The VT61A transmitting double triode was designed for use up to 150 MHz. It was made by Rogers of Canada and carries the Air Ministry markings. The VT prefix is an Air Force designation, also on the envelope is the old stores code of 10E/142.
The original VT61 is on a UX7 American base, but for the VT61A the valve is mounted on a B5 base.
The operating parameters, but not the heater values, given are from the 4074A and this was a UX7 based valve as this is the closest match to the VT61A we could find. The 4074B is a B5 based valve of pinch construction in a classic glass envelope. The anodes are brought out to separate top caps.
Triodes were favoured for transmitter work over tetrodes because of lower capacitances and an ability to be driven harder. This valve was designed to pass a grid current of 17 mA.
The twin triode sections, anodes connect to the top caps and the control grids have cooling fins. The plastic base carries the crown and letters AM for UK Air Ministry.
Face view of one triode. The cathode tube has a seam and the heater is a single hairpin.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 109 mm tall.