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-   -   McIntosh KT88 and 12AT7 tube impressions/experience (https://www.audioaficionado.org/showthread.php?t=51519)

80B 08-31-2022 04:07 AM

McIntosh KT88 and 12AT7 tube impressions/experience
 
We've owned a pair of McIntosh MC2301 mono block power amps now for 4 1/2 years now. I bought a set of new Genalex Gold Lion tubes (16 KT88s and 4 12AT7s) back then, but waited until now to roll the Gold Lions in and replace the stock McIntosh branded tubes (my understanding is they are made by JJ for McIntosh). There was no special reason to roll now, except that our son is in town, and was willing to help wrestle these heavy amps and do the rolling with me. After the Gold Lions burn in a bit, I may compare, but this post is about the stock tubes and hopefully may help someone thinking about buying tubed gear.

The amps have been in service for about 44 of the 54 months we've owned them (put in storage for 10 months while we lived in an apartment while building our current home). I'd guess that we've averaged between 2.5-3 hours playing per day, which may seem a bit high, but they are used in the home theater setup as well as stereo from the C1100 preamp. That puts our usage on the tubes at ~3,700 hours, likely a bit higher.

I have an Orange VT1000 tube tester, which does a number of tests, but only shows an output of Good/Worn/Fail vs. detailed readouts of the individual tests. All the tubes showed Good and a numeric ranking of 7-11, with over half of them rating an 8. The upshot of all this is that the stock McIntosh power tubes were still going strong, and I plan to keep them since they test well and sound fine, and can function as backups.

These amps were our first major foray into tubes, and I had a lot of questions about going over from solid state amps, not least of which were regarding tube life and the possibility of degradation in sound over time. The bottom line is that my experience with the McIntosh stock tubes is that they have performed well for something approaching 4,000 hours and still have plenty of life in them. This isn't exactly a revelation or unexpected, but may help someone considering a tube amp for the first time get an idea of what ownership entails.

FreddieFerric 08-31-2022 08:10 AM

Thanks for this post. It's long been suspected, but never comfirmed, that the stock Mc tubes are OEM branded by JJ Electronics. I have the MC1502 that is now only 4 months old, but I am logging hours on it mostly every day.

There's always been a bit of concern over tube life, especially for power tubes. The only other power tube experience I had before this amp was an integrated that ran 4 Tung Sol 6550's. It seems one tube went bad (never found out which one) as the amp kept shutting down after just a few minutes of use. Once I replaced all 4, the amp never had a problem again. My memory of that episode was that I didn't have a lot of hours on the amp, but at only 65 watts, I did run it pretty hard. 10 to 15 years ago tube prices were a lot less than today, so with 8 KT88's in the mix, longevity is a worry.

FWIW, my brother has the MC275 MkVI. Like you, he runs his amp several hours a day pretty much every day and has been doing that for the last 3 years. So far he reports no problems at all. Hopefully his and your good experiences will be echoed by me 4 or 5 years from now. Time will tell.

I realize you didn't start this thread for the purposes of comparing the stock Mc tubes to the Gold Lion tubes, but could you give us a short word or two on how you feel about the sound of the new tubes over that of the old. It would be much appreciated.

80B 09-01-2022 03:01 AM

So one more bit of data on the tubes, this time the 12AT7s (2 in each amp). One did test as just inside "Worn" territory vs. "Good", which was a bit unexpected given that it's the power tubes that are supposed to have a shorter life. It may have also contributed to a feeling the sound was maybe a bit flat, but not something I could put my finger on since any degradation would have been so gradual I would not be conscious of it.

Freddie, great question about a comparison and one I didn't want to delve into on the initial post because I had maybe 2 hours on the new tubes. Now I have 5! So, the caveat that they really don't have enough time to be broken in aside, there is a bit more detail and punch with the Gold Lions. In fairness to the stock tubes, we're comparing tubes with considerable use vs. new ones, with one testing "Worn" vs. "Good". My 26-year-old son, who has musical talent and a better ear than me, said he feels it sounds better. He showed more interest in listening this evening, which is telling. You've got to be enjoying that MC1502!

I know some folks have replaced their stock tubes with Gold Lions (or other tubes) right away, something I decided not to do since this was my first foray with tubes in the main system (replacing MC501s), and I wanted to get used to the "house sound" as well as "utilize the assets" of the stock tubes. It also seems wise to get used to a set of tubes before making a comparison, especially with 16 KT88s which isn't cheap and almost twice as expensive as it was in 2018, although you don't need four years to do that! I like what I'm hearing with the Gold Lions, but don't regret waiting since part of having tubed equipment is that it can be more of a journey and allow for some experimentation, depending on how much you want to roll tubes. That said, I can see why someone with the right experience would swap out stock tubes if he thought he had something better, just set it and forget it and focus solely on the music.

Formerly YB-2 09-01-2022 08:36 AM

Nice report. Thanks.
I've always enjoyed tube-rolling in my amps (though 2 power tubes per channel has always been my max). So much so I'm second-guessing my choice to go back to solid-state.
If there was not so much expense involved I would suggest trying a set of 6550s if the MC2301 allows for same (as does the MC275/MC75/etc.). In my MC275/MC75 amps I found the 6550s to have a bit better mid-range and not quite the bass.
My experience with GL tubes was the same as your report. A small, but noticeable improvement over stock. Plus, considering the cost of McIntosh branded tubes I always preferred to run GL or Tung-Sol and have the stock tubes on the shelf.

