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Lost a tube in my MC275 MarkIV, and the replacement is very hot

postman56

postman56

Active Member
I lost a tube in my McIntosh MC275 IV and thought nothing of it, figured it maybe ran it's course, but when I replaced it with another KT88 from Mac, it showed signs and felt it was getting much warmer and faster that the other three ,so I shut the amplifier down worried about loosing another expensive KT88.
Service depots are not all that close, so any help would be appreciated.
 

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The tube in the center of your picture has "gone to air" it is no good it has loss its vacuum. Kirk
 
Hi Kirk, yes that's the one that went on me, and I put a new one in, but quickly turned off the amplifier after the replacement tube was considerably hotter shortly after starting it up, and felt there may be something wrong with the unit.
 
Carefully swap the two left channel tubes for the two right channel ones, and see if the new tube that was getting excessively hot gets hot in the other channel, and the swapped pair are functioning normally in their new positions.

If so, your new tube may be defective. If it is now normal temp, and one of the tubes moved is getting hot where it wasn't before, you have a problem in the circuitry, such as a shorted resistor or capacitor allowing too much current to flow into the tube.

Your instinct was right, BTW-- don't let the tube stay in any longer then needed if it seems way too hot compared to the others.
 
I'd recommend checking the voltage on each pin using a standoff socket saver. Compare it to another output tube. It is possible that when your tube failed, it may have taken out or affected a resistor. I had a Telefunken 12AX7 driver fail in one of my MC-30's several years ago and it took out a resistor.
 
Those tubes don't need to be matched, but I'd use a set that are from the same manufacturer and KT-88s come in a couple different looking types - there are some w/ plates w/ 3 small holes, 3 big holes, and 3 rectangular holes. I'd try to use a similar type as well rather than mixing them.
 
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