KT88's vs. 6550

[HRobert]

Last fall I bought a 1967/68 Sunn Sentura II amp on eBay.  This is the second Sentura II I've owned - I got my first one new in 1968 when I was in high school.  If I remember correctly, my first Sentura had 6550 power tubes, but my present amp has KT88's, and the schematic printed off the web has KT88's shown.

 What is the difference between 6550 and KT88 power tubes?

Rob

[loudthud]

In the 1968 catalog the 60 and 120 watt amps were listed as having KT88s. In 1969 they were listed as having 6550s. The amps didn't change. I think that was the time Movalve in England stopped production of KT88s. Prices were going up, availability going down so Sunn switched to 6550s. The tube chart, a little piece of paper glued to the inside of the head box, should reflect this change.

The 6550 was really the American version of the KT88. The KT88 has slightly higher ratings. Some of that is due to different construction and some due to American manufacturers being more conservative. These days you can't really see any difference in the internal construction of the tubes (most examples) except the glass is different.

[EdBass]

I agree that it was a cost factor that probably caused the switch. I'm attaching the "official" Sunn reasoning in this letter from Robert Selby in March '69. IMO (and a whole bunch of other tube geek's opinion) the original Genelex KT88 was/is the holy grail of hi-fi output tubes, and the current pricing of a set, provided you can find a matched set, reflects that theory. I'm running a quad of re-issued Genelex Gold Lions in a Reeves Custom 225, they sound good but I only have about 8 hrs on them. One interesting note; I flamed one of them in about 3 hrs, which doesn't say much for the quality control.

[Soundmasterg]

I agree that it was a cost factor that probably caused the switch. I'm attaching the "official" Sunn reasoning in this letter from Robert Selby in March '69. IMO (and a whole bunch of other tube geek's opinion) the original Genelex KT88 was/is the holy grail of hi-fi output tubes, and the current pricing of a set, provided you can find a matched set, reflects that theory. I'm running a quad of re-issued Genelex Gold Lions in a Reeves Custom 225, they sound good but I only have about 8 hrs on them. One interesting note; I flamed one of them in about 3 hrs, which doesn't say much for the quality control.

Ed, I fixed a 2000S for a guy and he wanted me to put in those reissue Genelexes that he provided. Well after two sets from his dealer, he changed to JJ KT88's as the Genelexes had unstable bias and didn't keep their match for very long either. I've used the JJ's in my Sunns and Winged C in my vintage SVT and both have been rock solid and sound good.

Greg

[EdBass]

I have had good luck with JJ KT88's in my Sunn amps, I have them in a 2000S and a 200S with no issues. I would say they are a good "go to" tube, you pretty much know what you're getting and what they will do.
The hype for the new Genelex have intrigued me ever since I heard about them, and I've read reviews claiming outlandish performance akin to the original Gold Lions with out coming across any negative feedback. However, talk is cheap, and having first hand experience with British Genelex tubes I didn't blindly buy into the new "holy grail" propaganda, but I haven't been able to find anybody that I actually know that have tried a set.
Plus, they look really cool, so I finally broke down and ponied up for a matched quad.
I got a push when a Reeves customer ordered a Custom 225 with Gold Lions as an upgrade. One of the guys at Reeves (who had discussed the new Genelex with me previously) called me and told me that they were ordering some matched quads from the distributor, and did I want to add on to the order. 
JJ's are the factory KT88's in the Reeves, and I only have maybe 20 hrs on mine so the idea was that I break the Gold Lions in a little and then do some A/B comparisons. The bias held stable (last checked about 15 minutes before one red-plated  ) and my initial opinion was that the tubes sounded great.
I've got a couple of gigs on them so far, and if they turn out to be the "second coming" I'll post about it.

[biltmore]

I have had good luck with JJ KT88's in my Sunn amps, I have them in a 2000S and a 200S with no issues. I would say they are a good "go to" tube, you pretty much know what you're getting and what they will do.

+ 1

I<3 JJ Tubes

[Soundmasterg]

Ed, I think the initial run of the Genelex reissues had some problems. They did sound good, but then so do the JJ's. Jim McShane (tube dealer) really seems to like them and he's a standup guy, so maybe they're ok now. My 200S's JJ's are still going strong though (blue glass even!), and I haven't overhauled my 2000S yet, so it still has GE 6550A's in it along with original caps. I don't play it though.....one of these days I'll get around to overhauling it. Anyway, I'm happy with the JJ's so far and have no need to buy any new brands unless I repair an amp that calls for them and the owner wants to try them, and I don't get 6550/KT88 amps in often.

greg