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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
newbee
4,767 posts
 
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.
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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