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.
Looking into the anode the screen grid is seen with the wires passing flat across the face of the cathode. The beam plates are end-on in this image. The heater can be seen to be made from an insulated coil.
The strap joining the two halves of the anode is on the left.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 37 mm tall.
References: Data-sheet & 1040. Type CV3998 was first introduced in 1957. See also1957 adverts.
