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The MH41 is a mains triode that has the 1930s UK standard 4 Volt 1 Amp heater. The valve is a low impedance device and designed for audio frequencies.

The MH41 was a uprated MH4 and was M-OV's answer to the Mullard Type 994V as a high-gain detector triode. Unfortunately, by the time the MH41 hit the market the real need (for modern superhets) was for double-diode-triodes (eg. the M-OV MHD4, etc.).

However, the MH41 saw service as an audio pre-amplifier and in professional applications. MH4s and early MH41s were inclined to be a bit hummy for high-gain use so a low-noise triode, the MH40, was also produced for professional users. Later MH41s were redesigned to reduce hum and noise and were used by the BBC and other professional organisations.

The reason for the mesh anode is that fairly large amounts of gas are evolved when the coated cathode surface is activated during manufacture. Very much smaller amounts may be evolved when a valve is first used after a prolonged period of disuse. Making the electrode structure porous to gases helps evolved gas to flow away from the active regions of the valve and towards the pump stem or the internal getter, as the case may be. Even some modern valves have openings in the anode structure for similar reasons.

The difficulty of keeping metal loaded paint on a glass substrate can be seen in this exhibit where most of the zinc loaded paint has broken away.

The mesh anode.

The remains of the printed Marconi logo.

Here the heater can be seen to be double wound turn for turn to cancel out any generated magnetic fields and thus reduce hum. The heater is threaded with an insulated core. This bifilar winding was used in audio specific voltage amplifiers like the EF37A.

The shaped mesh anode.

The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 97 mm tall.

References: Data-sheet & ¹⁰⁴⁰. Type MH41 was first introduced in 1933. See also 1933 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.