Developed in 1952-1953, the EL84 was first used at JMI by Dick Denney in his AC15 (the AC1/15) of 1957/1958 and by Derek Underdown for the "Model H" organ also of 1957/1958. Both amplifiers were cathode biased.

Denney used cathode bias for the AC30 Twin too, the new design completed by April 1960. The EL84s initially were biased with an 82ohm resistor, later reduced (in 1963) to 50ohms. One also finds 47ohms, a fairly standard value for many resistor manufacturers. 47/50ohms runs the valves substantially harder than 82ohms, the aim being to extract as much power as possible from the EL84s within "bearable" limits.

Valves in cathode bias generally dissipate more power in the form of heat than those set up in fixed bias however - and AC30s do run hot, the heat reflecting back in particular on the mains transformer via the 6.3v supply lines to the EL84s. And this of course is in addition to the heat generated in the transformer itself by the fairly hefty current draw of the amp's power section.

Anyone who has played an AC30 for even a short while knows not to touch the metal vents.

Detail of JMI circuit diagram OS/065 for the AC30/6, the circuit originally drawn up in April 1960. Note the 50ohm resistor of the first design. Modifications were registered in a side panel on the drawing.

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