MC275 Tube Rolling...

I also realized that on the chassis it is printed 12ax7a. What is the difference between the 12AX7 and the 12AX7A? Will it be okay if I use a 12AX7 instead of a 12AX7A?

Genelexs are in three of my amps. Great tubes. Reasonable price.
Amperex ecc83 are the best of the bunch for sound and longevity. Also,can use-12ax7,12ax7a,7025,7025a,6681,ecc803,ecc803s, plus others.
Telefunken ecc81, slightly dark but very clear and clean. Also can use-12at7,12at7wa,12at7wb,ecc801s etc.
These tubes range from common use tubes to commercial to military grade.
You can pretty much tune amp to your ear.

Mullards, Telefunken and Amperex are the most common used for 12ax7 and 12ax7a in preamps and amps. Some tubes have higher gain or low microphonics with warmer or darker sound.

Magnumpi205 described it pretty well.

Do we really need to follow Mac's instruction for using 12AX7A? or the regular 12AX& and 5751 will work. I am talking about the v1, v2 and v5 position.

If I am using unbalanced input, is v1 in use? Will changing the v1 tube affect the sound more or the v2 and v5 changes the sound more?

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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