My MC275 has a clueless sound-change...

I don't think your problem has anything to do with tubes. Sounds to me like what you are hearing is a support change.

The mechanical characteristics of the shelves in most racks are totally different between the bottom and top shelves because of the inverse weight distribution. The top shelf will be much more wobbly and and less damped (within certain frequency ranges) than the bottom shelf - particularly with such a heavy amp providing the guiding force.

I have been experimenting with equipment supports and it is clear that changing the loading changes the sound of whatever component is on it. This is a well known effect with monitor speaker stands but it turns out to be equally applicable to all other components - especially tubed ones.

Arthur

Check the start up tube that takes most of the current when you turn on the amp. Had to change that tube to a 6681. That is the 2nd from the left of the controls and next to the covered tube (12ax7a).

oh forgot to mention, I have the current model.
Not the vintage one.

The machine really sounds normal, no pop, no cracking sound, no sparkles in the tube, no abnormal distortion at normal listening level.

*** It will be very difficult for me to get the tubes checked. (If someone in the NYC area is willing to help, I will greatly appreciate it.)

Is the top shelf made of glass ?
That coud explain your desscription of the changes.

If you are driving your 275 through the single ended inputs, the V-1 splitter/inverter tube (12AX7) is IN THE SIGNAL PATH and has BOTH CHANNELS running thru it, making it the MOST important tube for sonics. AND, because that model 275 doesn't have balancing pots on it, it's critical that both sides of the V-1 tube match closely for gain, or you may experience channel imbalance for which (in the Mk-V) you can no longer compensate. Therefore, IMO, you should be using the best 12AX7 or 5751 tubes you can afford for ALL THREE 12AX7 positions, and especially for V-1. The 12AT7/6201 quad of tubes is important for sonics too; not as much as the 12AX7's but in a McIntosh amplifier, more important than the power tubes.

IMO, amplifiers of all descriptions should be either a.) near the floor on an amp stand, or b.) on a VERY STOUT medium height rack (30" max. gives a decent height-to-footprint ratio,) but remember, even an extra 30" of speaker cable could be costly; both in dollars, and in damping factor, if you're using a tube amp ;-)
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