The output transformer is a huge and overspec'd Hammond (like amptech said, Garnets are solid) - not a problem. I will have to change the tube retainers out though (bear traps) - that's no big deal.
I was gonna use JJs; I find them consistently reliable and good-sounding.
Quite sure me tech said plate V not screen (he said 520V wouldn't be an issue there - the amp has them both at 520).
The tech wasn't referring to any data sheets or schems for the tubes or amp just talking generally. I showed him the schem for the power (amp and supply) sections of the amp after he told me that over 500V on the plate would be an issue, but a lot of other people that go to him use lower quality tubes so he may be going off that experience.
I've had the thing gone over so I know the caps are fine (no tonal issues - sounds great at jam and in the studio, just live I have to push it too hard sometimes, still sounds good, but it's further into the dirt zone than I would like) - like I said the pair of 6CA7s are ancient (can't remember if I have ever changed them, I doubt it because I probably would have used EL34s as 6CA7s, until recently, were harder to find, unless you went NOS). I did have the phase splitter/driver tube go recently - sorted that one out myself (amp sounded fine but 1/4 the volume; all 4 preamp channels affected equally so I though power tubes initially because they're so old but that wasn't the case - sounds right now.... though I am preparing for those power tubes to die any time now due to age, which is why this whole KT88 thing came to the fore). I know that that (driver tube going) can be a sign of other issues, but the thing has been piping along happily for 2 months since so I am not concerned. Funny thing is that nearly the same thing happened to my other (smaller - 6L6) Garnet around the same time. They are around the same vintage, but I suspect the smaller one has been used more heavily (had to change the power tubes a few years ago).
...anyway, looking at the spec sheet seems like a good idea, but am I reading this right - the JJ KT88 is supposed to handle up to 800V on the plate and 600V screen?
http://free-hosting.infodiv.com/audiotube/power_tetrode/power_tetrode0034/power_tetrode0034-01.htmlSovtek says 800V max too....
Looking at the JJ 6CA7 datasheet I see the same limiting values (and therefore no reason why KT88s wouldn't be OK in place of my 6CA7s, but I ain't no expert).
http://www.jjtubes.eu/image/data/powertubes/datasheet/6CA7-data-sheet.jpgDon't mess with something you like the sound of; just get more of it...
There is no more to be had; that's my problem.
Your guy must have been confusing KT88's with KT66. KT-88's are much more robust than EL34's. Go ahead and try them. If you're looking for more headroom, I think you'll like the results.
I told him the reason was lack of headroom so I doubt he would have confused KT88s with 66s but it's possible, old hippie and all that. ... actually looking at general specs online that does seem likely (max plate V for KT66 listed at 550V). I'll double-check that with him. Thanks!