So, I received got a Quad of KT66 from Decware a few weeks ago and have been enjoying breaking them in. Yesterday, after a brief morning listening session, I left the system up and running (idle) and did several hours of choirs outside and in the garage. I came back in around mid-afternoon and started listening to Tidel, after about 10 minutes I started to notice the some minor distortion in the right channel which progressively worsened over the next few minutes. I messed and fiddled with everything, restarting all my components down to the modem, the distortion persisted so I shut down the MKIV and let the thing cool down. I replaced the EL34's and fired the amp back up, everything was fine the rest of the evening. This morning I fired everything up and did little listening, EL 34's still in from the day before, everything ok, then the distortion started in again, hmm, grabbed an "ove'glove" from the kitchen and pushed around on the tubes just because and noticed my right channel OA3 tube flickering on/off, so I figured I found the problem. I shut down, pulled the OA3 out and grabbed a light to inspect the tube socket, everything looked good to my not really trained what to look for eyes and decided to switch the two around just for kicks, did that, fired the amp back up and no distortion for several hours, decided to go back to the KT 66 and see what would happen, switched them back in and ran them pretty good for about ½ an hour everything ok, let them sit and cook with no signal for a few hours while I finished yesterdays choirs and back to listening, so far even pushing them hard they are preforming great.
 So my question is does that sound like I probably got a new tube going on me or is it something I haven't even ponder yet. I'll obviously keep this updated as I continue to listen, but I have to think that with the extent of the distortion that something is up and the OA3 thing was coincidence or ??. Any thoughts appreciated, sorry for the wordy explanation, but wanted to hopefully describe all the circumstances. Good listening to you all tonight!

Best,
Scott

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That would be my guess too.  Or one of the OA3's has a loose/broken connection that stabilized after removing and reinserting.  If that is the case, it will likely happen again.  I would leave the suspect OA3 in the left channel socket and if the distortion re-emerges and the OA3 is the cause, it will shift the distortion to the left channel.

Have you switched the two right channel KT66's with the left channel KT66's?  I would do that asap.  Then if it happens again, if the problem appears on the left channel, then you know it is a bad KT66 tube.  If the problem occurs on the right side again, then could indicate and internal issue.

You did say that you also had distortion with the EL34s?  (before you moved the OA3s)

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This is why I thought you had distortion with both the KT66s and the EL34s.  If it's just with the KT66s then Jeff has the next step.  I've also chased tube issues that I couldn't reproduce.  Glitches in the machine!  I think when things get heat soaked you can have different effects.  I've had radical issues with my CSP3 tube sockets that simply aren't there anymore -- but may come back!  

Who knows, maybe some contaminant burning off inside a tube?

Cool!  It's always stressful when something odd happens.  Glad it was just a tube.