Tracking down a hum.

Today when I powered up my system, I noticed a hum from my tube pre amplifier, a Rogue Audio Model 99 (line stage only). It is worse when first powering up. It is absent through my headphones, (using an external headphone amp and the record button on the pre-amplifier)  Listening to speakers the noise is present but diminishes and eventually goes away as the system warms up.
I'm using the Rogue to send a signal to a Musical Fidelity X-P200.
I don't think the issue is the tubes as I'm using Tung-Sol 6sn7 tubes with less than 300 hours on them.
I bought this Rogue used and had to take it back to get it fixed as one channel kept cutting out because of a bad solder in the out put jack. Maybe something else failed?

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Well I plugged everything into a different outlet. So far so good. I'll have to give it some time to see if that really was the problem. As whart said. "It's usually something stupid."   It appears jond was right as well.

Story has a happy ending. Since the source of the hum has been found and dealt with It was the power outlet I was using odd in that it didn't affect anything else in the system Combined with the new parts installed by Rogue. This almost 20 year old Model 99 sounds AMAZING! Got the best of both worlds right now.