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Tube Issue on MA2275

Matt The Cat

Matt The Cat

Prowlin' At 78 RPMs
Hi all,

I noticed the other night as I was going to turn my MA2275 integrated tube amp off that the tube cage was WAAAAY too hot. I determined that the front, left KT88 was extremely hotter than the other 3 KT88s and also smelled bad. I immediately shut the amp off. Next day, I turned it on and sure enough, the front, left KT88 got very hot, very fast, while the other KT88s remained cool to the touch.

Hoping this was just a tube issue, I removed the 2 KT88s from the left channel and replaced them with the McIntosh issued tubes that came with the amp. I am currently using the reissue Gold Lions. The original Mac tubes worked fine, EXCEPT the front, left KT88 is still way too hot. Again, hoping it was just a tube issue, I replaced the 12AT7 for the left channel, but that didn't sold the issue.

So, did I blow a resistor or something?

I'm gutted as I hate to take this 77lb beast out for repairs. What a hassle, not to mention the financial setback. I bought the amp used in 2008, but it was produced in 2005.

Any help/advice would be most appreciated.

Thank you.

Matt
Salem, MA
 
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I never had a MA2275 but if you changed the tubes and sure enough, that tube gets super hot, I am afraid that it means there is something wrong with the amp.
When you mean super hot, do you see it red plating quickly?
It could also be the tube losing it's bias from a bad grid contact at the socket level. That happened to me once.

My 2 cents.
Brice.
 
Did the offending tube get red?
I had similar issue on one of my restored MC60 a month or do ago and definitely will need to open the unit, check the voltages, then resistors etc.
I think you showed issue is not the tube and it's a time for tech to look under the hood.
I hope you have one close by but i feel your pain; MAC amps are haavy and bear to move.
Good luck
 
Thanks guys,

I appreciate the responses. No, the tube wasn't red and it didn't look different from the cooler ones, but it was WAY too hot and there was a slight rumble in the left channel (same channel as the offending tube). It was just a little bit of distortion, which was not in the right channel AND did not get louder when the volume was turned up. After turning the amp on, I would say the tube got very hot after about ten minutes or so, while the other 3 KT88s remained cool and could be touched. There were no pops or anything in the left channel either. There was that sort of electric burning smell, which made me nervious that it was probably the amp itself.

Moving this unit from the attic is going to suck. It's times like this when I regret some of my audio choices...like whenever I have to move my record collection...bad choices, man.

Matt
 
Hi Matt,
I was wondering if you got your "hot tube" issue figured out. I believe I have a similar problem with hotter tubes in one channel at 180F vs 120F in the other channel. My own troubleshooting did not yield any definitive answers yet.

Here is what I did to narrow down the problem:
- switched tubes left/right — no change
- bias resistors measure correct at 220 Ohms
- heater voltage of the KT88s is same for all tubes 6.5V
- I switched the amplifier board left/right to rule out any problem with the transformers — problem moved to the other side with the board

My MA2275 plays perfectly but it will blow the internal fuse on the left amp board every few years, which I take as a reminder to change power tubes. I have been running Gold Lion KT88s ever since I switched for the first time after about 3 years of service.

I would also appreciate any hint where to look next. I’ll attach a schematic of the amp board for reference.

Thanks and good luck,
Oliver
 

Attachments

Hi Oliver,
Thanks for the response. Man, you have gone above and beyond my expertise on the issue. All I did was switch the tubes out and found that it didn't matter what tube was in that socket, they would all get way too hot. I'm in the process of having a tech take a look at it. I'll let you know what I find out as soon as I find it out.

This is my 2nd quad of Gold Lion KT88s. The first time, a tube died nice and quietly and didn't cause any issues. It looks like this time that a bad tube may have disrupted the circuit. I've never had the fuse in my 2275 blow though. The Gold Lion reissues are the only new tubes that I'm running in the 2275. My 12AT7s are 60s Telefunkens and my 12AX7s are Teles and RCAs. I've gotten some great sounds from tube rolling vintage NOS tubes.

I'm sorry I couldn't be more specific or of much help to answer your questions. I'll let you know what I learn from all this.

Matt
 
Hi Matt,
I’d love to hear what your tech comes up with.

Last night my amp blew another fuse and that killed the right channel completely. I have a suspicion that my problem has something to do with one of the 4 NTN transistors Q3-Q6 in the signal path. One of them does not measure the same as all the others. I ordered a few of these transistors and will replace them as a last diy attempt before I call the doctor.

I actually have a great Mcintosh service lab in town (Ft. Worth Audio) but I do loath to move this monster and do not even try to imagine loading it somehow into my 911.

I’ll keep you updated on my findings, please do so as well.

Cheers, Oliver
 
I hear ya, man. I don't know how I'm going to lug this behemoth from my attic room to the car. I thought maybe I'd put it back in its box and gently roll it down the stairs....carefully. But then I'd have to pick up the box with its added weight (total weight 110lbs in box) and move it outside. It's not the 77lbs of the amp that's the problem, per se, it's the fact that you have to be so careful with the glass. I love owning McIntosh equipment UNTIL stuff like repairs or moving are necessary and then I wonder why I ever paid so much, to be so put out? Ugh!

