On this page, a Jennings 30 Watt Organ Amplifier, probably from c. 1955-1956, one of two currently known to survive (the other still in place in a "Model A" organ). Derek Underdown later used the circuit - his original design - as the basis for the power section of the single-speaker AC/30.

He drew from both it and the 50 Watt Organ Amplifier too when he came to design the power section of the Vox AC50 in the latter part of 1963.

Left to right, Haddon mains and output transformers and choke. As in the case of the Jennings Organ Power Pack no. 3 illustrated a few days ago, valve sockets and other hardware are for the most part riveted onto the chassis plinth.

Sockets left to right: mains input and audio out (to speakers); bias adjustment; a blanked off hole (for another pot?); signal in and mains out; volume preset pot.

Main filter capacitor, dated October 1954.

The circuit diagram (drawn by Derek) from a Jennings Organ Company Service Data handbook of 1958. Thanks to Steve for the image. The sheet is undated.

Some facts and figures: the chassis of the 30 Watt Amplifier was slightly larger than that of the Jennings Power Packs it accompanied. It measured: 13 1/8" long; 7 3/4" wide; and 2" tall. Sockets were provided for five valves though the circuit required only four: a GZ32 rectifer, two EL34s, and a 6SN7. The vacant space (corresponding with the fifth socket) can be seen in the photo above. Compare with the shots of the 30 Watt Amplifier in the "Model A" organ pictured below.

The fifth valve socket is completely unwired. It is likely that chassis were prepared in this way to allow for the assembly of a 50 Watt Amplifier, which did require five valves.

Below, a rear view of the Jennings "Model A" organ mentioned above, the 30 Watt Amplifier to the left of the central well for the volume pedal, a Power Pack no. 1 or no. 2, with a heater supply transformer finished in blue hammerite similar to that used for early AC1/15 chassis, to the right. The labour in these organs was naturally the wiring up.

Thanks to Tony, detail from a photo taken in the late 1990s / early 2000s.