This magnificent valve is a cooled anode modulator. The CAM valves were developed from the CAT, cooled anode transmitter, valves of similar design. The CAM 3 being based on the CAT6 transmitting valve.
The CAM3 was designed for modulation stages in high power amplitude modulation transmitters. It runs in class A and was designed to be very linear. It is believed that this exhibit was used in the BBC Radio 4 long wave transmitter at Droitwich.
The close-up shows how these valves were constructed. The nuts and bolts testify to them being hand assembled. The re-entrant top provides an internal glass cylinder to hold the grid assembly. The rings at the top of the envelope and the top of the anode are anti-corona rings. A third such ring would have been mounted on the grid side contact.
The filament is of pure tungsten. It operates as a bright emitter and the expansion due to temperature increase is balanced by the tension spring seen in the centre of the image. Pure Tungsten provided stable and predictable characteristics, and fixed transmitters moved away from pure tungsten bright filaments in the late 1950s when the longevity of alternatives had been established, and the savings in electricity costs were important.
The valve has a glass to metal seal and the top of the copper anode can be seen inside the envelope. In use the anode was contained in a water jacket. The water used was purified to reduce electrical conductivity, and minimise hot-spots caused by the adherence of impurities to the anode.
With the electrodes being fixed only at the top the valves had to be transported and stored vertically.
During manufacture they would be x-rayed to check for accurate alignment of the filament basket and grid cage. such an x-ray photograph is shown above.
The glass envelope is 70 mm in diameter and, tip to tip the valve measures 710 mm tall.
References: Datasheet & 1005. Type CAM3 was first introduced in 1929.