The MSP4 amplifier pentode was released in 1933 and produced in two versions, we have the B7 type but a B5 base alternative was available, also released in 1933. The MSP4 was essentially a redesigned pentode version of the MS4B.
It has slightly straighter characteristics and therefore performs better than the MS4B in applications where the output signal is large, as in superhets using a double-diode pentode in the output stage. It was also suitable as a general-purpose RF or AF amplifier.
By the time the MSP4 was introduced in the mid 1930s it was already obsolescent for use in domestic radios since by that time AGC was de rigeur and vari-mu types (such as the VMP4) were required instead.
It had some applications in early TV receivers but its gm was too low. EMI used this valve in their early pre-war sets.
An improved version (MSP41) was marketed but had few takers since the real requirement was for high-slope types such as the KTZ41 (which had a nominal gm of 12.0). The MSP4 was used professionally to some extent but was over shadowed by such valves as the AC/SP1. All in all, the MSP4 was something of an ephemeral also-ran.
The Marconi paper label.
The internal coating makes photography difficult. Inside the top the screening is visible but little else.
The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 105 mm tall.
References: Data-sheet, private communication & 1043. Type MSP4 was first introduced in 1933.