The High Vacuum Valve Company [probably] began production in 1932. They specialised in miniature valves for hearing aids and portable radios, although they did also make full sized valves of very high quality - e.g. this PX230. Their boxes were particularly attractive.
I have some data and/or prices for the following types:-
XD, XL Review, Curves, Advert 1, Advert 2, Advert 3
XSG Review and Curves, Advert 1, Advert 2, Advert 3
Here are thumbnails which link directly to the adverts, which are shown in approximately chronological order, and contain some data. They are all from the T&R Bulletin between 1933 and 1937. The Transmitting and Receiving Bulletin was the official journal of the Radio Society of Great Britain before the Second World War.
