2. amps-preamps
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Tube amp down, need guidance from the experienced

I listened to my McIntosh MC275 Mk.IV all day yesterday without issues, as usual. Today I turned it on and left the room. Didn't pay attention at startup. Ten minutes later I realized it was not on. Eventually I figured the fuse had been blown, and replaced it. Turned it on again and found one KT88 was not glowing and cold. The rest of the tubes, both small signal tubes and KT88, were all on. Didn't attempt to play anything, turned it off, and here I am.

Obviously I need to replace the KT88, preferrably all of them. Unfortunately I don't have any spares on hand. What I'm anxious about, though, is to figure out if anything else was damaged. Is there reason to believe something else might have been damaged when this tube went off? First time a tube fails for me, and have seen a number of horror stories told on the net.

Thanks much!
lewinskih01

Showing 4 responses by lewinskih01

lewinskih01 OP
906 posts
 
Thanks all for the input. Exactly what I was looking for.

My KT88s are probably 5 to 6000 hours old. I was once told these last about 9000 hs, so I'm getting towards the end of life I guess.

I did aas jakegt3 suggested and swapped two KT88s: the failed unit and another one. Upon turn on, the "failed position" had the new tube glowing and the failed tube was not glowing again. Hopefully Jake is right and this shows the resistor didn't go.

BTW, the failed tube shows black markings on the top of the unit, whatever that means.

Will look for new tubes and double check with McIntosh.

Thanks all again!
lewinskih01 OP
906 posts
 
OK, lesson learned: power tubes should be replaced after 2000 hours or so. How about signal tubes, like the 12AT7, 12AU, 12AX, 6922, be it them in the amp or pre amp?

I have no music but sort of feeling lucky it only seems the tube went bad.

Thanks all!
lewinskih01 OP
906 posts
 
Thank you.

Opinions re what KT88 to buy? Psvane black bottle, Genelec (Jim McShane recommended this one), other? Don't want to spend over $400.
lewinskih01 OP
906 posts
 
I thought I should give you guys feedback after all your help: I installed a KT88 in place of the failed tube, played the system for an hour, and everything seems to be as usual. Thank goodness!!

Beyond the anxiety the tube failure created, this thread was most helpful to me in that I learnt several things. Great forum, great answers, thank you!!

So now I'm running my amp with 3 tubes that have 5 to 6k hours...I need to replace them ASAP. After the reading and exchanging with Jim McShane, I'll go with new Gold Lions.
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