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The 6550 is the output tube used in Leslie model 122, 147 and various other amplifiers. (Actually, they're used in pairs.)
For pictures of the guts of one, see 
http://www.dominik-termeer.de/6550.html.
The 6550 tube was developed by Tung-Sol Electric, Inc. as a high-output audio tube in the mid-1950's. 6550's might be found in Leslie amps with the Leslie private-label brand applied but they can be identified as Tung Sols by the EIA (Electronic Industries Association) manufacturer's code "322." _NewOldStock_ ("NOS") Tung-Sol 6550s are widely sought-after and command high prices.
A variant of the tube, the 6550A, was developed by G-E. The G-E tubes can be differentiated from the Tung Sol tubes by shape: The G-E tubes have a straight-sided glass envelope; Tung-Sols have the cooler-looking "shouldered" or "Coke-bottle" envelope.
6550 tubes are no longer manufactured in the US, but are still being produced in Russian factories.
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