The Twin Reverb was first introduced during the Blackface era of Fender amplifiers which was from 1963-1967. During this time the amplifier's output was rated at 85 watts into a 4ohm 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 0-10.
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 0-10 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 (Hartley) 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 channels 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.
In 1968 the Fender amplifier line switched from a black faceplate to a brushed aluminum faceplate with light blue labels (excepting the Bronco, which has red) and changed the color of the grillcloth from silver grey to silver with sparkling blue threads embedded within it, ushering in the Silverface era. Other blackface cosmetic features were retained.
The first silverface Twins made used the Blackface AB763 circuit until May 1968 when they began using the Silverface AC568 circuit. The fact that after this change Fender still for some time used printed tubecharts saying AB763 has caused some owners to think they have AB763 circuits when in fact they are AC568s.
The Silverface Twin Reverb had an aluminum frame (trim) surrounding the sparkling blue grillcloth and black lines on the brushed aluminum control plate until early 1970. The Fender logo on the top left corner of the grillcloth used the underlining tail found on the blackfaced amps of the 1960s. This feature was offered on models produced prior to the "tailless" period in 1973. Some Twin Reverbs made in the mid-1970s came with an unusual non-standard sparkling silver/orange grillcloth.
The rating of the amplifier's output power was also upgraded to 100 watts. Fender factory schematics show slightly higher voltage on the output tube plates when compared to the older AB763 circuit. Some say that this accounts for the higher output power. It should be noted however, that the power transformer part number for the AB763 and the later circuit designs are identical, being part number 022756 (125P34A), and the specification of that part number did not change. The transformer output is rated at 640vCT at 450ma when its primary receives a 117v input. Other parts of the power supply are essentially the same, so some mystery surrounds the increase in power. One possible answer is voltage delivered to the amplifier from the wall plug. In North America, wall outlet power is considered to be 117v nominal, plus or minus 10 percent. Fender may have originally rated the amplifiers when plugged into the voltage available at their factory, which may have been as low as 105v. Maximum operating voltage would be about 127v. Quite possibly CBS saw fit to rate the amplifiers power at maximum input voltage, thus gaining an "upgrade" with no change, and at no cost whatsoever.
From about 1973 forward, a master volume with pull-boost (on a push-pull control) became a standard feature on all dual-channel silverfaced Fender models (usually known as "master volume" amps). Original master volume amps from late 1972 were made for a short time without that "pull boost" circuit on the master volume control.
Between 1977-1982 the power was increased to 135 watts. This increase was partly due to the output section being changed to the ultralinear topology, as different power transformer and power supply design resulted in much higher plate voltages. During that period the "tailless" Fender amp logo was redesigned with a "Made in USA" script added on the bottom side.
During the 1970s and to a point, the late 1960s, the American amplifier companies were all engaged in an undeclared "wattage war". Each manufacturer would rate and or produce amplifiers of increased power as a means of gaining market superiority (or the illusion thereof). American amplifier companies used a philosophy of bright clean tones and the elimination of distortion was a key design factor. Another problem facing the tube amplifiers of the day was the costly need to match the output tubes for proper balance of the circuit. Fender approached this problem with a modified output tube bias control which achieved balance at the expense of overall bias level. Between 1968 and 1973 other circuit features were used to achieve a sort of self balancing of the output, namely a combination of grid and cathode bias. All of these features were frowned upon by the musicians of that time, who generally preferred the more edgy tones of the original blackfaced amplifiers.
George wrote:First off, should I go black face or silver? What are major differences?
If silver, should I go before or after addition of master volume ('72, i think)? Any years to avoid?
Finally, what should I look for to make sure I'm getting a good amp and not something that is going to crap out on me? If buying on ebay, any specific questions to ask or things to look for or beware of?
Anyone here selling one or know of someone who is selling?
Many thanks in advance for any and all help and advice for my current acquisition project.
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