MC-275 Mark VI -- Left Channel Distorting

oregonalex said:
1.
4. I would be concerned if the amp blew a resistor without the sentry tripping. If it were my amp, I'd have it checked by a reputable tech even if it is working fine now. My sentry has tripped twice so far (in three years). Once on a catastrophic KT88 failure, once on a transient glitch in a different KT88 tube - in neither case was the amp damaged. The first tube was a goner alright, the other one initially failed a Maximatcher test, but retested OK multiple times, so I put it back into the amp and it has been chugging along for two years since.
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If I lived in Central New York or a big city with a great McIntosh specialist, I would certainly take your suggestion. But I live in South Bend and the thing weighs a ton. If and when something else blows, I'll deal with it.
 
feinstei said:
If I lived in Central New York or a big city with a great McIntosh specialist, I would certainly take your suggestion. But I live in South Bend and the thing weighs a ton. If and when something else blows, I'll deal with it.
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Understood. This is the one thing that bugs me about tube power amps - the potential of a power tube failure taking out other parts in the amp. Still, I tried living with a bunch of SS power amps, even Macs, and ultimately decided that dealing with this is worth it. Very much worth it...
 
(This is the original poster...)
I found out the underlying cause of the failure that was causing the problems that I had with the MC-275 Mark VI resistors blowing out. I was playing a classical CD with very high soprano voices and I noticed that the right channel was distorting badly. I found that one of the right channel resistors blew up. Replaced the resistor and the KT88's (with new-production quad matched Electro Harmonix tubes that I bought at Amazon) and the distortion disappeared. Even though it was the resistors on the left-channel that I initially reported as blowing out, I had switched the channels of the KT88's so that the left tubes were on the right and the right tubes were on the left. The fact that the right channel blew up was good evidence that those two McIntosh branded tubes were the cause of the problem. Those tubes were OEM'd by McIntosh and were from the time that McIntosh was using a pretty unstable run of Electro Harmonix tubes. So far (after two hours of listening), the new tubes sound heavenly.
 
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I am going to be sarcastic! So fair warning to all. Don't you just love tubes amps. Thats why I gave up on my pair of 275's 48 years go. Of course I should have waited till the MC 7200 came along. But bad tubes made me change to 2100's and then DC 300 A's and PSA2. But they needed rebuilding every ten years. But at least I didn't have to replace tubes every 6 months or so. And watch d Davie try to make Mac tube amps meet specs and then when they wouldn't he had to attach a letter to each Mac he tested saying good tubes were no longer available. The 70's was a rough period for Mac tube amps and I so loved the MC 3500. No wonder they stopped production after building close to 200 units.
 
A 63 cent part failure. Seems easy enough, the question is how do you prevent that in the future? Design change? Part improvement? Usage change?
 
Although any part failure can be inconvenient, sure beats the expensive melt-down an MC2300 would experience if its driver board runs away or an output rails. I’ve seen and repaired a lot of carnage on SS amps over the years including McIntosh, if my repair were a simple resistor and no tubes were damaged I’d be pretty damn happy.

I guess those decades as an Engineer doing DFMEAs and focusing on collateral damage & unintended consequences, my thought is that the resistor is sacrificial (either by design or by accident) and that is a good failure mode.

Tubes? Don’t dump on ‘em if you don’t like them, just move on IMO.

Kind of tired of the monthly “don’t you love tube amps” sarcasm and hearing about what tube amps you don’t like and what you’ve decided is better, ... need to un-latch the repeat button on the CD player.
 
damacman said:
Audio is all about camps. Vinyl vs digital. Tubes vs SS. Media choices - jazz is better than rock, etc.

Gets old.
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yeah, the internal conflict became so unbearable that I ended up with parallel McIntosh s/s and tube systems , 4000 CDs, 4000 LPs and a catalog evenly split between jazz,classical and rock. Not to mention ladder DAC/DDC streaming .

But I'm saving a bundle on psychiatrist bills!
 
Yggdrasil said:
yeah, the internal conflict became so unbearable that I ended up with parallel McIntosh s/s and tube systems , 4000 CDs, 4000 LPs and a catalog evenly split between jazz,classical and rock. Not to mention ladder DAC/DDC streaming .

But I'm saving a bundle on psychiatrist bills!
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Some might be concerned about the cost of a divorce Attorney if you go too deep!
 
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