44B), and to Relay K3B.
The dry and wet signals sum together through R54 and R55. Relay K3 is controlled by the Effects Select switch in
the Both position, K3 will be in the Normally Closed position. Any signal available at the Effects Return jack will pass,
whether in Ch-1 or Ch-2. The Mix control will be active. With the Effects Select switch in the Ch-1 position, the
Effects Return will be active while the amp is in the Ch-1 mode. When the amp is switched to the Ch-2 mode, the
control voltage from U2B will energize relay K3, K3A will toggle, bypassing one half of the Mix control (R44A), and
K3B will break the connection from the Effects Return. Therefore the Effects Return will be active only for the channle
that is selected by the Effects Select switch.
REVERB
The signals from Relays K3A & K3B sum through R54&R55, then split to feed the Reverb Drive circuit and the
summing amp for the Wet and Dry signal. V5 (12AT7) and T1 make up the standard Reverb Drive circuit. The