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My DR504 has this intermittent popping noise that appears every minute or two. Louder/softer with master volume.

Nothing plugged in to amp, happens with preamp volumes turned down or up.

Cap job last year, they still look in good condition. I replaced power tubes last week, but the issue persists. Other than that, amp works great.

Here is an MP3 of the noise: https://drive.google.com/file/d/0B7Gmbc ... sp=sharing

Any ideas?

Thanks!

I couldn't get your google drive file to play.

Try all the usual things, remove and re-seat all the preamp tubes. If you have a known good spare, try swapping out the preamp tubes one at a time. You could also look at the inside of the chassis in a darkened room to see if you see any flashes anywhere that correspond to the popping.