The W148 is a variable μ amplifier pentode. It is a high impedance device with a medium mutual conductance. Variable μ devices are primarily designed for IF and RF amplifiers in radio receivers. The W148 in particular, was designed for use in the second IF stage of conventional domestic AC superhets, and in car radios. The 'W' prefix was used by the Marconi/Marconiphone/M-OV /Osram brands to indicate a vari-μ RF or IF pentode.
The W148 was copied from the US/International Type 7H7 but renamed W148 to conform with the house nomenclature. It also had the effect that the uninformed punter needing a spare for his set would have to buy one at BVA cartel prices rather than a 7H7 at a more competitive price.
The anode current is quite high and with the large electrodes, this suggests that the device may have been designed for low noise input stages of HF communications receivers.
The base shell has rusted. Within the solid screen hides most of the details but the dark anode can be seen and on the top right is the outer grid support.
Here the locating pip is on the left and within can be seen the copper of the control grid supports and the notch supporting the wire of the suppressor grid.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 58 mm tall.
