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MC275 V Blown Output Tube(s)

nodim2

nodim2

Member
I recently (August 07) traded in my two MC275 GG Editions for two of the Mk Vs. Since that time, I've had two of the KT-88 output tubes blow. The first was covered under the 90 day warranty for tubes, the most recent (yesterday) will not be covered unless McIntosh feels this failure rate warrants some consideration. That's a 25% failure rate in six months (seems unusually high). I'll call Mc tomorrow. At this point, I've almost decided to bite the bullet and replace all the output tubes with another brand. Fortunately, it hasn't had an impact on my listening (except for the inconvenience of isolating the problem) since I have a spare set of the original output tubes (Richardson) that came with the GG Editions. I'm looking for suggestions for the replacements, brand and source. Thanks in advance for your inputs.

Harry
 
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nodim2 said:
I recently (August 07) traded in my two MC275 GG Editions for two of the Mk Vs. Since that time, I've had two of the KT-88 output tubes blow. The first was covered under the 90 day warranty for tubes, the most recent (yesterday) will not be covered unless McIntosh feels this failure rate warrants some consideration. That's a 25% failure rate in six months (seems unusually high). I'll call Mc tomorrow. At this point, I've almost decided to bite the bullet and replace all the output tubes with another brand. Fortunately, it hasn't had an impact on my listening (except for the inconvenience of isolating the problem) since I have a spare set of the original output tubes (Richardson) that came with the GG Editions. I'm looking for suggestions for the replacements, brand and source. Thanks in advance for your inputs.

Harry
Click to expand...

You might want to have the amp checked out to make sure there is no defect in the amp itself that would cause the tubes to fail. You might ask Mc about that possibility.
 
Harry, you have now experienced what roughly 1/2 the owners of the 275 IV and V have experienced. I'd bet a hundred bucks there's nothing wrong with your amp, the OEM Electro Harmonix power tubes are consistently unreliable and regularly put on light shows! I always recommend folks replace them right away. They also have the weakest gain (transconductance) readings of any KT88's new or vintage. So if you want the most power out of your 275, the EH's won't do. The little Chinese tubes are more reliable. But I did have one start to go gassy, which can take out a power tube if not caught in time! Besides, they don't sound very good.

The Richardson KT88's are supposedly terrific tubes (never heard them myself) but you only have enough for one amp, and I almost never see them available on eBay or anywhere else. If you want to use the Richardson tubes (I sure would!) the only problem will be that (even used) they'll still be more powerful (and probably better sounding ;-) than even a quality current manufacture KT88, like the re-issued (sic) "Genalex Gold Lions" or the PentaLabs KT88SC's. So unless you could find more Richardsons, the closest match would be a quad of NOS Gold Lions, Gold Monarchs, or GEC KT88's (they're basically all the same, and all expensive, roughly $200 - $400 per tube, depending.)

On the other hand, if you'd be happy with a couple quads of currently made yet reasonably good-sounding and reliable tubes, then you should get the reissued GL's (~$200/quad) or the Penta KT88SC's (~$150/quad) www.dougstubes.com is an excellent and reliable seller. (He also sells EH KT88's, like the ones that blew up ;-) You could then put the Richardsons on eBay -- they'd probably bring $800 or more if they're in good shape.
 
my 2102 blew some outputs.

I would have it looked at as there might be a few diodes in the circuit that may have been compromised or blown (happened in mine and a easy fix).

Call Mc and ask to speak to Keevin Kenyon. He can tell you about known issues or things to have your tech look over.

I echo the Doug's tubes vote- try the GL reissues or try to find NOS
GE 6550a's
 
(Hi Neil!)

My personal first choice:

The EAT KT88 has an designed maximum anode dissipation rating of 35W and is recommended for use in the output stage of an a.f. amplifier. Two valves in class AB1 give a continuous output of up to 100W. The KT88 is also suitable for use as a series valve in stabilized power supply.

