The CV3998 is a high-slope RF beam tetrode primarily intended for use in wideband amplifiers in telephone carrier systems, radar equipment and measuring equipment. The CV specification shows that the Government requirement was for the GPO telephone system. The prototypes were the 5A/170K from STC and the E180F from Mullard.
This very small size helps to reduce inter electrode capacitance and thereby raises the maximum usable frequency at which the device can operate.
The lower part of the heater wire is coiled. Within the insulation the coil will be close wound. The STC logo and the commercial Type 5A/170K is also visible.
The anode looks indented but is in fact flat.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 33 mm tall.
References: Data-sheet & 1040. Type CV3998 was first introduced in 1957.