Mixed driver tube for Mcintosh MC275

I just wonder if anybody know which sections of Mcintosh MC275's driver tube are most crucial? Is it all sections of equipment are the same in terms of the importance of the driver tubes. Another thing, is it using a tube that has a sonic trait that is different than another like combining a thinner more detailed tube like a Telefunken can actually work very well with a warmer tube like a Mullard?

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

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 ;-)


Anybody have any opinion?

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?
in my MC275 V with very good results. Musicality and instrument definition greatly improved with the Mullards over the stock 12AX7's. Also found that the amp benefitted from using Audio Research tube dampers on V1, V2 and V5. Tried the dampers on the 12AT7's and really didn't notice much of an improvement, at least not compared to the dampers used on the CV4004's.
I am using Mullard CV4004's in place of the stock 12AX7's in my MC275 V with very good results. Musicality and instrument definition greatly improved with the Mullards over the stock 12AX7's. Also found that the amp benefitted from using Audio Research tube dampers on V1, V2 and V5. Tried the dampers on the 12AT7's and really didn't notice much of an improvement, at least not compared to the dampers used on the CV4004's.

I have the MK IV so maybe it is different
The input stage of MC275 re-issue is completely different to old design. In balance input of re-issue, the positive and negative phase is push-pull amplified directly and coupled to output transformer to form a completed output waveform. This new design is to avoid the phase-splitter design in the traditional tube amplifier design. In single-end input of re-issue, the input itself forms +ve phase and use a half of 12AX7 to invert input to form -ve phase. In this case, the signal of +ve and -ve phase is not mirror inverted since -ve phase is suffering distortion produced by an extra 12AX7 amplifier stage. This can explain why the balance input sounds better than the RCA input in the case of MC275 re-issue.

Schematic of MC275 re-issue see http://farm5.static.flickr.com/4037/4204605299_bdfa12827f_b.jpg
I think the RCA sounds better
I wrote the post Mr. Yewlock quotes. There are only three electrical differences between the Mk IV and the MkV:
1. The Mk IV is a regular lead based chassis. The Mk V is lead free. This is nice for the environment (maybe) but there have been some fracture issues with lead-free solder joints.
2. The Mk V has modern five-way speaker cable binding posts, the Mk IV has the old barrier strips.
3. The Mk IV has input level attenuators (pots) -- just for the single ended inputs, that previous models had. The Mk V does away with them. McIntosh says they ran out of room on the Mk V after changing to the five-way binding posts -- but I just think they realized how ridiculous they were in a modern amplifier. If you have a Mk IV and you're running single ended, you should just turn the pots all the way up (clockwise) to get them out of the signal path.

If you look at a schematic for either the Mk IV or the Mk V, you will see that the V-1 and V-2 12AX7's share a 24V filament supply. So even when operating in the balanced mode, with XLR inputs, if you want your amp's left channel to work (the V-2 12AX7 is the left channel "gain-multiplier" tube) then you better have a 12AX7 with a working filament in the V-1 spot -- but it doesn't have to be a particularly good 12AX7, or even a new one ;--)!

The Gordon Gow Commemorative Edition MC275CE was the first 275 to offer balanced inputs, HOWEVER, the circuiting was such that the single ended input wiring was still connected to the amplifier sections when the mode switch was in the 'BAL' position, so you could not switch between two sources, one SE and one BAL. In the IV and V, the SE and BAL inputs are completely independent, so in theory, you could connect different sources (or two preamps) one to SE and one to BAL. Frankly, I don't recommend it, that's what preamps are for.

The amp WILL be quieter (better signal to noise ratio) using the BAL inputs; first because (as Hifiguy said) simply eliminating the V-1 tube from the signal path eliminates a source (the V-1 tube) of potential noise and distortion. There is a second reason for the sonic improvement however, and it's the fact that the BAL input signal is twice the strength of the SE signal, further improving the S/N ratio. And there is a THIRD reason as well! In BAL mode, the amplifier's input impedance is twice that as in SE mode. This guarantees flat frequency response, regardless if your preamp's output is a little on the high side, as it is in many tube preamps.)

