MC275 Tube Rolling...


I would love to roll some tubes with my MC275 (the current production version V) It is highly appreciated if all you who are experienced drop a few lines... I would like to know about the tubes that you have ever rolled on this machine from the beginning until the end, the sound characteristic for the each of them and the one which you settled down with and of course... why the choice... Yeah, I am clueless of what tube to get since there are so many out there... All your inputs will be extremely valuable to me. Thank You So Much!
infinity_audio
The "A" designation simply refers to a filament configuration that allows the tube filaments (there are two "tubes" in a single bottle) to be connected in series or in parallel (to accomodate 6.3V or 12.6V power supplies.) In the 275, you can use any 12AX7 or 5751.

As for tubes, I have arrived at certain preferences (none of the ones previously mentioned here;-) but there are lots of opinions and it gets a bit like discussing cables (though not quite so bad!!) I will agree the stock tubes uniformly suck! Now that wouldn't be such an issue, and ordinarily I'd just mention it in passing and let people discover alternatives on their own (or not) except that besides sounding crappy, they can be dangerous -- to the amp! I can't tell you how many new (and usually new to audio ;-) MC275 owners have had KT-88's arc/short and blow fuses (and minds!) And if the fuse doesn't blow fast enough, or you don't hit the power switch fast enough, it can fry a plate resistor or worse and then it's going to be more complicated than just replacing the faulty tube(s). So I recommend changing out the power tubes immediately - for insurance reasons, not so much for listening pleasure - that's just icing on the cake ;-) Either the Shuguang/Penta KT-88SC or reissue Gold Lions are very reliable and sonically more robust than the OEM Electro Harmonix (although the "sound" of an MC275 comes first from the choice of 12AX7's, then the 12AT7's, and last the KT-88's. There is also a new (haven't heard it) Shuguang 50th Anniversary black glass KT-88, very sexy looking ;-) http://grantfidelity.com/site/Shuguang+Treasure+KT88-Z and $258/pair. Heck, if you're a shopper, you can find a pretty good used pair of original GEC Gold Lions for that kind of money and really be in business! But as I said, I haven't heard them, and the KT-88SC's I tried were quite impressive.

As for tube matching. McIntosh is semi-correct in saying it's not critical to have closely matched power tubes in their amps. That's because the Unity Coupled Circuit design doesn't rely on critical tube bias adjustment. The Mac PT bias is actually set to a fixed value at the factory and that's it! One reason they (McIntosh) can get away (most of the time) with sloppy matching of the KT-88's is because the OEM EH KT-88's have a rather low power curve (5000 Gm max.) whereas Shuguangs and new/reissue GL's are 7000, and NOS GEC's or Gold Lions are 9000 - used!! And even at 5000Gm, the the stock tubes often fry, and I'm convinced (though I can't prove it) that it's because they're not better matched. So if you're getting new PT's, get a matched quad and be done with it (better yet, get 5 matched tubes and in the unlikely event one goes bad, you can drop the spare in anywhere without worry or having to buy a new matched pair for that channel ;-) Otherwise, you can rotate the 5 tubes every 6 months like tires -- I'm serious! ;-))

The 3 latest MC275 models (the GG/Gordon Gow Commemorative, the Mk-IV and the Mk-V) have balanced circuits (I finally realized that's why they're the only three models with XLR inputs -- duh!) If you are driving the amp single ended, the V-1 tube IS in the signal path in order to convert the incoming un-balanced signal to a balanced signal to drive the rest of the amp. Each side of this double-triode tube handles one channel. So it's CRITICAL that both sides of this tube (regardless what brand) have matching Gm (which stands for transconductance or gain) otherwise your channels could be unbalanced; and since they dropped the gain controls on the model V (Infinity-Audio,) you can't be fixing an out-of-balance tube at the amp anymore. V-2 and V-5 should be reasonably matched (within 20%) both internally and with each other. Ditto the pairs V-3,4 and V-6,7.

If you eventually acquire a preamp with balanced outputs and can drive the amp balanced (highly recommended) you will still need to keep some 'ole 12AX7 in the V-1 position to complete the filament circuit, but it won't be doing anything with the signal, so it need not be something expensive -- even one of the OEM 12AX7's will do ;-)
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The folks at Grant Fidelity have asked me to update the link to the Shuguang Treasure series KT88 which appears in my previous post. The new link for these tubes is http://grantfidelity.com/site/Shuguang_Treasure_KT88-Z
Thanks for such an interesting comment! I've a MC275 MKV and I would like to retube it. Now, my main concern regards bias, as I've been told that electric parameters between valves can change significantly. In a nutshell, I would love to replace factory tubes with Genalex 12AX7 and 12AT7 and Genalex KT88 or Shuguang Treasure KT88 or Tungsol 6550 instead of factory KT88, but I'm quite scared of potential bias consequences if it can cause any harm to my beloved Mc.
Could you kindly give me some suggestion or assurance before deciding what to do as you seems much more expert than me....
Many thanks
Lots of talk about this tube so I bought a quad set; hopefully, what I've read will be duplicated in my system...
5 years later, I have settled down to the following tubes...

KT88 Gold Lion
(will try black treasure one day)

V1 (12AX7A)
RCA GP 12AX7A

V2,5(12AX7A)
Mullard Blkburn GP 12AX7A

V3,4,6,7 (12AT7)
Sylvania Gold Brand TMBP 12AT7wA (aka 6201)