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 ;--)

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.

Neil
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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.