My MC275 has a clueless sound-change...

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 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.