Casouza mentions the 12AT7's as being possibly overworked ;--) Thats not really true, but it's especially not true in McIntosh amps which are extremely easy on tubes -- they run cool and last a very long time. The main reason for this is the McIntosh 'Unity Coupled Circuit' which takes current from both the plate AND the grid of the power tube. This requires TWO separate primary windings on the input side of the output transformer(s) AND it requires TWO 12AT7 driver tubes per pair of power tubes, instead of the usual one per pair, as in conventional push-pull power amps. This gets more power out of a pair of KT88's with less stress. It also eliminates the need to adjust/maintain precise bias; in fact McIntosh amps have their bias fixed at the factory, and that's it! This is great, especially if you have fluctuating wall voltage, however it makes it MORE important IMO, to use well-matched power tubes. McIntosh denies this, but that's because they don't even TRY to match the tubes they supply with their amps, just discarding the obviously bad ones ;--)
The Mark IV also has gold plated balanced inputs. The balanced inputs sound far more electric sounding IMHO in comparison to the natural sound of RCA's in any component. The balanced inputs have more gain but I am not sure that makes them better, different for sure. Some manufacturers still do not even use balanced inputs in their components. It was a major disappointment the new amp does not have the RCA's and attenuators. The lead free design is a good thing. I also think the old barrier post allow for a better connection, just not as many options. I could have had either but went with the older model so I can run a phono amp directly into the amp. I like this option. From what I am told, the mark IV and earlier models had their tranformenrs wound by the same person/people that wound the original models. Does anyone know if that is true?

I still have to say that putting a better tube in V1 does make a difference when using balanced inputs. It is not small either. You have to try it. Better bass and well, everything. I don't care what the schematic says, my ears tell me different. Take the stock tube out and put a spare tele in there and come back and tell me you do not hear a difference.
Tzh21y, do you know how to read schematics? I'm guessing you don't, because if you can make a statement like "I don't care what the schematic says" etc., then you'll never understand that there is a cause-effect relationship behind ALL audio experiences. Whether a person prefers one kind of sonic experience over another is their call! But that doesn't change the science.

I've used all kinds of 12AX7A/5751 tubes in the MC275 V-1 slot while driving the amp in BAL mode; and they don't have ANY effect on the sound; a result which understanding the schmatics would make clear for you; unless of course you subscribe to 'audio voodoo' ;--) And if you are in fact being sincere and objective (and good manners demands I assume you are ;--), the only explanation I can offer for your "experiences" it that you've got some 12AX7's with badly damaged filaments, gas or perhaps shorts!

Further, it's been my experience that when a listener says they sonically prefer SE operation over BAL operation, it's because;
1.) Some other hardware in their system is skewing the result for reasons contained in that hardware.
2.) Or the quiet and more accurate BAL operation is revealing some nasty condition or component elsewhere in the system that the less precise and less dynamic ("natural sound"?) of SE mode is masking.

My point here, is that instead of throwing up one's hands and pronouncing that the Gods of Science and Audio somehow or other work differently in THEIR listening room, it would be far more instructive (and productive of better sound) to learn some of the science behind the things we hear (both good and bad) and not just chalk it up to the weather or the color of the listening room ;--) I've looked at your system and would be happy to discuss these things in greater detail if you want to send me an email.

I spoke to McIntosh today and their tech support told me that V1 is definitely in the signal path when using balanced cables on the Mark IV so I now I know I am not tone deaf. According to Mcintosh, it is the circuit that determines whether you are using balanced or unbalanced. If you do not believe me, call them, they are easy to reach. I would like to see the schematics for this amp because I can read them. Are they available? Where would one find them? I know you can get the schematics for the original 275. So there you have it!

I know that I am not the only one that feels that RCA inputs sound better. I have spoke to others and they feel the same way. In the case of the MC275 which we are discussing, balanced cables offer more gain, otherwise, a waste of money. The MC275 is not a fully balanced amp so why would I want balanced inputs. If my amp was fully balanced, then it would make a difference, otherwise, it is a waste of money that I could use to buy records. I would not call it masking the sound. I also think the masking many hear is due to other factors such as the power cords. It is amazing what good AC power cords can do.
Tzh21y the balanced inputs on the MC275 have less gain not more as you stated in your last but one post.
MC275 gain: Balanced - 4 ohm = 16.8dB, 8 ohm = 19.8 dB, 16 ohm = 22.8dB
Unbalanced - 4 ohm = 23.2dB, 8 ohm = 26.2dB, 16 ohm = 29.2dB
These figures came from Mcintosh tech dept.
That may be true as I do not have the gain specs in my manual. It does say that the input sensitivity is 1.2 volts unbalanced and 2.5 volts balanced. The only time I would use XLR is if I needed to use a longer interconnect from the preamp to the amp. It is not a fully balanced amp so what benefits are there to the balanced connection other than if you need a long run to your preamp? it makes absolutely no sense other than spend more money that I could put towards more records.

