Replace both KT88 tubes and hopefully the fuse stopped further damage. You can take off the bottom cover and look for further damage. Call Mcintosh and get their opinion. With tubes many possibilities exist, not all good ones. Hopefully you were using fast blow fuses. Slow blow respond too slow. Jallen |
The tube arced a resistor may be open. I suggest you contact the McIntosh service dept. for further assistance. |
less likely you'll need to replace all power tubes and hopefully the tube only took out fuse. you may place live tube onto the same slot(use dummy load resistor instead of speaker) and if one is blown, chances that you have a bad dc cap are big. however you can still visualize the circuit components and see if you will spot leaky dc-cap or anything fried.
good luck. |
I would just change the power tube and see if it works.
If so, then I would investigate perhaps replacing power tubes if they are getting weak. Arc-over failure is a common failure mode of weak power tubes.
If the amp does *not* work I would get the amp serviced. So you can approach this in a simple yes/no manner. |
For a test, you don't need to buy more KT88's. Swap one of the good tubes that's in there now into the bad position. Turn the amp on and see if the good tube comes on. If not, then there is something inside the amp that needs fixing. I would the guess the original bad tube suffered an internal short, which is not uncommon in power tubes. |
Ah, the worry of a tube amp owner, your days of carefree listening are over when you get a tube amp. Anyway, I had a vac avatar super that used kt88 and I have had a tube go bad and take out the resistor along with it as hi-fi geek mentions and I have also had it where just the tube went. It depends on how long you have used the tubes, if you are close to the beginning I would replace with a matched one and keep going, but if you are past the mid-way mark, it might be better to replace the entire set. Unless the resistor is really gone you should be ok. I think I might be over protective but I don't leave the immediate vicinity with my tube amps left on. Let us know how you make out. |
When a blown tube takes out the resistor on my amp, new tube will work but can't bias and just runs very hot. |
Being a current owner of an MC275 mk.V and having had several issues with tubes, I'm fairly certain that the circuit layout and tube designations are the same and I can tell you that you have one of 3 most likely causes for the KT88 tube to be dead.
1- The KT88 has failed outright.
2- The 12AT7 tube that supplies bias to the KT88 tube is defective.
3- The fuse that protects the biasing of the KT88 is blown
most likely from a defective 12AT7 tube.
Things to try before opening up the chassis to check for a blown bias fuse:
Swap one of the known good KT88's into the position of the dead KT88. Does it light up and hold steady? If yes, KT88 is the culprit. Replace dead KT88 tube.
KT88 tube swap results in the known good tube to be dead.
Swap 2 of the 12AT7's and if the known good KT88 in the suspect position is now functional, the 12AT7 is the culprit.
If the known good KT88 is still dead after the 12AT7 tube swap, you most likely have a blown bias fuse.
There are 4 bias fuses inside the chassis one for each output tube. Remove the bottom cover and check for a blown fuse and also check for any signs of component damage/overheating on the PCB. (It goes without saying but if you are not comfortable/familiar/knowledgeable with working on tube equipment, STOP and get experienced help.)
If no damage is noted, replace the fuse and the bottom cover. Retaining the swapped 12AT7 tubes and with the known good KT88 in the suspect position, apply power to the amp. Does the KT88 in the suspect position light up and hold steady? If yes, the circuit for that tube position is good.
Power down the amp and swap the 12AT7's back in their original positions. Power up the amp. Does the known good KT88 in the suspect position now light up and hold steady? If yes, the problem was the fuse. If no, you have a defective 12AT7 which probably took out the fuse again. This would also be indicated by the KT88/bias fuse being dead in the circuit that the defective 12AT7 was swapped to earlier.
Depending on what you find, replace the defective tube(s) but I would suggest you replace them all in a set/pair whether it's the 12AT7's or the KT88's. At the very least, buy and keep on hand a replacement tube for all the positions as well as fuses. That way you minimize your downtime and have known good tubes/fuses to troubleshoot with.
Cheers |
Thanks all for the input. Exactly what I was looking for.
