The EL37 audio output valve is one of the classic valves. At a time of generally low power audio amplification, these valves used in push pull could deliver up to 69 Watts at 2.5% distortion. The EL37 appears to be the successor to the EL31, albeit at a lower voltage and output power. Mullard also described the EL37 as suitable for use as drivers for large triode push-pull output stages. The EL37 has a high value of mutual conductance compared with the valves it is sensibily equivalent to. At 11.0 mA/V it is almost twice the value of the KT66 and the 6L6. A Mullard engineer said of the EL37 'that unlike similar valves it only needed a whiff of signal to drive it to full output. A single triode was all that was required to drive it'.

Mullard gave example operating conditions that produced 20 Watts with a 250 Volt HT rail, and 35 Watts with a 325 Volt HT rail. The anode to anode transformer impedance would have been 4,000 Ohms in both cases. The anode dissipation was 25 Watts. This valve was designed for operation as a pentode but it was popular to operate audio valves as triodes for lower distortion. The ultra linear or distributed loading concept was not developed when these valves were used for new designs.

This valve was rendered obsolete in 1954 with the introduction of the EL34 that also has a gm of 11.0 mA/V.

The classic envelope is 47 mm in diameter and, excluding the IO base pins, is 114 mm tall.

 
Pin Connections

IO	1	2	3	4	5	6	7	8
	m	h	a	g2	g1	nc	h	k,g3

 
Absolute Maximum Operating Conditions

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm	Pout	D
	6.3	1.4	250	250	-13.5	100	13.5	13,500	11	10.5	10%

Thanks to Frank Philipse for supplying the above datasheet

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