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The ACM3 was employed in the video modulator stages of the old high power Band I TV transmitters. The ACM stands for Air Cooled Modulator or AC filament Modulator Triode.

The cooling ring at the centre is for the anode, the grid is the brass tube at the bottom and the cathode is the tube at the top as well as being one side of the heater. The flying lead is the second heater connection. Construction is as concentric cylinders.

For its type it had a high mutual conductance. The anode ran at a few kV and the anode dissipation was of the order of 1,500 Watts, suggesting significant cooling effort would be required of the mounting arrangements.

Their characteristics made them very suitable as video modulators. An example given is for the Sutton Coldfield transmitter. Here four ACM3's were used in parallel in cathode follower configuration as the modulator output stage. The video swing was 1,200 Volts peak-to-peak at an output impedance of 11 Ω. All of this performance was delivered over a video bandwidth of 3 MHz, which as Ray says is not bad for those days.

Visually the anode ring to outer ring is filled with folded metal tape to form a high performance radiator in the same way as is used to the present day. Thus the valve was to be cooled by forced air blast.

Thanks to Ray Cooper for providing correct information on this exhibit.

The wide glass tube envelope is 83 mm in diameter and, including the base connections, is 229 mm tall.

Reference: Data-sheet & Ray Cooper. Type ACM3 was first introduced in 1954. See also 1954 adverts.

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.