it's pretty common with many amps. I generally ground my preamp only, and lift the ground on power amps, assuring they have an appropriate ground connection through the IC to the preamp.
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This can have something to do with the wiring/grounding of your place. I used an MV55 and had the same problem, fixed by the same cure, when I lived in one place, but at the next place no problem even when the amp was grounded. (In both places the preamp and power amp were each plugged into the same central power distributor.) The only thing that puzzles me about your experience is that you say you don't hear the hum until you turn on the preamp. With mine the power amp hum was heard whether or not the preamp was turned on.
I wish we have some scientific explaination abt hum/ his/pop from tube pre and SS amp or vise versa. My tube pre gets loud hiss from speakers when mute off. It also pop when volume up and down at my house. I took the same pre/ amp/ ICs to dealer's place, no pop no hiss. Blame it on my eletrical wire ? Very convenient, temporary answer to a permanent problem.
Thank you very much for sharing your experience. I do own other tube amps such as audio research, rouge, audible illusions, Cayin without having this problem. Zaikesman, sorry for mis-informing you about no hum when the preamp is off. That is NOT true. The hum is definitely present and louder than normal when the preamp is not in operation or in mute mode during the soft start process and gets much louder (5X) when the preamp is ready for use.
I've owned two CJ amps (and 3 of their preamps),. The Premier 12 monoblocks exhibited no hum problems at all, and I frankly don't recall any with the pair of MV75A1 amps I had way back in the 1980s, but I THINK I'd remember.
A couple years ago I bought a Hsu subwoofer amp and was surprised that it came packed with a cheater plug and directions in the owner's manual to use it if necessary. It was, and I did. Good luck, Dave