The EL37 audio output valve is one of the classic valves and its predecessor was the EL35. The EL37 was rendered obsolete in 1954 with the introduction of the EL34 that also has a gm of 11.0 mA/V and a 25 Watt anode dissipation.
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 and EL31 are both 25 Watt anode dissipation output valves from the mid 1940s. The main difference is that the EL31 has the anode brought to a top cap as the maximum anode voltage is 800 Volts and it is the extra anode voltage that gives the higher 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 sensibly equivalent to. At 11.0 mA/V it is almost twice the value of the KT66 and the 6L6G. 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 Ω in both cases. 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.
Seen through the coating on the inside of the glass is the anode placed well above the pinch.
The pair of top micas with lower one holding the tops of the electrodes and the upper one gripping the shoulder of the envelope. The cathode tube is oval and the control grid is wound with copper wire unlike the EL37 that we took apart. Note also the large heat-sink fins on the top of the control grid supports.
The classic envelope is 51 mm in diameter and, excluding the IO base pins, is 118 mm tall.
References: Data-sheet, 1040 & 1043. Type EL37 was first introduced in 1946. See also1946 adverts.
