70s SVT - 12BH7 and 6550 Question

[bassboysam]

My amp blew a few screen resistors and a few of the power tubes gave up the ghost. I had Sovtek KT88s in there but I'm thinking of going with either Ruby 6550s (apparently very similar to the GE 6550) or JJ 6550s.

Not many people have experience with the JJ as they are fairly new, but can anyone comment on the Ruby. I know they are just selected and rebadged Sino/Shuguang tubes, will they last in an SVT? Chinese tubes are supposed to be much better than they were 10 years ago, but the SVT can be hard on tubes as well.

Also since I'm re-tubing the power section I figured I would get some fresh 12BH7s. Do they need to be matched for optimal performace, they each drive half of the power amp so my guess is that they should be matched so the both sided of the power amp are even but I could just be making things up too...

[coreyfyfe]

Having the 12BH7s matched to each other would be optimal in this amp, but if you can't find matched 12BH7s you can also use match 12AU7s, I have a nice NOS pair of them in mine. They may wear faster than 12BH7s in the long run but serve the same purpose.

As far as the Rubys it depends on which Rubys. I have a sextet that came to me in my SVT that are winged C with Ruby matching labels, so they may sell those as well. If they're rebadged shuguang or valve art tubes they would probably be ok. I personally haven't tried them in an SVT but have had poor experiences with their other tubes so I tend to steer clear.

[JGR]

Not true, matching triodes is pointless and a total waste of money - each side of the OT primary isn't balanced to begin with. This is a myth that came over from the HiFI world where they have bifilar wound trannies and are going for absolute lowest THD. And while we're on the subject of BS tube marketing, let's call bull on the cryo treated tubes too. Cryo treatment can affect the matrinsitic transformation which doesn't apply to most of the metals used in tube construction.

If you have the original 22 ohm screens, they really need be replaced with a larger value or your will likely kill more tubes. Ampeg later went to 220 but it wouldn't hurt to go to 500 or 1K to add a bit more protection to your substantial investment in glass.

[ddbassGA]

Originally Posted by JGR

... more protection to your substantial investment in glass.

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[bassboysam]

Thanks John, I'm pretty sure they are the original resistors in there so I'll look at upgrading those.