Mixed driver tube for Mcintosh MC275

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 ;--)

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.

Neil
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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! ;--)

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?

Neil
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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.
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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 ;--)