Cost? $1,400 list. I paid $1,200.

~~~~~~~~~~~~~

If you're interested, I'd be wiling to sell my not new old stock but still very powerful Genalex KT-88s for what I paid for them, $800 plus shipping. (I'm too lazy to list them on eBay.) I bought mine from the same guy Neil ('nsgarch') bought his first quad. Shortly after buying them (a month or so. what a dope!) I bought and have since used the EATs.

From the seller (to me):

Here is the background on the tubes:

They were removed from a custom amplifier built by an old friend of mine in the early 1960s (1961-62 era). He built the amplifier using the then massive Dyanco 120 Watt tertiary primary output magnetics using a quartet of Genalex tubes in each channel. The amplifier made use of sequenced power supplies where the filaments were powered up for 60 seconds prior to any plate voltage and then the plate voltage was sequenced on via a series of time delay thermal relays to avoid any shock to the tubes etc. As a result, when the amplifier was shelved a few years later the tube were in excellent condition. They all measured 9000 uMho on my Military TV-10D/U tube tester and show no signs of gas or shorts. I was actually surprised at how well the units had held up as well as the balanced GM tube to tube to tube....

I am currently repackaging the amplifier to reduce the bulk weight of the unit's monolithic construction which weighs in at around 150 pounds for the amp and power supply. I'll be using my stash of GE 6550s and Tung Sol 6550s in place of the Genalex KT88s.

As mentioned, one pair is Genalex Golden Lions and the other pair is Genalex Blue Label KT88s. They all measure in at 9000 uMho as noted earlier. Bases are fine and locating keys are present. These are triple getter units which are nice and shiny.
Click to expand...

Send me a private message if you're interested.
 
I have about 80 hrs. on my 2-275 V and so far so good. I am however considering changing them!
 
Ditto on Doug's Tubes...speedy delivery too (plus some equipment as well)!

Funny (tragic) about the EH's--a friend has recommended that I change a couple of EHs on a recent new amp purchase.

If EHs are seemingly so unreliable one would think that OEMs would use other tube manufacturers?
 
Been listening to my 275 for close to 2 years on and off and no problems what so ever with the Svetlana SED KT88's. :thmbsp:
 
I've had my 275 V for a year now and nothing wrong so far with factory tubes.
I bought a spare quad of Gold re-issued KT88s jsut in case of. Waiting for something to go wrong to try. I don't like to fiddle with tubes, especially if it is not broken ;)
B.
 
if you blown an output tube on an recent model MC-275 you may want to have the driver tubes checked. I had one blow a few tubes and it turned out to be related to bad driver tubes.
 
Thanks for the replies and sharing your experience/suggestions. I failed to mention in the initial post that both tubes failed from the same amplfier. Both times it was just a matter of replacing the fuse and tubes. Having said that, I don't think it's the amplifier itself. After replacing the first set under warranty in early October, the second set had been performing fine, averaging 2-3 hours daily since then. If it had been the amplifier, I think the second bad tube wouldn't have lasted that long. Perhaps it is one of the small tubes. I have a spare set, so I will replace them also. I have decided to replace all the output tubes in both amplifiers. Ordered eight Gold Lions from Doug's Tubes. Hope to have them this week. Will be interested to hear the difference between them and the OEM tubes. This has been somewhat of a bad experiene in that I had one of my GG Editions since 1993 with absolutely no issues, though I had replaced the original Richardson tubes with Svetlanas (through choice, not necessity). Two years ago, I purchased a second GG Edition from Audio Classics, and had absolutely no issues with it either. I'm keeping my fingers crossed.
 
I have had over 2000 hours on the stock EH tubes in two 275's and had only one failure - these are the McIntosh branded tubes with circular holes in the anodes - the Gold Lion tubes seem identical in construction to these and to stock EH tubes (take a good look at the internal structure) - I would bet they are the same tubes with a different brand...
 
Users are reporting good results from the Gold Lion reissue KT88 tubes. They certainly look cool!

Thanks,
Ron-C
 
not the same

The construction of the New Sensor Electro Harmonix (McIntosh OEM) KT88 is considerably different than that company's "re-issued" (and re-designed I might add ;-) "Genalex" "Gold Lion" KT88:
The GL has 3 getters, the EH has one.
The GL bottle is much heavier than the EH because the GL has higher vacuum.
The GL has plate cooling fins, the EH does not.
The GL has copper grid posts and gold grid wire, the EH does not.
The GL has a tranconductance (gain/power) of 6500 to 7500 mMHOs. The EH has 4500 to 5500 mMHOs.

If however, you want an actual duplicate/replica of the original British-made Gold Lion, that would be the Pentalabs/Shuguang KT88SC which is almost identical, right down to the solid black plates.
 
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