Blackface
The Twin Reverb was first introduced during the blackface era of Fender amplifiers which was from 1963 to 1967. During this time the amplifier's output was rated at 85 watts into a 4-ohm speaker load.
The so-called blackface amplifiers have black faceplates, black tolex covering on the cabinet, and neutral to slightly amber tinted silver sparkle grill cloth with a small ribbed rectangular pattern.
Twin Reverb amplifiers came standard with "tilt back legs" which allowed the amplifier to be tilted at an angle backwards, so the speakers faced at a more upward angle, promoting better distribution of their output to an audience when placed on a low stage.
The circuit used is commonly known as the AB763 circuit. Fender Twin Reverb amplifiers use four output tubes, of the 6L6GC type. They use six preamp tubes, consisting of four 7025/12AX7 types and two 12AT7 types. The 7025 is a lower-noise version of the 12AX7.
The Fender Twin Reverb has two independent channels, labeled Normal and Vibrato. The controls have black-skirted knobs numbered from 1 to 10.
The Normal channel has two inputs, a "bright" switch (which compensates for loss of brightness through the volume control when the control is set lower than about "6" on its 1-10 scale), a volume control, treble, middle and bass tone controls.
The Vibrato channel has a duplication of the same controls as the normal channel, plus the addition of reverb, vibrato speed and intensity controls. Reverb is accomplished with a tube/transformer driven low impedance spring reverb tank (made by the Hammond company) or its spin-off company Accutronics. The addition of the reverb circuit requires an additional "gain" stage in the preamp, and as such, the Vibrato channel is capable of a bit more distortion when the volume is set high. Vibrato (tremolo) is accomplished with what is known as a "vibrato bug" by Fender amplifier repairmen. The vibrato bug is a four-wire device consisting of a neon tube and light-dependent variable resistor, packaged in a short section of black tubing. It resembles a water bug, with slender wispy legs. The tube generated vibrato oscillator pulsates the neon light, which varies the resistance on the resistive element. That variable resistance is applied to the audio signal on the Vibrato channel, creating a pulsating increase and decrease of that channel's volume. The speed controls varies the rate of the oscillator. The depth control limits the amount of application the variable resistor has on the audio signal.
The front panel also has a bright red pilot light lens (better known as "pilot lamp jewel" for its multiple triangular-shaped facets), covering the pilot light (made by the Chicago Miniature Lamp Co.). Other colour schemes (amber, white, green, purple and blue) are also available.
Rear panel controls include a 2.5-amp "slo-blo" fuse, an accessory AC outlet, an earth ground selection switch, on/off and standby switches. Additionally there are two speaker output jacks, and jacks for footswitches to activate/deactivate the reverb and vibrato effects.
The black faceplates of the "blackface" era Twin Reverb will say either "Fender Electric Instruments" (FEI) or "Fender Musical Instruments", (FMI) depending on the date of manufacture. Those units made before the CBS take over of Fender in 1965, will be marked Fender Electric Instruments, and be worth a bit more on the collectors market.
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