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McIntosh MC275 IV distortion on channels

entreat11

entreat11

Member
Hello,

I recently purchased a used McIntosh MC275 IV amp, and upon hooking it up I noticed distortion on one channel right away, seems to get louder as the volume goes up. The whole unit is not as clear or as good as i expected it to be.

I have swapped the original McIntosh KT88 tubes and the distortion seems to follow that ( I will verify this again later today when I am able to turn up and test again fully after clearing my head!). Checking the tubes they seem to be in good shape and have the silver at the top. I also notice the red glow on the tubes, seemed normal but did not check very closely to see if it is red-plating.

My question is does this seem like the tubes are bad or is there something more deeper on the amp that needs to be looked at? Also anymore suggestions on how to trouble shoot?

Also out of the 4 KT88 tubes which are the left and the right? Could the 12AX7A/12AT7 tubes have any fault in this as well? All tubes look pretty good and are silver at the top of them.

Any help would be great.

thank you,

michael
 

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We'll you are learning about tubes. I was fortunate to have test equipment available at the radio station where I worked while in High School and College and the high fi shop had test equipment too when I owned tube amps and pre-amps in the 60's and early 70's. Mcintosh Amplifier clinics conducted by Davie Obrien visited every other year so I could get free tubes when necessary to make the 275's, 240's , MC 60's and MA230 work as they should. In between those times I had a complete replacement set for each amp. And even then once in a while when I checked my amps every 6 months I had to buy additional tube or two. Fortunately it was very seldom for the Gold lion KT 88's. The GE 6l6GC weren't that pricey and the telefunken tubes were readily available. I just paid cost to the radio station as they had hundreds laying around. A tube tester won't reveal all the issues either. You're lucky, they even bias the output tubes at higher levels with the newest model. I was fortunate to grow up in a time when tubes were the only alternative and replacing one or two now and then wasn't a big deal. Yes I had to make a 40 minute drive to work or the nearest tube shop was 8 blocks away if it were an emergency. My system ran 12 hours a day whether I was home or not. My parents loved music, too. 3000 to 5000 hours a year was not un common. The Previous Fisher and Scott units couldn't stand up to that amount of use and were always having capacitor issues and even an out of spec resistor or two. As much as I like the sound of 2301's, C-500T, and the mighty MC3500. I'll never own tubes again unless I win the lottery and can afford the Luxury. Now that my town no longer has a tech familiar with Mac or an authorized dealer, I am up the same creek as most folk. I dread the day when I'll have to return one of my 207's to Audio Classics for repair. But I'll keep my fingers crossed and hope the 207's will last as long as my almost 50 year old 2505. The MR 80 has been updated as have the C-34 and MPI-4. The MX-119 and MAC-3 were replaced with a MX-151 and RFD2. The 7083 has been touched uo, too. MVP 861 had a new mechanism installed. So you can see Audio Classics and I have quite a working relation ship. Especially when UPS tampered with my MPI-4 and lost my MX 119 for 6 weeks.
 
Thanks for your response, yes you are correct, this is my first venture in the tube life! Seems like you have had a lot of experience with it. So update and closure, i was able to figure out the issue, the distortion did follow the bad tube when it was switched to the other sockets, and I was able to narrow it down to one of the tubes being bad because it was not getting as hot as the other 3 and moving it repeated that symptom when moving it to a different socket. Also the distortion followed that. I purchased a new McIntosh KT88 tube and once I put that in all worked like it should and sounded good. Ran it for about an hour and no changes or issues and all tubes seemed to be about the same temp. I am so glad the fix was that simple and not something more.

Hopefully this unit will last a long time with no issues other than tube replacements!
 
entreat11 said:
I have swapped the original McIntosh KT88 tubes and the distortion seems to follow that ( I will verify this again later today when I am able to turn up and test again fully after clearing my head!).
Click to expand...
If the distortion moved with the tube, it's the tube. Try to figure out witch one it is since you don't have a tester by moving one at a time. Get a new set and save the good old ones for backup.
 
Thanks for the response, I was able to figure out the tube that was bad and I purchased a new KT88 tube and replaced it, after that everything worked and sounded. So glad to know it was not the socket that had an issue.
 