You can find schematic of MC275 re-issue in http://farm5.static.flickr.com/4037/4204605299_bdfa12827f_b.jpg. You can study from the schematic that V1 is not in the signal path.
Well Tzh21y, I don't know how you worded your inquiry to the McIntosh tech, nor did I hear his response. However, I'm a little suspicious of his words as you reported them
their tech support told me that V-1 is definitely in the signal path when using balanced cables on the Mark IV
Could he perhaps have meant "the V-1 tube is (still) in the SINGLE-ENDED signal path, even when you are using balanced cables"? -- which would be true. But the V-1 tube is definitely NOT in the balanced signal path. EVER! Further, "using balanced cables" has nothing to do with whether the V-1 tube is in (even) the single ended signal path or not. It ALWAYS is. It has to be. What determines whether the V-1 tube is in the (single-ended) signal path (before the signal goes to the rest of the amp) is the position of the BAL/UNBAL switch.

And I can tell you this: I'm right now looking at the McIntosh service manual for the MC275 Model IV (which I ordered from McIntosh after purchasing my amp) and the schematic clearly shows there is NO WAY for the balanced input signal to go through the V-1 tube (and on to the rest of the amp,) when the the input switch is in the BAL position. It's not a matter of whether I believe you. Frankly, I think there was some miscommunication going on when you spoke with whomever at McIntosh.

You can order a service manual from McIntosh for $10, or (now that the Mk IV is not a current model ;--) download a pdf version for free at this site (which I just tested for you): http://safemanuals.com/user-guide-instructions-owner-manual/MCINTOSH/MC%20275%20MK4-_E

OR! you can prove it to yourself by sliding the input switch on your amp to the 'BAL' position. I guarantee you won't hear anything out of your amp (which you WOULD if the RCA signal could get to the rest of the amp (via the V-1 tube) with the input switch in the BAL position.

Furthermore, you can 'double-prove' it to yourself by doing the reverse: unplug the RCA cables and plug in balanced cables (carrying a signal of course) and slide the input switch to UNBAL. Again, I guarantee you'll hear nothing out of the amp. The balanced input signal neither goes to, or through V-1. Why would it? There is absolutely no reason for it to do so when driving the amp with a balanced signal -- in fact, going through V-1 would actually screw up the balanced signal!

If you still think driving the amp single ended sounds better than driving it balanced (assuming you have balanced main outputs available on your preamplifier, and have made an honest A-B comparison,) then it has to be for one or another of the reasons I suggested previously, and you should investigate the cause. If, as you say, "others" feel the same way, then they should also investigate! As for balanced cables being a "waste of money" unless the amp has balanced circuit design, you are simply dead wrong! First of all, studios have been using XLR's since 'the dawn of time' because of their proven superior performance, regardless of the circuit design of the various equipment they use. And second, any conventional push-pull amplifier, be it SS or tube, requires a balanced signal in order to amplify! If you feed it a single ended signal, then you need to first create a copy (splitter) and then reverse the polarity of one of the copies (inverter) and then feed that BALANCED SIGNAL to the amp, or it won't amplify! This is what the V-1 (splitter/inverter) circuit does. Nothing wrong with that except it is a lower impedance input, and adds another layer of processing (and noise, and distortion) to the signal. However, if you are already supplying the amp with a balanced signal, the V-1 tube is not needed, WHICH IS WHY IT REMAINS OUT OF THE SIGNAL PATH WHEN DRIVING THE AMP WITH A BALANCED SIGNAL!

It would have been fun to listen in on your conversation with the McIntosh tech, just to discover where the miscommunication started. In any case, I stand by all my previous remarks regarding the basis for your BAL vs. UNBAL listening experiences. So there YOU have it! ;--)
Okay, after viewing the schematic, it is clear that V1 is not in the signal path. Then what am I hearing? Voodoo? I am obviously crazy! Now I am convinced as to not buy anymore tweeks because my hearing is somewhat flawed.

I thought I noticed a more full sound with the other tubes I used. This is crazy. I am going to put my one pair of balanced cables back into the amp and see what I hear. I did ask the tech and he did say it was in the signal path and would have an effect on the sound. I felt I was pretty clear on how I asked the question. My mind is obviously playing games with me. This is a very informative thread. Maybe I will try different balanced cables and see what happens. There I go again, tweeking.
Tzh21y --

1.) I don't think there is anything at all wrong with your hearing! I'm 68 and I KNOW my hearing isn't what it used to be (to say the LEAST!!) however my ability to LISTEN is better than ever ;--) I wish I could go over your system with you in person, because I know I could find out what is keeping balanced operation from sounding completely marvelous and far superior to single-ended.

