Might be time to get them serviced. Tubes don`t last forever. Just because they light up doesn`t mean they`re putting out their rated power.
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Thanks Warnerwh for the input. I talked to Alfred at KR Audio products. He was really helpful, and given that one tube has gone, and at least one other is on it's way out, I ordered a fresh matched quad of tubes. He recommended the KR 842 tube though, to better drive by average-efficiency Virgo IIs (89db, 4 ohm). Still, it is very worrying that two tubes in two seperate monoblocks with a supposed life of 4k hours would appear to blow at the same time - coincidence, or more likely some other factor at play - I"m just worried about some other power issue that may plague my amps even when I get the new tubes up and running. Anyway, any input very welcome.
Your situation is not unique. Could this simply be that the tubes have reached the end of their life? In just the last 2 months I have also had 2 power tubes fail in my CAT JL-3 amps. These tubes just happen to be 2 years old. And with previous ARC amps, after a couple years, I have learned that once one tube goes, others soon follow. If the tubes are manufactured by the same company at the same time, it stands to reason that they should all pretty much fail at the same time. So I suspect you are due for a new set of tubes for both amps.
It's like Ken Stevens of Convergent Audio recently wrote to me: when one tube goes, you should replace the whole set. When your car needs a new tire do you buy only one?
And if tube replacement is only $1060, consider yourself lucky. It's a whole lot more expensive for me!
As for the issue of power conditioning, I strongly doubt this has anything to do with your issue here.
Thanks for the advice and input - all good stuff. Hopefully the problem is isolated to tubes that are on their last legs. In which case my new quad should get me up and running again with no other issues to worry about. it looks like I'll need to have the amps manually biased when I get the tubes, in which case I'll get the technition to evaluate the voltage too - good tip that one (the high voltage issue is likely one that killed a tube in my CRT projector a few months back ;-) Damn, these hobbies can get expensive - a labor of love I guess.
In your case I think is time to buy new tube's, since your are the second owner and you been enjoying the amps for a couple of years now using 89db speakers which I think you be run out the tube's life quicker than a 92db speakers. Also Alfred at KR Audio products is a very nice guy to deal with and is a really helpful guy too.
I'm not a PRO but you should keep your amp's in the same wall outlet if possible, that way you get the same result. Later