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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
czarivey
5,941 posts
 
less likely you'll need to replace all power tubes and hopefully the tube only took out fuse. you may place live tube onto the same slot(use dummy load resistor instead of speaker) and if one is blown, chances that you have a bad dc cap are big. however you can still visualize the circuit components and see if you will spot leaky dc-cap or anything fried.
good luck.
jakegt3
40 posts
 
For a test, you don't need to buy more KT88's. Swap one of the good tubes that's in there now into the bad position. Turn the amp on and see if the good tube comes on. If not, then there is something inside the amp that needs fixing. I would the guess the original bad tube suffered an internal short, which is not uncommon in power tubes.
lewinskih01 OP
909 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!
swampwalker's avatar
swampwalker
5,138 posts
 
You should be good to go. The "bad" tube position cannot have a blown resistor if a tube lights up at that position. The fact that the tube from "bad" position does not light up when plugged into the "good" position shows that tube was bad.
knghifi
1,668 posts
 
When a blown tube takes out the resistor on my amp, new tube will work but can't bias and just runs very hot.
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