Bottlehead just mentioned the 3B7 in one of their custom 2A3 amps, for all-DHT operation.
Loctal base, glass envelope, 1.4 / 2.8VDC filament @ 220/110 mA.
http://frank.pocnet.net/sheets/127/3/3B7.pdf
Looks like it could have some other interesting uses.
/edB