The VT61A transmitting double triode was designed for use up to 150 MHz. It has the Air Ministry markings. The VT prefix is an Air Force designation in use before the CV register was introduced in 1941.
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 A M and Crown of the Air Ministry.
The twin triode sections with the grids taken to the top caps.
The anodes are partly blackened and the control grids can be seen to be wound on copper rods with heat radiating fins at the top. The closed cathode tube has a reduced diameter at the top that passes into the top mica. This allows a greater separation between the holes in the mica and thus is mechanically stronger. The heater is double helix wound on a central ceramic rod - usually reserved for audio work where cancelling out the heater's magnetic field reduces induced hum.
The classic envelope is 45 mm in diameter and, excluding the B5 base pins, is 102 mm tall.
