When I had my SVT-CL, it came with these tubes in it. They were put in by the previous owner.
Week after I got the amp, one of them blew.
Took it to my amp tech and he said it looked like a one-off tube failure.
Instead of just replacing one tube, I opted for a whole new sextet of the Tung Sol Reissue 6550s.
I have these Sovteks just sitting around. Amp tech said they are in great shape with plenty of life left. I don't know a lot about amp circuits, so I'm going on his word, and he's never steered me wrong before and I trust him completely.
So, for sale are the 5 working ones of the original matched sextet. The blown one is in the odd box in the picture and I'll send it too. Boxes are not original, but are the boxes from the Tung Sol Reissues I used in the re-tube. The tubes you are looking at are Sovtek.
Any questions, feel free to ask.
$60 OBO shipped to your door, and very well packed, in the lower 48 USA.