The CV2798 is the commercial QQV03-10 designed by Mullard. For UK Government service it was issued a CV number and thus an open specification so that it could be made by a range of suppliers.
This valve type was much used for mobile VHF transmitters, as well as drivers for the larger power valves, such as the QQV06-40A.
We had a broken copy of this valve which allowed us to take a look inside. The outside of the anode is a uniform grey to dissipate heat. The working face of the anode can be seen to be bright. The anode was also found to be attracted to a magnet.
The grids are fixed to substantial copper rods. There are four supports close to the cathode as the central core is surrounded by two separate control grids and a common screen grid.
The visible grid is the screen as the control grid is exactly aligned with it and very close to the rectangular box cathode.
The heater is formed as two coated coiled hairpins within the cathode tube. In keeping with the Mullard practice the heater is centre tapped to enable operation from 12.6 volts as well as the normal 6.3V, thus making them directly suitable for operation from vehicle batteries.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 70 mm tall.
References: Data-sheet & 1040. Type CV2798 was first introduced in 1957.