Mcintosh MC275 - Tube Temperatures

John Flagg

New Member
I have a MC275 stereo power amplifier that I bought new in 1998. It's the Gordon J. Gow Commemorative Edition. I used a Fluke Infrared Temperature Meter and shot all the tubes. All appear to be in the same temperature range except for the KT88 in socket V8. It comes in at approx. 125˚F. The rest of the KT88s are running at approx. 200˚F plus. I swapped a few around with the same results. Any idea what's "cookin' " here?
 
May be due for two new matched pairs of output tubes.
I'd recommmend a full bench check of performance by a qualified tech, like Dave O'brian did with the Mac clinics.
 
Just to be clear - is it that socket (V8) that measures lower temp regardless of tube arrangement, or a particular tube that the lower temp follows around? Does the tube appear different while in operation, as far as what is glowing and how brightly (assuming you have a matched set)?

If it follows the tube, it's probably the tube. If not, more likely the socket - or some inherent temperature difference/measurement inaccuracy peculiar to that tube slot. You always have to suspect systematic effects on measurement.

Position V8 is at the very front, so no adjacent power tube on one side. It's worth noting that the power transformer is at the back, and that's the one that does get toasty - both output xformers stay cool in operation. So V8 is as far as it gets from the output xformer. I'm not sure where the power resistors are under the hood on this version, but the chassis may not heat entirely evenly. Depending on how air flow works in your measurement position, that could also easily play a factor. Any of these could be playing a role.
 
I have a MC275 stereo power amplifier that I bought new in 1998. It's the Gordon J. Gow Commemorative Edition. I used a Fluke Infrared Temperature Meter and shot all the tubes. All appear to be in the same temperature range except for the KT88 in socket V8. It comes in at approx. 125˚F. The rest of the KT88s are running at approx. 200˚F plus. I swapped a few around with the same results. Any idea what's "cookin' " here?

Yep the cold tube is weak and should be replaced.
Ron-C
 
Good morning! I'll have to check the socket to make sure it's making a proper connection. The KT88s are the factory (R614A) matched set. And, to clarify, if I swap any KT88s to socket (V8) the tube temp goes low. By the way, if I looking at the factory print/schematic correctly, it shows V8/ socket as being the farthest right KT88 tube.
 
The thing with tube technology is that there is an arc of deterioration in the service life of the tubes themselves, and thus the equipment that uses them. There is also the possibility of accellerated deterioration or spontaneous failure in individual examples. A technical performance check can determine the state of the amp meeting its specifications as a check of all tubes and associated components, operating as intended in their normal condition of usage.
 
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