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Old 12-31-2021, 11:36 PM
paltomare paltomare is offline
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Default Need troubleshooting advice for a MC275-v6

Happy New Year!
Need some help with my MC275 v6.
I have noticed a raspy distorted sound coming from the left channel at moderate volumes, +/- 20 dB (40% volume setting).
I swapped the R/L balanced cables from my C2700 to the amp and the distortion stayed with the left speaker. I then swapped the R/L speaker cables, and the distortion followed the left channel of the amp to the right speaker. This tells me it is not the preamp or the speaker. This is most prominent with a vocal-piano track (Underworld - Louisiana). To be sure it is not the track I played it on a second system and did not hear the distortion. I'm hoping it's just a tube but not sure of the correct way to swap tubes to see if it follows the tube. Would appreciate any advice. Thanks, Paul
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Old 01-01-2022, 12:22 AM
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I have a MC275MkVI also. The only advice I have is to contact Chuck Hinton at McIntosh - he has helped me to diagnose an issue with a MC252. It is important to have matched tubes on a stereo tube amp. Are these the original OEM tubes? A little Google searching may even give you the answer you are looking for. I have 2 full sets of tubes ready to go when I have a tube problem.
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Old 01-01-2022, 12:36 AM
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Swapping the tubes is easy. More likely to be a power tube than a small-signal tube, so would start there. Just swap the left channel KT88s with the right channel and see if the problem follows. If not, swap the small signal tubes. Of course, you need to turn the amp off before doing any tube swapping.
Be aware that their may also be corrosion on the tube sockets/tube pins. Just pulling and reinserting the tubes may cure the problem. Power tubes are generally good for around 2000hr and small signal for 10,000, or so.
Always good to have a spare set on the shelf. I burn-in my spare tubes for about 100hr before putting them on the shelf. Never know if you might get a bad one.
Let us know what you find out.
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Old 01-01-2022, 02:13 PM
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Quote:
Originally Posted by Formerly YB-2 View Post
Swapping the tubes is easy. More likely to be a power tube than a small-signal tube, so would start there. Just swap the left channel KT88s with the right channel and see if the problem follows. If not, swap the small signal tubes. Of course, you need to turn the amp off before doing any tube swapping.
Be aware that their may also be corrosion on the tube sockets/tube pins. Just pulling and reinserting the tubes may cure the problem. Power tubes are generally good for around 2000hr and small signal for 10,000, or so.
Always good to have a spare set on the shelf. I burn-in my spare tubes for about 100hr before putting them on the shelf. Never know if you might get a bad one.
Let us know what you find out.
Follow Glenn's advice. Be sure to wear cotton gloves. DO NOT get finger oils on the tubes.
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Old 01-01-2022, 03:22 PM
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^ gloves indeed
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Old 01-01-2022, 04:35 PM
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Default Need troubleshooting advice for a MC275-v6

I had an MC275VI do the exact same thing. I replaced all the tubes and it didn’t fix it. I ended up having to take it in for service and apparently some of the internal components had been damaged due to a tube failure. It’s been a while since that happened so I don’t recall exactly what had to be replaced. Like yours, my amp would play but the left channel was distorted. Hopefully yours is just a tube failure because the repair shops don’t exactly handle the amps with white gloves like I do and mine came back with a huge scratch across the chassis.
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Last edited by Stahle; 01-01-2022 at 04:38 PM.
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Old 01-01-2022, 08:43 PM
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The easy way to find out if it is a tube is to swap the output tubes from left to right and see if the problem follows the tubes.

If not then swap the small signal tubes. Most are dedicated to a single channel. If the problem follows the tube then bingo.

If not, you can pop off the bottom cover and take a look for any obvious component damage.

If none of this works it’s off to a repair facility.
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Old 01-01-2022, 09:42 PM
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Quote:
Originally Posted by Stahle View Post
I had an MC275VI do the exact same thing. I replaced all the tubes and it didn’t fix it. I ended up having to take it in for service and apparently some of the internal components had been damaged due to a tube failure. It’s been a while since that happened so I don’t recall exactly what had to be replaced. Like yours, my amp would play but the left channel was distorted. Hopefully yours is just a tube failure because the repair shops don’t exactly handle the amps with white gloves like I do and mine came back with a huge scratch across the chassis.

I'm hoping the OP doesn't have to go this route, but in truth if it is a tube failure wouldn't the amp shut down? Even my old Jolida shut down when a power tube failed.
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Old 01-01-2022, 09:47 PM
paltomare paltomare is offline
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Thanks for all of the suggestions. I do have an email into Chuck, but with the holidays, I'm not expecting to hear back for a while. I was hoping it was a tube even though I only have between 500-800 hours on this amp. I swapped V8L/V10R and V9L/V11R today and let the amp run for a few hours. Happy to say I no longer hear the distortion, even pushing it harder than before, not sure why but if that does the trick, I'll be happy. And yes, I handled the tubes with gloves.
I like the idea of keeping a spare set on hand, are most of you keeping output tubes only or signal tubes as well?
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Old 01-01-2022, 09:58 PM
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Quote:
Originally Posted by paltomare View Post
Thanks for all of the suggestions. I do have an email into Chuck, but with the holidays, I'm not expecting to hear back for a while. I was hoping it was a tube even though I only have between 500-800 hours on this amp. I swapped V8L/V10R and V9L/V11R today and let the amp run for a few hours. Happy to say I no longer hear the distortion, even pushing it harder than before, not sure why but if that does the trick, I'll be happy. And yes, I handled the tubes with gloves.
I like the idea of keeping a spare set on hand, are most of you keeping output tubes only or signal tubes as well?

Glad to hear the problem seems to have resolved. But with only swapping out tube assignments I'm wondering if the "fix" is permanent.

Not sure what others do for spares, but I'd be interested to know since I have an MC1502 on order and it utilizes 8 KT 88's and 8 signal tubes.

I haven't thought to consider whether Mc covers tubes under their warranty or whether they're considered a consumable and not subject to warranty.

OP, is your amp still under warranty? I'd query Mc on that subject if it is to see where they stand on the topic.

Edit: Just read the warranty info. 90 days on tubes, so with 8 of them I'll probably invest in a tube tester.
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Last edited by FreddieFerric; 01-01-2022 at 10:05 PM.
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