The CV3990 is a small VHF transmitting beam tetrode that was typically used in mobile transmitters. As a single ended Class C RF amplifier it would generate up to 5.5 Watts in the VHF range to 125 MHz. If used in push-pull with up to 500 volts on the anodes the pair could produce up to 50 Watts but 27 would be more typical. The maximum dissipations of 10 Watts anode and 2.5 Watts for the screen had to be observed and so it is probably that they would have been used for telephony rather than CW.
As with other transmitting valves the design allowed them to be used as AF modulators as well. For AM modulation the CV3990 would be used as either class AB1 push-pull or single ended for low powers. With this dual use the transmitter would need fewer valve types as spares. This was particularly required in mobile communications.
The box anode looking along the grid axis.
The reverse clearly shows this to be a Mullard 2E26 made in Britain.
The screening box above the top mica is secured to the beam plates.
The modulator from a Plessey PTR175 aircraft transceiver. The CV3990 and a 2E26 as the power stage. The modulation transformer is to the lower right. The module measures 112 mm square and 74 mm front to back.
The rear plate shows the layout of the driver components. All the valves are small wire-ended types.
The main valves operatated with 425 Volts on the anodes.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 76 mm tall.