Loose Tube Incident

Turned on the stereo tonight and heard a low hum. Uh oh. It's just a little Douk Audio ST-01 pumping 100 watts-per-channel, but still...

Looking at the amplifier my first instinctive reaction was that it was a loose tube. The one on the right looked crooked. Must have bumped it while messing around with a USB cable connection near the amp earlier this evening. Switched off the power to it, pulled the tube, felt its pins. Hot. Let it cool, plugged it firmly back into its socket and switched the amp on.

Problem solved. I love it when my instincts are right on, and that I have wisdom to trust them.