Mixed driver tube for Mcintosh MC275

Hi,

After some research, I found this answer from one of the previous posting.

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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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Anybody have any opinion?

Thanks.

Every tube matters in a tube amplifier, but you knew that already.

In general, the higher the gain of a tube, the more it affects sound quality.
The current-version Mc275 uses three 12AX7s (high-gain) as input tubes and four 12AT7s (medium gain) as driver tubes.
There were five versions of the Mc275, some used 12BH7s as drivers, some 12AT7s.
My definition of a driver tube is one that provides the necessary voltage and current to drive the grid of the output tubes.
So, the 12AX7 are actually input tubes or voltage gain tubes and the 12T7s are driver tubes (at least two of those).

The above post is correct, V1 converts single-ended to balanced, it is the phase-splitter tube. If you use XLR cables, V1 is not important for SQ.
If you use RCA cables, V1 is the most important tube on the amp, buy the best you can afford and make sure that its two triodes are well matched.
The remaining 12AX7s are also very important due to their very high gain.

The 12AT7s are less crucial to sound quality, but they will wear out faster than the 12AX7, because they run at high voltage and high current. Therefore, replace or test the driver tubes every couple of years or 2,000 hours. This is valid advice for most tube push-pull amps.

The Mc275 uses a unit-gain output stage (unity-coupled), so IMHO the output tubes do not make or break the amp's SQ as in most of the competive PP amps.
The reason is simple: all of the amp's gain is provided by the 12AX7s and 12AT7s, the KT88s only amplify current.

In summary, if it is a new amp, spend your money first on better 12AX7s.
As a second priority, buy better 12AT7s.
Your third priority would be better KT88s and my vote goes to Gold Lion reissue KT88s in matched pairs.
As for NOS input and driver tubes, there are many options and the sky is the limit.
Search "12ax7 rolling" or "12at7 comparison".

Whatever you buy, do not try the mixed approach until someone shows you your Mc275's schematic and proves why that mixing brands is doable.

I used to own an Mc275 MK V but I did not have access to an schematic, therefore I chose to be conservative and not mix and match tube brands.
I chose three Phillips Holland 12AX7s (ecc83) and four Miniwatt 12at7 (ecc81), all matched.

I hope this helps

I disagree with the discussion regarding V1. I was using the stock Mcintosh tube in this location. I was told that you did not even have to use a tube here if you were using XLR. WRONG! My amp would not even play in one of the channels without a tube in this socket. Then I put a telefunken tube in the stock tubes place. WOW, it made a huge difference, not a little one, so don't let anyone tell you that a good tube in V1 is not important because it is very important. If you do not believe me, try it for yourself

Tzh21y, I did not write that one can simply pull off V1,

If one pulls a tube, most likely a series string of tube heaters will be interrupted and the amp will malfunction.

My experience is with a MK V version, other versions may vary. For example, on the schematic available on the internet (most likely MK I), V1 is essential. The amp will not play without it.

That's why I ended my post as follows:
I used to own an Mc275 MK V but I did not have access to an schematic, therefore I chose to be conservative and not mix and match tube brands.
I chose three Phillips Holland 12AX7s (ecc83) and four Miniwatt 12at7 (ecc81), all matched.

As you can see, I kept V1 in place.
Enjoy the music!

With balanced inputs, does MC275 combine the signal first and then split the phase for push pull output stage?