|Phase Inverter||7025 (1/2)|
|Vibrato||7025 (1/2) 12AU7, 2 varistors|
|Output||25 watts peak|
The 421 is a single channel and tone control. While the smaller 401 and 411 amplifiers have tremolo, the 421 has both tremolo and vibrato. The tremolo uses the oscillator to vary the bias of the power pentodes. A tremolo/vibrato switch moves the oscillator over to the two varistor based vibrato used on the Custom Series M4, M6, and M8. While the rest of the circuit is somewhat economy engineered, the advanced vibrato is clearly the 421's strong point.
The triode section of the 6GW8 is used as the oscillator for the vibrato.
The early amps had gold control panels, but this was changed to black in 1965. In 1966, the 421 became the 422.