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Tube Replacement: Single, Pair, or Quad?

Hello, can anyone help me?

I have a new (to me) pair of Cary SLM-100s. Without fail one of my amps will blow a "tube" fuse after approximately 45 minutes of listening. Immediately prior to the fuse blowing, two of the tubes (positions 3&4) become very bright. After this happened a few times, I switched Tube #4 in one amp with Tube #4 in the other amp with similar results in that amp.

I am using Tesla KT88 power tubes (not JJ Tesla.) I have them biased to approximately 150ma.

My initial diagnosis was that I have a bad tube. However, my question is, if the tube is OK, why would it continue to operate correctly in 45 minute increments? Could something else be wrong? If I need to replace the tube, do I have to replace tubes in pairs (e.g., 3&4) since this is a push-pull design? Do I have to replace all four? Can I just replace the one bad tube?

Also, the original owner did not have this problem. Further, when I installed the tubes I did so randomly not knowing which tubes he had in which amp in what order he had them in. Is this relevant?

Thanx in advance for your assistance.
pawlowski6132
nsgarch's avatar
nsgarch
2,524 posts
 
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
4,767 posts
 
Nsgarch, Do I recall correctly that the 'old Svetlana tubes, now also referred to as SED's, Winged C's, and even before that I think they had a big 'S' logo on the bottle, were made in St Petersburg? I've always been a bit confused over the sale of the use of the name "Svetlana" in some but not all countries. Hard to keep up with all of the name changes and licensing agreements. :-)
nsgarch's avatar
nsgarch
2,524 posts
 
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.
piezo
304 posts
 
sounds like a bias problem to me. my brothers marshall head did the same thing when tubes were changed with disregrd to biasing
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