My tech is south of Boston and an hour away from home. I guess I could ship it, but that would add probably an extra $300 to the bill for shipping.

I'll let you know what the tech says, but I'm honestly in no hurry to move it. I haven't moved it yet. Thankfully, I have a trusty solid state amp that's been doing a great job, so my listening hasn't been interrupted.

Matt
 
I just got done replacing the 4 NPN transistors (MPS A42 at $0.30 a piece) on my offending amp board and … drumroll please … so far so good. The “hot” running tube stays within 20F of all the other power tubes at around 130F (measured on the silver top). I had the amp running now for a couple of hours and it sounds and behaves as good as ever.

I knock on wood that this is a permanent fix. I wish you, Matt, that your amp will be back to its glory soon … without too many headaches until then.

Cheers,
Oliver
 
You are lucky call Audio Classics and see if Steve will meet you on a Saturday at Audio Classics. Its only 322 miles via the interstates. In my part of the word thats 4 hours. Or call him and he will send you a call tag for a pick=up by UPS. Its much cheaper than taking the unit to UPS office.
 
Hey Oliver,
Is your repair still going strong? That's fantastic news, man! Congrats. I bet I have the same problem with mine. I tinker a little, but I'm not confident enough to replace transistors in a McIntosh. I've replace capacitors in an old Westinghouse radio from the 40s, but not a McIntosh. Ha ha! I'll be driving it down to my tech very soon. I'm really missing the MA2275. My old NAD sounds pretty good, considering, but it's no McIntosh tube amp.
 
Hey Matt,
So far so good. I have been running the MA2275 for at least 6 hours a day since the repair and it still works perfectly. I still have one of the tubes running a little warmer than the others but I think it stays within ballpark specs. After I had put it back together — which was surprisingly simple — the amp sounded a lot better than immediately before. Maybe one of the tubes was really on its way out and caused the transistors to fail.
I will keep a supply of those transistors and the glass fuses on hand just in case.
Hope you get yours sorted soon. Please keep us updated on the diagnosis.
Cheers, Oliver
 
Hi guys,

I know it's been a long time, but I wanted to give you an update on my 2275 since I just got it back from the shop AFTER SIX MONTHS! All it needed was the 2 transistors that regulate voltage to the affected KT88 replaced. I got it back on November 5th and noticed A NEW PROBLEM. Please chime in if you have any ideas. So I hook it up and the KT88 is fine. HOWEVER, now there is a much louder noise floor in the phono stage.

I mean, of course there's going to be a bit of a noise floor, but this is WAY too loud and can be heard from my listening position. So I swapped our 4 sets of 12ax7s in V4 and V5 and the distortion was the same every time. No change with varying tubes. No issues on any of the line level settings. When you touch the ax7s or even put your hand close to them, a hum is introduced. The general distortion I'm talking about sounds like something oscillating and when the volume is turned up all the way, it even registers to about .075 on the VU meters. So I thought maybe something went wrong during repair. I brought it back in and the authorized Mac tech told me it was a microphonic tube. I didn't believe that since I checked for that. He invited me into the shop and we changed out several tubes, but he was only monitoring on an old pair of '60s or '70s cans. No speakers were hooked up. You know the headphone amp in the 2275 isn't very powerful and you have to turn the volume up real high to get a normal listening level? So I don't think he really heard the distortion I'm talking about. I got it home again and nothing has changed. I'm thinking maybe EM or RF signals are being picked up on the phone stage? Does anyone has experience with this? This was obviously not here six months ago before the amp went to the shop. I suppose some new EM or RF signals may have been introduced in my neighborhood over the past six months. Of course I've tried shutting everything down. I've tried plugging the 2275 into different electrical outlets.

I've ordered some shorting plugs, but I don't think they'll make a difference as the sound doesn't change AT ALL when phono RCAs are plugged in or unplugged. I've tried disconnecting my speakers thinking the cables might be antenna for these signals...no change.

Am I forgetting something? Please let me know if you have any ideas. I'm frustrated and lost. The amp sounds great, even the phono sounds great, excepting the noise floor. I'm so happy to have the amp back after six months, but so upset by this new issue.

Thanks in advance, everyone.

Matt
 
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Here is a link to what the phono stage distortion sounds like.

http://www.mattthecat.com/MacPhonoDistortion.wav

I recorded this in front of the speakers with my phone to demonstrate how loud it is. Doesn't this sound like EMF or RF interference?

Please let me know what you think. I'm thinking of trying tube shields for the first time to see if that reduces the noise, but please let me know if you have any ideas on this. It's driving me nuts. I'm also contemplating hooking up an outboard phono stage, but every time I think about it, I get angry, because I have this top-of-the-line piece of stereo equipment and everything should be working perfectly. ARRRGHHH!

Thanks,
Matt
 
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