The 210HL from Cossor was listed in their 1935-6 manual and we have an advert for it from 1931. Cossor say of the 210HL that it probably the most popular valve in their battery series for use as a detector.
Its characteristics are such as to suit it for use as a leaky grid detector in combination with small LF transformers. Under these conditions sufficient output is obtained to load a small output pentode or triode before overload occurs. In addition, the anode consumption is small, a consideration in receivers using HT batteries.
The valve, in common with most other Cossor battery valves for use as HF or IF amplifiers, or as detectors, is fitted with seven point filament suspension. No microphony need therefore be feared as filament vibration is completely damped.
Cossor describe the 210HF as similar but with greater gain.
The balloon envelope is 40 mm in diameter and, excluding theB4 base pins, is 81 mm tall.
References: Datasheet & 1040. Type 210HL was first introduced in 1931.