12DT8 Valve

[Mike Watterson]

Evidently there is  a 0.1amp heater version of the ECC85 or 6AQ8.

The HCC85:  https://www.radiomuseum.org/tubes/tube_hcc85.html

Geordie McBoyne.

Not 0.1A
That's the 0.15A for USA/Japan 110V markets, usually actually a 17EW8 (Click on 16=1963 etc)

The 0.1A for 220V-250V is the UCC85
https://www.radiomuseum.org/tubes/tube_ucc85.html

I had one of these. Any Series heater AM/FM Radio set in UK from 1955 would likely use UCC85. They'd all be 100mA (TVs P series, 0.3A, so there is a PCC85).
https://www.radiomuseum.org/r/ekco_u243_u_243.html

The total heater voltage gets too high for 110V countries using 100mA series on AM/FM radio. The 200mA was pre-WWII C series for AM only sets.