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.

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

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Newbee: The "Pemium" Svetlanas are the ones made at the St. Petersburg plant. They are not relabeled Sovteks.

From their blurb: All tubes in new blue style original boxes produced and professionally matched by Svetlana factory in St. Petersburg, Russia. Each tube has 90 days replacement free guarantee since date of purchase (if the tube physically not damaged).

If you go to the "Tube List" at ConusAudio and click on KT88 specs, I think you'll be quite impressed: Very high cathode emission, gold-plated grids, a three plate anode, etc. etc. All tubes are aged at the factory (I think that means burned-in, but I'm not sure.)

Newbee, I'm a little confused too, but I think what happened is Svetlana bought Sovtek and started putting their name on the products coming out of the Sovtek factory. The ones from St. Pete's have the Winged "C" and a red band where the bottle attaches to the pins.

If you want to find out for sure, call Euguene Karna at Conus. He's the owner (and Russian.) (905) 853-8808 or you can email him from the website.