First KT88 failure in MC2301

[jdandy]

Last night I was listening to music on the studio two channel system, enjoying Lee Ritenour - Wes Bound. I had the lights off, including the meter lights turned off on the preamp and power amps. I was sitting in the sweet spot drifting with the music, gazing at the soft orange glow of the KT88's in the windows on the front of the MC2301's. All of a sudden one of the KT88's in the right channel amplifier arched in a flash of white and bright orange. Just as quick as that happened the McIntosh Sentry Monitor circuit shut the amplifier off, fortunately without any damage to the amp or speaker. I immediately powered down the entire system. I only heard a low click noise in the right channel speaker when the amp turned off. I believe the suspect tube was the second one back from the front on the left side of the amplifier (V4+), but I replaced the first and second KT88's (V5+, V4+) just to be on the safe side. After replacing the two tubes I turned the system back on and the right channel MC2301 came back to life.

The two MC2301 amps are loaded with Gold Lion KT88 tubes, 16 total for the two amplifiers. I checked the hour meter I use to track the time on tubes and it displayed 760 hours. Now I have two KT88 power tubes I need to check because one of them is still good. 760 hours is low service for these KT88's which usually have a lifespan of 3000+ hours, so I want to identify the good one to keep and the bad one to toss. My problem is I don't have a tube tester. That moved me today to order a new MaxiMatcher II power tube tester and while I was at it I also ordered the MaxiPreamp II tube tester from Maxi Test in Seattle, Washington. They both ship tomorrow so I should have them by Monday or Tuesday of next week. I should have purchased these two tube testers a long time ago. Maxi Test has recently upgraded both tube testers, so it is working out in a good way. Now I can test all of my tubes myself any time I wish including new tubes that I purchase. I also ordered another four Gold Lion KT88's to make sure my tube locker maintains a sufficient quantity. That will give me a total of 20 spares. I'm pleased I decided to get the MaxiMatcher power tube tester and MaxiPreamp tube tester at the same time. Being able to test and match power tubes and small tubes is a necessity for someone who owns tube preamps and power amps.

I can't say enough about the McIntosh Sentry Monitor that guards the amplifier from tube and circuit failures. It shut the MC2301 down so quickly that if I hadn't been looking right at the amplifier in the dark when it shut off, I might not have known what happened. The Sentry Monitor circuit in McIntosh amplifiers is worth its weight in gold. Nothing would irritate me more than having to pack up one of these beautiful beasts, all 116 pounds not counting the 25 pound shipping boxes, to return the amp to the factory for repair. Even worse would be having the studio two channel system out of service while waiting for the amp to be returned, plus the stress and worry associated with shipping.

As the old saying goes, all's well that ends well. The MC2301 amps are singing as I type. This incident became the catalyst to get me off center on purchasing the tube testers I have wanted for years. It's about time. The care and feeding of tube amplifiers can be challenging from time to time, but the sound.....oh, the sound, that's what makes it all worthwhile.

[Masterlu]

Dan... Interesting read; this of course prompted me to play catch up. I just ordered a MaxiPreamp II tube tester for myself, as I already have the MaxiMatcher tube tester.



I'm glad to hear all is well, and your amps are singing once again.

[MyPal]

A good tube tester is an invaluable tool for anyone with tube driven equipment. Used in conjunction with an hour meter will take the guess work away.

Additionally, periodic readings over time will allow you to plot the diminishing performance curves of tubes that are in commission.

Dan....I know you prefer to keep your grills on your MC2301s. I have chosen to keep them off to facilitate such maintenance, plus I like the look. This has sparked a thought though....

I'm thinking that it would be worthwhile conducting some further EM emission tests on the radiating sides of the MC2301s with the grills both on & off. Say, using a Tri-Field 100XE meter. In some situations, there could be benefits to leaving the grills on as they may work like a faraday cage, offering additional EM shielding.

If by chance there was an unshielded power cord radiating a large electric field (say 200mm radius) of which was running along the side of a tube amplifier, I'm thinking that this could be very bad for the sonic performance of the tube amplifier.

[jdandy]

Ivan.......I think you'll like the MaxiPreamp tube tester. MaxiTest has been manufacturing the MaxiMatcher power tube tester for nearly 15 years I think, but their small tube tester has only been around about four years. They both have a solid reputation for easy use, solid construction, and reliability. We're both set now.

[jdandy]

Originally Posted by MyPal

Dan....I know you prefer to keep your grills on your MC2301s. I have chosen to keep them off to facilitate such maintenance, plus I like the look. This has sparked a thought though....

I'm thinking that it would be worthwhile conducting some further EM emission tests on the radiating sides of the MC2301s with the grills both on & off. Say, using a Tri-Field 100XE meter. In some situations, there could be benefits to leaving the grills on as they may work like a faraday cage, offering additional EM shielding.

If by chance there was an unshielded power cord radiating a large electric field (say 200mm radius) of which was running along the side of a tube amplifier, I'm thinking that this could be very bad for the sonic performance of the tube amplifier.

Steve.......I have not given any thought to EMI radiated into the tubes or away from the tubes until you mentioned it. I would be interested in the results of any emissions testing you might perform. I leave the tube cages on primarily because I prefer the look, but there may be some real world benefit from the tube cages with respect to EMI. Please keep us posted on any findings.

[arien]

Good to hear all is well Dan....this prompted me to save for the tube testers too...

[Volks]

Thats a relief Dan that its only the tube that gets damaged.

[Clad 7]

Well ended for you jd !

[djwhog]

Glad all came back to life Dan!

Dan on a tester at that level will it allow to test working tubes and thus give an indication of weakness and or how much life is left even in a working tube?

I think at this level of tube gear/ownership it is like a race car tech having modern computer aided diagnostic tools to test and configure equipment and also fine tune to the limits of the gear one owns.

Nice looking unit

[Hangar18]

Originally Posted by Volks

Thats a relief Dan that its only the tube that gets damaged.

You got that right. I've had to repair countless amps without McIntosh's Sentry Monitor - the end result is usually a fried resistor. Kudos to McIntosh Labs.