conusaudio.com red base Svetlanas
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Pretty much the 6550 gold standard is the Tungsol 6550 - the original was developed and made in the USA. It was introduced in 1955 and was specifically designed to provide inexpensive high power for home audio. There were three variants on the design. You can expect to pay about $75-125/tube though you will find less then perfect quads for less which is what I did. The tube was repackaged by Raytheon, RCA, Sylvania etc.
GE also makes a nice one, the 6550A which is a different design that was introduced in 1971 (these continued in production through the 80s), and Sylvania also introduced one at that time though the Tungsol is "the one". The reissued (new Sensor) ones have been out a couple of years now and are supposed to be quite good - they are certainly a whole lot less expensive - I haven't heard them.
The KT88 was developed across the pond in 1957 to cash in on the growing demand for high power. The Genalex is the undisputed king of this heap and are priced accordingly.
At least on my Cary, the EL34, 6550 and KT88 are interchangeable. Not sure about the 90's and the 100's.
the new Tung Sols aint the same animal as the old by everything i've heard about them. I just retubed with a matched set of winged C 6550s from upscale audio (and most of the matching was good to the second decimal point and only out by about 1.5 on the bias number at the worst out of 12 tubes)and the effect was killer. admittedly i swapped the driver tube (nos mullard at7) and inverter at the same time.