The old Tung-Sols were always very strong tubes and could often exceed their ratings and hold up. The GE 6550's were not very good as a comparison, so they came out with the GE 6550A, which was better, but they still don't hold up all that well when their screen voltage ratings are exceeded. They sound very good in an Ampeg SVT, but while these have a 700v plate voltage, the screens are pretty much hard set down around 300 to 400v, and thats within the ratings of the GE tube and they work well and sound fine there. The Sunn stuff often has a plate and screen voltage around 560v, and that is often too much for the GE 6550A, so they don't hold up well in this application. Some of the later GE 6550A tubes have pretty spotty quality....late 80's especially.
In modern 6550 types, the Winged C 6550C sounds quite good, and works well in an SVT, but again, doesn't like the Sunn circuit very much because of the high screen voltages. Much the same story with the other modern 6550 types out there...Chinese and Sovtek/EH ones. The new russian-made Tung-Sol 6550 is supposed to be very good at holding up under punishment, but I don't have any direct experience with those yet. The EI KT90 is very robust and decent sounding, but prewar is MUCH better than postwar in quality. A lot of the hi-fi guys like these. Pretty much any of the new KT88's will hold up just fine in the Sunn circuit, with the JJ being exceptional and very nice sounding in this application. Bob at Eurotubes is a big Sunn freak and sells only JJ tubes, and of course he uses them in his amps too. The KT88 isn';t all that much more expensive than the 6550, and since they last longer and sound as good or better, that is the route I choose to go with my Sunns. Besides, original issue on the Sunns was either Tung-Sol 6550's or Genelex KT88's, depending on the era. Can't go wrong with either choice.
