Type VMS4B typifies the swansong of the screen-grid receiving valve with anode top cap. It is an improved, vari-mu RF/IF tetrode intended for domestic superhet receivers but it represents the final milestone on the road which started (barely 10 years earlier) with the Type S625.
From this point on the screen-grid type was superseded by kinkless tetrode or pentode types based on American prototypes and with grid top caps.
The main advantage of the pentode or kinkless tetrode in comparison with the simpler screen-grid type was the much larger undistorted output signal handling capacity, not limited by the characteristic screen-grid kink.
This meant that kinkless valves could easily handle IF signal levels of 40 V or more without objectionable intermodulation distortion. An IF signal level of 40 V may sound excessive but it can be needed if, say, -40 V of AGC bias has to be generated in the detector stage in order to suppress an over-loud local transmitter.
The identification is within the standard M-OV ellipse but too feint to be seen.
The lower screen. The four stands of the heater can be seen and the two sets of grid supports. The mesh anode can also be seen although it is not the main focus of the image. Close-ups have a very small depth of focus, and the glass envelope does not help the resolving of fine details.
The top mica showing the bridge across the top of the anode and the connection to the top cap.
The classic envelope is 43 mm in diameter and, excluding the B5 base pins, is 122 mm tall.
References: Data-sheet & 1005 Type VMS4B was first introduced in 1933. See also1933 adverts.