My KT88s are probably 5 to 6000 hours old. I was once told these last about 9000 hs, so I'm getting towards the end of life I guess.
I did aas jakegt3 suggested and swapped two KT88s: the failed unit and another one. Upon turn on, the "failed position" had the new tube glowing and the failed tube was not glowing again. Hopefully Jake is right and this shows the resistor didn't go.
BTW, the failed tube shows black markings on the top of the unit, whatever that means.
Will look for new tubes and double check with McIntosh.
Thanks all again! |
You should be good to go. The "bad" tube position cannot have a blown resistor if a tube lights up at that position. The fact that the tube from "bad" position does not light up when plugged into the "good" position shows that tube was bad. |
5,000 to 6,000 hours on the KT88's is a *lot* of hours. My amp (ARC VS-115) has 6550's and ARC advises that 2,000 hours is the "expected" life of these power tubes. |
With most pentode or tetrode power tubes these days 2000 hours is about right. After that you are lucky. |
Ditto advice ARC gave me. 2000 on power tubes -- that's a max Q!! |
5 to 6000 hours on the tubes???? Wow. Who told you that the tubes have a useful life that long? I think you really were pushing your luck. Manufacturers will give an estimated life range for the tubes they use and will include a "fudge factor" amount of time, for the people that are too stuburn (not you) to get up and go get tubes replaced. But, I think you really went over the edge on your tubes. Once you get this taken care of, you should really pay attention to the manufacturers recommendations. Also, Ralph (Atmasphere's) advice is spot on. |
OK, lesson learned: power tubes should be replaced after 2000 hours or so. How about signal tubes, like the 12AT7, 12AU, 12AX, 6922, be it them in the amp or pre amp?
I have no music but sort of feeling lucky it only seems the tube went bad.
Thanks all! |
Small tubes can last 5000 or more hours. I replace mine when the sound becomes dull or noisy. I keep extras about so when I question the effects of age I simply substitute in a new tube(s) and see if there is a meaningful improvement. |
Thank you.
Opinions re what KT88 to buy? Psvane black bottle, Genelec (Jim McShane recommended this one), other? Don't want to spend over $400. |
The Gold Lion Genelex reissues are my favorite kt88, excellent at everything. |
Before you decide which tube to use, consider it's reputation, in general, for the tone it will produce (with your equipment - it is about synergy). I think the Gold Lion Genelex is an excellent place to start if you really don't want/need something specific in a tube, that is a tonal color to match your speakers and the rest of your stuff.
FWIW I've never used your Mac. I assume that it can use other tubes in the 6550/KT88/KT90 (and maybe KT120) family of tubes. If that is so, you have a lot of tubes to consider which can help you achieve a tone to match your system. |
My favorite KT88/6550 tubes are the old stock GE JAN Solid Plate 6550 tubes. Another favorite are old stock Sylvania grey plate 6550 tubes. A recent production KT88 tube that I like was the solid black plate Penta Labs KT88 tubes. |
I thought I should give you guys feedback after all your help: I installed a KT88 in place of the failed tube, played the system for an hour, and everything seems to be as usual. Thank goodness!!
Beyond the anxiety the tube failure created, this thread was most helpful to me in that I learnt several things. Great forum, great answers, thank you!!
So now I'm running my amp with 3 tubes that have 5 to 6k hours...I need to replace them ASAP. After the reading and exchanging with Jim McShane, I'll go with new Gold Lions. |
So now I'm running my amp with 3 tubes that have 5 to 6k hours... I've never heard KT88 last this long. Either you are VERY lucky or miscalculated. My old amp uses matched quads per mono. The sound was getting weak, several tubes just stopped glowing and died peacefully ~2500 hours. I never had any 6550 blowing but several reissued Gold Lion KT88s failed and took out resistors only after several hundreds of hours of use. If I were you, check out KT120 ... only heard positive comments in terms of sound and durability and NOT just more power. |
If there was lots of idle time, I guess the number of hours increas drastically. 5...6000hrs is darn lot. I'd never idle tubes and always would turn them off when not listening. It takes only 10m for tube to reach normal DC offset. |
