Tube Replacement: Single, Pair, or Quad?

I think it may be something very simple like how you are doing the the bias adjustment. Especially if (you didn't say) the 45 minute "blow time" is unrelated to listening levels. In other words, something besides the driving voltage is causing the tubes to begin drawing too much power after warm-up. (I could be wrong, but.....)

You should adjust the bias with the speakers connected, but the inputs disconnected, and only after the amp has been running for 20 minutes or so. If you don't do it this way, you may be inadvertantly setting the bias too high. Also, the bias for 6550's and KI88's is generally somewhere around 35-45 ma, Does the manual specify 150? I understand what Newbee says about 150ma for the quads, but I had two sets of quad 6550's in each of my ARC monoblocks, and only biased each set of quads to 32ma!

If re-biasing doesn't solve the problem, it could be a bad tube. You could replace the one tube without much loss in performance. A matched pair is better, but matched quads are really overkill.

If you think it is the tube, and/or the bias suggestions aren't relevant, I'd recommend getting a matched pair, and saving the one good one from the previous pair for a spare.

IMO, you should go to the Cary website, get their phone number and call Kirk on Monday with your question.

Thanx Lak. I did that already. They want me to send all eight tubes in and they'll test them for $125 bucks. I'm 99.99% sure I know which one is bad. Newbee, unfortunately, I can only balance the whole quad. The manual states 175ma, Cary technicians suggested 150ma. I keep it somewhere in the middle. Nsgarch, thanx for your suggestions too. I think I'll try to replace just a pair although, I can't find Teslas KT88s ANYWHERE. I'm sure the tubes are the issue now becuase I had a spare set of eight Svetlana 6550s which I listed to for four hours last night without interruption. Thanx all.

Pawlowski6132: I think the Premium Svetlanas are the BEST (IMO) and you can get a matched pair from Conus in Canada for about $75. I used 16 6550 Svetlanas in my ARC monoblocks (ARC uses Svetlanas in their new amps, the last time I checked) and they're the bomb!

Website:

www.conusaudio.com

FWIW

.

Nsgarch, "Premium Svetlanas". As opposed to SED's, Flying 'C', or the just plain old Svetlana? Just trying to distinguish them from the new Svetlana which I understand is just a Sovtek in disguise. Just so the folks won't get confused.

I highly indorse the SED's and Flying C's. I've never cared for the old Sovtek's.

Pawlowski6132, FWIW one of the reasons, but not the only one by far, that I think so highly of the SED's (by any other name) is that they are rugged. I've yet to have one short out and I've used a bunch of them. Can't say as much for several other popular brands. (Unfortunately for me when a tube shorts out it frequently takes a bias resistor and I have to haul it into the repair shop - I don't solder worth a crap). When you are faced with balancing a quad this becomes important - its hard to integrate a new tube into a used quad and get the best sound.