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This ML4 looks to be an early example as it is enclosed in a balloon envelope in the same way as ML4 which is labelled Marconi. We have another ML4 in the later classic envelope.

Although sometimes listed as a triode output valve, the ML4 was the definitive descendent of the famous (or infamous) Type KL1 of 1926 and was primarily a professional and military type, little used in domestic radios. This exhibit is labelled Osram and this was a domestic brand of M-OV with Marconi being the brand of choice for professional valves.

The ML4 was quite widely used as an RF oscillator, as a single-valve phase splitter, and as a driver for larger valves.

This view of the dome shows the printed identification. The top mica shows that the electrodes are essentially a flat assembly. The outer supports are for the mesh anode and the inner supports are for the grid wire. The centre electrode is the oval cathode tube. The heater lies within.

This view of the envelope shows the pinch and the long leads from external pins to the electrodes themselves. Also visible is the Osram label and the heater leads entering the cathode tube.

The mesh anode, grid, cathode tube and heater are all visible.

The mesh anode seen full face. On the left of the picture the insulation on the heater leads can be seen.

The balloon envelope is 50 mm in diameter and, excluding the B5 base pins, is 111 mm tall.

References: Data-sheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type ML4 was first introduced in 1929. See also 1929 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.