80B 02-17-2024 04:51 PM

Bad Tube?
 
For those of you with a lot of tube experience and expertise, I could use some advice. This is really the first time it seems I've got a tube go bad, and welcome any comments. Since the MC2301s are my first tube gear, this new territory, and it doesn't seem like there's a lot of commentary about trouble shooting or dealing with bad tubes. The system has been a dream since I bought it. I don't drive the volume levels very high with the rare and brief exception, and the amps are well-ventilated.

My left channel MC2301 went "pfffft" and the Sentry Monitor kicked in, turning the amp off, with the red indicator light blinking. So, it looks like there's a bad tube somewhere. Same thing happens after shutting down the amp and turning it back on, with the Sentry Monitor kicking in a few moments after the unit is turned on.

I have an Orange brand tube tester, and test all eight KT88s and the two 12AT7s, the Gold Lion tubes that I installed in August, 2022, ~18 months ago. All the tubes tested "Good" (for those of you with that tester it was all Good/8 or /9). My guess is there is about 1,500 hours on this set of Gold Lions. At this point I either have a tube problem that doesn't register, or something has gone bad with the amp.

I have the original tubes since they still have life in them, as well as another set of Gold Lions, so I'm able to test by swapping out tubes.

Swapped out the 12AT7s, and got the Sentry Monitor kicked in, so it looks like it's a KT88 or the amp.

Next, put the 18-month-old 12AT7s back in and swapped out the Gold Lion KT88s with the stock KT88s, and it is now working fine, and has run for about 45 minutes now, no problem, so it seems one (or more, however unlikely that is) of the GL KT88s is bad.

My next step will be to replace stock KT88s one-by-one with the Gold Lions until the Sentry Monitor turns it off again, to identify which tube is affected by an undetectable gremlin. I'll do this over the course of a couple days, as I turn the system on to listen to it, thus avoiding extra on/off cycles and wasting time waiting while tubes cool down.

Questions for those of you with more experience with tubes than me: Does this sound like the best approach? Is it "typical" if not common, that a tube could test as "good" but be a problem? Any other things to keep in mind?

I'll update this once I've resolved the issue.

https://www.audioaficionado.org/pict...pictureid=5947

vintage_tube 02-18-2024 01:09 AM

Don't subject your MC2301's to repeated on/off just to find a bad tube. I'll send u a PM.

I sold my MC2301's around 6 years ago & my memory from Ron Evans (designer of the amp) told me the following 15 years or more ago:

First though -- As far as immediate troubleshooting - not your case now since you reinserted the originals back in.

I believe you can remove the front four KT88's (left 2 & right 2) and operate the amp gingerly. If all okay, turn amp off, reinsert those 4 and remove the rear 4. Operate gingerly & within common sense volume level.

The amp should cut off in scenario 1 or 2 (pointing to the front or rear quad).

This will at least isolate it to 4 verses guessing at 8.

Bob

vintage_tube 02-18-2024 01:42 AM

I found this I wrote back in 2010:

Had a couple emails w/Ron-C and a very short conversation w/Ron Evans (MC2301 designer/engineer @ McIntosh) regarding the power tubes for the MC2301 and thought I'd share what I know and what I learned so owners and prospective owners are a bit smarter & a wee-bit more knowledgeable when it comes to the power tubes in a MC2301 (and the 12AT7’s).

**First off, as we have heard here and elsewhere, matching of KT88's is not a big deal for this amp. "Don't get hung up on testing/matching power tubes as this will not increase their lifespan".

**What's critical for amplifier performance is based on two designs of the unity coupled output:
a. How well matched the idle current is on the right and left power tube banks.
b. The maximum cathode and plate current capabilities.

**Degradation of a power tube(s) will display itself with clipping as power is increased (there is a way in which to troubleshoot to a degree w/o a tester --keep reading).

**If you notice the 8 power tube slots are labeled V1-/V1+, V2-/V2+, V4-/V4+ and V5-/V5+. Each - (right bank) and + (left bank) pair works together push/pull, cathode and plate since they are unity coupled.

** Now, this is the most significant tidbit I learned -- those 8 power tubes are further grouped into a quad -- the front four (V1-/V5+, V2-/V4+) being separate from the rear four (V4-/V2+, V5-/V1+) as a design feature of the balanced design. What's cool about this is, each quad (per se) can be removed and the amp powered up and operated (carefully) -- this allows one to help isolate at least down to a quad which tube may be susceptible to giving the owner fits.

**Again, stock power tubes should be "looked at" after around 3000 hours and expect them to last between 5K & 10K hours.

** The most critical tube in the MC2301 are the two 12AT7A’s (slots V3-/V3+) which "work hard as they drive the KT88s". These are preferred to be matched and in optimum operating condition.

**Unlike all other McIntosh tube amp designs, the MC2301 is balanced with separate hot and ground connections for 2, 4, and 8 ohms. All other McIntosh tube amps use one common ground for all three impedances.

80B 02-18-2024 11:45 AM

Bob,
Thank you! This is very helpful and insightful.

I'm already down the path of testing a tube at a time each time I start up the amp to listen, so I'll avoid excessive on/off switches of the amp.

The tip about the four front/rear KT88s will be a real time saver in the future. Really appreciate you finding that correspondence. I did a bit of searching here on AA and missed that.

vintage_tube 02-18-2024 11:56 AM

Quote:

Originally Posted by 80B (Post 1080992)
Bob,
Thank you! This is very helpful and insightful.

Cool Beans & glad to have assisted. :thumbsup:

Bob


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