2.) My long experience talking with McIntosh technical people (even those at the top) has been 'highly unsatisfactory' to be diplomatic about it ;--) You would have better luck talking with one of the engineers; at least they understand English, but they are harder to reach. See if you can get hold of Ron Evans, V.P. Engineering.

I have a couple of system specific comments:

1.) JPS interconnects are OK (as in: decent)
2.) Everything "Cardas" sucks! Way too much capacitance (especially for your Merlins) and terrible time-alignment. There may be a couple pieces of equipment on the planet that one could connect with Cardas cables, but nothing I'd want to own! ;--))
3.) I'd be interested in knowing what kinds of (good) tubes you own for the amp, and -- I almost forgot -- what (if any) tube changes you've made to the C220?

I have the Cardas on the CD player. I am listening ( auditioning) to JPS superconductor 3 cables connected to the turntable. I can see how some can be critical of them. I have NOS Telefunkens in the C220, all Telefunkens in the 12ax7 position in the amp, GE 12at7's and Tung Sol 6550 in the 275.

I am going to plug the XLR's back in try that V1 thing again. If it still sounds different,
Tzh21y -- If you like the JPS product, you should stick with them, perhaps throughout your system for synergy. I just don't like (almost) any of Cardas' products because of their high capacitance (especially of their speaker cables) and the excessive time smear of their interconnects -- all due to all that "Golden Section" multistranding ;--)

Did your MC275 come with 6550 tubes? And what vintage Tung Sols are they? I ask because you might want to try a quad of KT88's which are more powerful and some say more accurate. I haven't tried 6550's in my amp, so I haven't compared the two. The best price/performance current KT88's are the SED's (yes, I've listened to them and tested them ;--) You can get a matched quad here for about $170 http://www.conusaudio.com The other tubes you are using are OK for now pretty much, except if you are using the C220 phono section, I would recommend tubing that part with 5751's -- either Sylvania or RCA triple-mica blackplates for ultra-quiet, great dynamics, and smooth top-to-bottom frequency response.

In order of bang for buck:
1. KT88 power tubes $170
2. JPS speaker cables (??)
3. Sylvania blackplate 6201's (12AT7's) $150/quad
4. JPS for the other cables (??)
5. RCA or Sylvania TMBP 5751's (12AX7's) for the phono section. $140 or $200 per pair respectively.
I have 2 RCA tall plate 12ax7s that I am not using. Maybe I should try them in the phono section. I do not find the Cardas to be as bad as you are saying. They are decent cables. I may try the Anti Cable interconnect. I cant go wrong for $100.00. If I do noy like them, I will sell them here. Many here are tossing their expensive cables in favor of them. They have spent much more than I on their systems.
Tzh21y -- if you go to the Cardas website >> products >> speaker cables >> Golden Presence, you will see that speaker cable has a capacitance of 168 picofarads/foot. Just as a reference, the maximum suggested capacitance for electrostats is around 20 pf/ft. Which shouldn't be a big factor with your Merlins, which use Israeli Morel cones/domes anyway. However, most Cardas speaker cable is 350-450 pf/ft, and because I personally believe this is, well, not a 'good' thing (and is pretty much a uniquely "Cardas" characteristic) I always recommend to people using Cardas speaker cables that they experiment inexpensively by picking up some insulated solid copper 10AWG wire at Home Depot (poor man's Anti Cables, and just the opposite characteristics of Cardas!) and doing an A-B. You might be very surprised, and it will only cost you a couple dollars and a little time. If I were you I'd do this speaker cable experiment first, and hang on to the Cardas IC for now. If you like what the Home Depot wire does, you can get some Speltz Anti-Cable speaker cable (but mid-priced Virtual Dynamics solid core would be better ;--)

The RCA 17mm longplate, blackplate 12AX7's are one of my favorite 12AX7's They have a lot of balls, and if you use them in the V-2, V-5 spots, you don't want anything too strong in the 12AT7 spots or the sound will be in your face! Tellies and/or Mullies would be fine for the 12AT7's.

For phono stages, unless you're using a very low output MC (like .2mV), my favorite 12AX7 is either a Sylvania or RCA triple-mica blackplate 5751. Very quiet and SMOOTH sound (no rough edges like almost ALL 12AX7's ;--) and they always seem to have closely matched triodes right out of the box. Of course they'll cost between $150 and $225 a pair these days! I have some of those too ;--)
Maybe I will try the RCA's in the phono section. The tellies are quite good and very neutral.