Power Pack

The power pack provided stabilised voltages for the various circuits (generators, oscillators, vibrato, and so on); the amplifier amplified the resultant signal. The unit in view is a "Power Pack no. 3", designed by Derek Underdown for the colossal "Model G" organ in late 1956. The circuit diagram was drawn out neat in August 1957 and modified in 1958.

The valves in the power pack are: GZ34 rectifier; two EL34s; a ECC83 (12AX7); and a Mullard 85A2 voltage reference valve. These provided two stabilised (regulated) voltages: 300v for the filters/voicing circuits and generators; and 500v for functions at present unknown (perhaps for the operation of the pistons and couplers).

Sept. 2026: - a further Power Pack no. 3 has recently come to light, details to follow.

50 Watt Amplifier

Below, the circuit diagram for the power amplifier, drawn up in 1957, the date unfortunately cropped away in the shot below. It will be supplied from an original copy in due course.

The valves in the amplifier are: 6X5 rectifier; GZ34 rectifier; two EL34s; and a 6SN7 phase inverter.

Just as Derek used the 30 Watt Organ Amplifier as the basis for the new single speaker AC/30 in 1959, he used elements of the 50 Watt Amplifier for the new AC50 (late 1963) - see this page on the AC50 website.

Power pack at the top of the picture, amplifier near field.

Amplifier, Haddon mains and output transformers, and choke.

Amplifier chassis from above

Amplifier, under chassis.

Power Pack

Power Pack, under chassis.