Today, a question from South Korea - what is the best Mullard input valve for a Leak TL12.1.  Well, you can fit as per original, the EF36 or the improved version, the EF37 or the super improved version the EF37A or the mega special version the ME1400. 

The Mullard Type EF36 was designed and introduced to market in 1940 as a general-purpose RF/IF/AF pentode with a non variable mu characteristic.    There were two famous uses for the EF36, firstly, Colossus, Tommy Flower's computer at Bletchley Park used rows n rows of them then of course in 1947 the Leak TL12.1 used them as the input valve.  It was found that when used as a high-gain audio amplifier the EF36 proved to be microphonic.

The EF36 was developed further and a sturdier version, the EF37 was developed from it and introduced in 1946.  The problems still continued especially as the valve aged and gave an unacceptable  hum and a high noise floor so, it was back to the drawing board again.

 The EF37A was introduced in 1950 especially for made for sensitive applications and was hailed as a low noise non microphonic valve.  By careful redesign of the ‘hairpin’ heater such that it took a bi-filar form, induced mains hum was minimised.  The hum was reduced further to minimal levels with a beefed uo electrode cage in the designed for electrometer applications variant much loved by the GPO, the ME1400.

All three types have the same nominal characteristics and when Mullard's finally got their design and production problems sorted they made only EF37A's and supplied these as replacements for the other two types.