For new production try Svetlana's (C logo's). Rugged and dependable. I've yet to have one fail prematurely.
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Those GE's are going to be hard to beat!!! However, if you haven't already done so, check out the Audioasylum (www.audioasylum.com). Look in the tubes section and do a search for 6550. You'll get lots of information and lots of opinions. If you don't mind spending the money, it will be very difficult to find anything better sounding or more reliable than NOS Tung-sol or GE.
Issue #19 of the Vacuum Tube Valley Journal has a review of 6550/KT88/KT90 tubes.
They ranked the sound from each variant in an Air Tight Amp. Unfortunately, I can't find my copy to give you all the results, but this was a very informative article. There were a couple of current production models that rated very well. One of these is the JJ KT88 for about $50 each.
I am still using NOS GE 6550A's so I can't give you comparison form my own experience, but I will try the JJ's when the time comes to retube.
Try to get a copy of the Journal...or maybe someone remembers the complete results and can relay them here.
Unless you don't play your amps very often, after close to 20 years of use on your old tubes, any of the better quality, compatible new production tubes out there will likely sound like a revelation compared to what you're hearing now (especially given around 40 hours or so of burn-in on the new set). I haven't tried them all, but the Svetlana 6550C's that Newbee mentions are quite smooth, extended, dimensional, clean'n'tight, and natural sounding, and have so far been perfectly durable for me. Nevertheless, I am also interested in trying either the Svet or JJ KT-88's at some point, which according to some might be a bit warmer, if not quite as tight and extended.
Thanks everyone. Particulary the recent article comparing 6550's sound interesting. My guess is that I have 1,000 to 2,000 hours after 20 years. I did not count. Recently listening as much as 10 hours per month. My voltages are very high 620 Volt. My repair shop said this makes it much harder to find a tube that will last. He said the Tung Sol are not very good with high voltage.
OK, now I've had one of my Svet 6550C's fail before its time was due. Luckily, I happened to walk back into the room and glance over toward the rig just as the tube began lighting up bright orange inside - I was cranking pretty loud at the time. I ran over and killed the volume and the amps. The music had actually continued to play without distortion right up until I doused it, but after allowing things to cool down and swapping out the light-show tube, the output fuse was blown when I tried to turn the amp back on. Fortunately, a new fuse took care of the situation - there were no blown resistors or anything. Why the tube went however, at only about 10 months old, I cannot say.
I just retubed my Conrad-Johnson Premier 11A amp with the new Svetlanta (now a three letter logo) 6550 C tubes. Wow, what a difference over the stock GE's. The tubestore has them in stock, got a 'perfect quad', which means the tubes are all matched to one plate voltage (52 in my case). This is a great feature if you have to adjust gain. The old tubes were not shot, but C-J recommends changing them every 3 years of normal use.