Sounds like a good plan on the surface, except for one small detail...power tubes in SVT's are tied together in triplet sets, not pairs
But hey, doesn't matter to you now.
Back then, it did not matter if all six are matched. What matters is that each pair matches, working from both sides, and if there is a single bias, the "happy medium" was selected when trimming the pot. And back then, there was more consistency between tubes. If a player kept with the same brand of tubes, the amp did not need to be re-biased every time a tube was changed, only if after a few years there was some "drift," and a little bit of distortion started to be noticed when the amp was cranked. Notice I said "a tube," because it didn't matter. Tubes were consistent. "Matched pairs" from a marketing standpoint is an early '80's sort of thing starting with Groove Tubes, after the USA tube factories had shut down and everything had to be sourced from Russia and China. Initially, quality was so bad some tubes would flash out the first time they were turned on, so they had to be tested and matched. So the "matching" has gone from being a way to make sure you got a pair of tubes that would actually work, to now being a religion.
I never cared for the tone of 6550's other than in an original run SVT. The original SVT preamp stack, including such gems as 12DW7's, was the only preamp that I thought tamed the harshness. I preferred, for guitar purposes, Sylvania big-bottle 6CA7's in my 1975 Marshall first-year 2203. They have 6550 plates, but a different grid structure, so they bias like EL34's, but are cleaner, like a good RCA 6L6GT, but with more cajones. I prefer the clarity over the Mullard midrange, um, er, "complexity" that Les Paul beaters prefer. I never bothered with KT66's or KT88's, and the other lesser used octal beam power tubes, like 7027's, because of other wiring upstream, including bias, that had to be done to make them sound their best, and because some of the KT66's & KT88's are simply re-branded 6L6's and 6550's, and are not worth the premium.
You're right, partially. When I sold the SVT because I didn't want to carry it around anymore, the kicker was when it was due for a complete re-tube, and when I priced the tubes then, twenty-odd years ago, the cost approached what I paid for the head used. Instead, I purchased a Carvin PB300 that was my only head for the next twenty years, until I decided I did not want to carry it around, either. Now I have a class D Carvin MB12 combo on top of a homemade 1X15 on casters.
I stand by my statements and recommendations. There is no reason to purchase all six power tubes if only one goes bad. Only a pair is needed, and bias just really is not that critical, although it is obviously important for efficiency and tube life. So if the mantra is going to be purchase all six and match them every time, then here is the link to the RCA tube manual bible. 6550's are described on pages 483-484. You can read for yourself the wide parameters they can operate under.
Invalid Link Removed If a player is that picky, he can read the numbers off the labels on the other four, and order his replacement pair accordingly from one of the on-line specialists, like The Tube Store or Tube Depot. Then go back and read page 9 of the SVT-CL manual and read about how the bias can be set on a pretty wide range, depending on what the player wants out of the amp. Or, you can go here and do it the old fashioned way:
http://ampworkshop.yuku.com/topic/1976/Biasing-an-Ampeg-SVT#.VDotZGPu3IU . Notice all they say is get good tubes; they do not obsess on how they match, because the biasing will take care of it. I always biased mine a little cooler so the tubes would last longer.
My SVT was as reliable as the sun rise, and that is how my tech and I took care of it. The only reasons I sold it were 1) I didn't want to spend the money keep feeding it, and 2) it was heavy, and I hadn't had children yet.
On re-reading the above posts, it is very apparent that nobody has bothered to mention, just taking it for granted, or worse, misinterpreted, that when you replace the pair, you replace the blown tube and its counterpart on the opposite side of the stack, not two that are side-by-side, unless it is the pair in the absolute middle. So if the outside tube on one side is the blown one, you purchase the pair, and replace the outside tubes on both sides. If it is the #2 tube in from one side or the other, you replace both #2 tubes counting in from both sides, and if it is a tube in the middle, you replace the two middle tubes so the push-pull load remains balanced so there is actually something there to bias.
