My MC275 has a clueless sound-change...

I would suggest you put the amp back on the floor to see if the "proper" sound returns. This kind of thing can happen with solid state amps. I have a YBA amp that I can dramatically change the sound on by simply changing the material I use under the amps feet i.e soft vs. hard material. Your amp, because it is transformer coupled, is less susceptible to changes in sound due to long speaker runs. That being said, the rule of thumb is to keep your wire runs as short as possible since there is no such thing as a straight wire with gain. All speaker wires introduce some coloration. The longer the run, the more pronounced that coloration becomes. I always think of speaker wires and interconnects as tone controls. G

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

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.)

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).

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

Do you have dedicated AC for the MC275?

Tubes like stable power.

My fuse blew in the first couple of weeks which really bothered me so I ended up upgrading the fuses first. Later I added Mullard 12A7T and 12X7A along with Genalex KT 88/6550.

Expect more depth and bottom end with the Genalex Gold Lion KT 88/6550. Brings your power rating closer to 90 to 95 watts a channel. Since I have oversized bookshelf speakers I could really tell the difference.

If we need to talk offline just send me an email.

I had a similar experience and moved my amp back to the lower rack. Could not explain it, but rationalized that maybe the lower rack offered better vibation control.

The 12 type tubes were mostly new.
Mine is the V generation. The current production.
V1 I left the original tube, because im using single end.
V2 and V5, i upgraded to a new pair of RCA 5751.
V3 and V6, V4 and V7, I installed a matched quad of RCA black plate 12AT7 which all tested around 75-90.

It is very hard to believe that the tubes are out.

Can it be the power tube?
I am interested in changing the tube to the Gold Lion reissue. How are they compared to the stock tubes?

My connections are as tight as they get...

Examine 12 type tubes and double check the fuse.
Double check connections to loudspeaker and amplifier.