The extremely rare Rev A (first produced on June 20, 1967) with serial numbers 101-125 is the original 1176 design and features a silver brushed-aluminum face and blue stripe. It has a 600-ohm input transformer made by Peerless that was later replaced with a UTC unit. The signal preamp after the gain reduction FET uses a FET followed by a bipolar transistor amplifier circuit and the Output level control. The Output level control feeds essentially the same circuit again, but follows it with a 2N3053 transistor line amp operating in Class A. The original UA-5002 output transformer has a split secondary, a tertiary winding for negative feedback and a separate emitter winding. This transformer was revised in following units to the improved UA-5002A and is the same transformer that is used in the 1176LN reissue.
Rev AB (November 20, 1967, serial numbers 125-216) features several changed resistor values in the signal preamp stages, which improve stability as well as noise. Also added are bypass capacitors around the resistor feeding the gain-reduction FET.
Rev B (November 1967 to January 1970, serial numbers 217-1078) includes an all bipolar design to replace the FET-based circuit in the signal preamp section. A tap off the emitter of the first transistor of this stage provides feedback back to the FET gain reduction stage.
Rev C (January 9, 1970, serial numbers 1079-1238) introduces the first of the black-face models and the LN suffix, which stands for low noise. An additional circuit designed by Brad Plunkett (and epoxy-encapsulated to keep it secret, pending a patent filing) reduces the drain-to-source voltage on the gain-reduction FET to keep it within its linear range designation. The LN circuit epoxied module and the Q-bias pot are soldered and mounted directly onto a Rev B circuit board.
Rev D (into 1973, serial numbers 1239-2331) has no circuit changes but features a redesigned main printed circuit board incorporating the LN circuitry and Q-bias pot.
Rev E (up to March 15, 1973, serial numbers 2332-2611) adds a power transformer that can be switched between 110V and 220V mains operation.
Rev F (March 15, 1973, onward, serial numbers 2611-7052) changes the output amplifier from the original Class-A design to a push-pull configuration (based on the 1109 preamp) that provides more output drive. Also, the metering circuit uses an op amp that simplifies meter calibration and final testing.
Rev G (production dates unknown, serial numbers 7053-7651) replaces the input transformer with a differential amplifier. (Op amps lowered manufacturing costs and were very popular solutions for audio manufacturers at the time.)
Rev H (production dates unknown, serial numbers 7652 and above) changes the front panel to silver.
