6CA4 / EZ81, 6BW4 / 12BW4, and 6V4 / EZ80; far more "exotic" is the Bendix Red Bank 6754.
"Exotic" doesn't necessarily mean expensive, I snagged a few of these from AES, several years ago, for relatively cheap money.
6754s have a very slow warm-up, which is good for preamp applications, I built up an "exotic" VA-CF line stage using 6754 rectifier,
and WE396A tubes, the WE396As came from the "great barn plinker haul", several years ago, price was free, just "sweat equity", lugging them
down from the loft, into my car and then into my cellar. Dead bugs, spider webs and mouse-turds, included at no extra cost
.
http://frank.pocnet.net/sheets/131/6/6754.pdfhttp://frank.pocnet.net/sheets/127/6/6BW4.pdfhttp://frank.pocnet.net/sheets/127/6/6CA4.pdfhttp://frank.pocnet.net/sheets/155/e/EZ80.pdf5993 is the Bendix version of the 6X4, 7 pin miniature.
http://frank.pocnet.net/sheets/131/5/5993.pdf/ed B