My Guess :Time for new tubes!!!!!!!
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No, I don't think that is normal.
Given that you've changed the tubes with no improvement, I'd have to say that unfortunately it will probably be difficult to isolate. I would think the most likely bet would be a capacitor or other component that has to handle high voltage, and is starting to break down under that stress.
But external sources would be possibilities as well, such as rfi/emi being picked up through the air or the power line when an appliance or other high current device elsewhere in the house cycles on or off.
Sorry I can't offer any more concrete suggestions.
If you gently tap on the faceplate do you get any noise? If your tubes are going microphonic, they'll definitely be noisy.
Sometimes, however, this could be a sign that the tube sockets themselves have some slight corrosion. Removing and re-inserting the tubes a few times might clean up the problem (and it's a cheap fix!)
Same thing happened to me. New preamp unit and some hissing with popping noise once in a while. Even after tube rolling.
Happened to be an interconnect problem. The connectors themselves got a little loose. Tightened them and the problem was solved.
Another possibility that was brought to me by a technician is older cabling. An old interconnect that has remained plugged and untouched for many years can break inside its shielding sleeve without any apparent marks once it is moved or twisted again. Witnessed that problem at my friend's place when he changed is amplifier.
Thanks for all the advices. This amp weighs 110 lbs with box so want to go over all possibilities before I take it in. For now, I think I can live with it given I have 2 more years of warranty and can take it in if it gets worse.
I don't think it's interconnect since it happens in all channels and don't think it's tubes since 2 sets (1 old 1 new) have same issues. Might be a bad socket or capacitor or fuse. Is McIntosh MA2275 easy to open up to check for capacitor or fuse? Thanks.
Ron from McIntosh recommended I remove my power conditioner and plug it straight to the wall outlet and surprisingly this seemed to have mitigated the problem noticeably. Not sure if it's the power conditioner or other components I had plugged to it, but now I just have the amp plugged in wihthout any power conditioner and the popping is less frequent and much quieter when it does happen. Pretty depressing knowing I could've saved $500 and lot of headache by not getting one.