So I had another follow up question, it seems like the amp lived its life in bridged mode and after I started playing it and spending more time with it I am noticing that the left channel is a higher than the right channel, the left channel also sounds a little punchier. I am currently using the unbalanced inputs on the amp.
Also on the tubes the 12AX7A/12AT7 which are the left and right as well as the KT88 which are left and right? I am tring to understand some of this stuff but I figured someone has to have a ton more experience than me!

Thank you
 
entreat11 said:
So I had another follow up question, it seems like the amp lived its life in bridged mode and after I started playing it and spending more time with it I am noticing that the left channel is a higher than the right channel, the left channel also sounds a little punchier. I am currently using the unbalanced inputs on the amp.
Also on the tubes the 12AX7A/12AT7 which are the left and right as well as the KT88 which are left and right? I am tring to understand some of this stuff but I figured someone has to have a ton more experience than me!

Thank you
Click to expand...
As I said you have to get 4 matching power tubes before anything else
 
Thanks for your response I do understand that is the best practice but would like to avoid spending another 400.00+ and just try to troubleshoot with what I currently have. All the tubes on the amp are McIntosh tubes and are not mismatched. Which tubes on12AX7A/12AT7 which are the left and right as well as the KT88 which are left and right?
 
I assume you have the balanced / unbalanced switch on the amp set to unbalanced, that any jumpers between output taps are removed, that both speakers are connected to the same impedance taps on the amp and that the MC275's gain controls are set in the same position.

I assume you double checked your preamp control settings and nothing is out of balance there.

Also, have you isolated the left / right imbalance to the MC 275 i.e. by connecting a different amp in its place and then not hearing the imbalance?

If you have systematically isolated the imbalance to the amp and all connections and controls are correct, then the amp has a problem which may or may not be related to a bad tube. You might again systematically, one at a time, try swapping tubes between channels to see if the imbalance moves. If the problem moves (swaps channels) with a tube, you've found a bad tube. If the problem stays put, the amp may need service by a tech.

Here is a list of the functions of the tubes for the type IV MC275:

V-1 (12AX7A) is the splitter inverter when using single ended interconnects; when using the balanced inputs, it's not necessary and is out of the signal path (but it still has to be in the amp to complete the filament circuit )

V-2 and V-5 (12AX7A) are the L and R channel signal multipliers. They drive the pairs V-3 + V-4, and V-6 + V-7 respectively.

The two pairs, V-3 + V-4, and V-6 + V-7, (12AT7) drive the two pairs of power tubes (V-8 + V-9, L) and (V-10 + V-11, R) (KT88/6550) respectively.

Good Luck.
 
Thank you for your help on this. Yes all jumpers are removed, speakers are corrected to the right impedance and the gains were set to the right position, they were just turned all the way up on the 275. I previously had a 2100 hooked up to the preamp and did not notice this issue. That works perfectly with the gains set at 12 o'clock.

I did try a different preamp and did notice the same thing, but might retest that to double check.

I will try the above tonight and try to move the tubes and see if things work any differently.

Thank you
michael
 
Where are you located michael?

There might be someone nearby with a tube tester, depending on what model tester, maybe look for a weak tube.
 
entreat11 said:
So I had another follow up question, it seems like the amp lived its life in bridged mode and after I started playing it and spending more time with it I am noticing that the left channel is a higher than the right channel, the left channel also sounds a little punchier. I am currently using the unbalanced inputs on the amp.
Click to expand...

4-2-7 said:
As I said you have to get 4 matching power tubes before anything else
Click to expand...
I was short because I was at work and using my phone.

Ok you already had a bad kt88 power tube so bad it was making a distortion sound. You now replaced that one tube with a new one and got rid of the distortion sound. I told you that you don't just replace one tube out of the 4, these are called "Power Tubes" two for each channel. You now have three weak tubes and one fresh one and that fresh one is only in one channel. Do you not think a fresh "power tube" is not going to have more power? making one channel sound louder? giving you a out of balance sound?

entreat11 said:
troubleshoot with what I currently have.
Click to expand...
Your not trouble shooting, your having problems because you need to buy tubes and trying not to. You have a nice mac amp and it's going to work for years and years...if it's taken care of. You can harm your amp trying to run it with bad power tubes<<Clear?
 
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