This ML4 looks to be an early example as it is enclosed in a balloon envelope. 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.

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

As close a view as is possible with the Olympus E20 plus 4 dioptre lenses.

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

The Marconi Valve logo is etched into the envelope as is the BVA logo.

This close-up was taken with a new Nikon D80 camera. The electrodes are fully revealed.

The balloon envelope is 50 mm in diameter and, excluding the B5 base pins, is 105 mm tall.

 
Pin Connections

B5	1	2	3	4	5
	a	g	h	h	k

 
Absolute Maximum Operating Conditions

Triode	Vh	Ah	Va	Vg	mAa	ra	gm	Pout
	4.0	1.0	200	-13.5	17	3,700	2.7	0.65W

Thanks to Frank Philipse for supplying the above datasheet

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This exhibit was last updated on 02 December 2007 {find valves by name}