Hum in left channel when I plug in a headphone

[trojan2900]

Great amp, built by someone else. However, when I plug in a headphone into the jack, I usually get a hum sound. If I touch the top plate by the jack, it might go away or lower in volume. If I tap the top of the left handed tube, it might get louder or go away. Sometimes, I just tap the tube a few times so it moves barely, until the hum goes away. Very strange. When I remove the headphone cable from the jack, sometimes the hum returns to the left channel over the speaker, and then I go through the same routine (tapping the tube, etc.) If I pound on my desk (its on my work desk), or close a desk drawer hard, I can trigger the hum.

I've tried different tubes and it didn't help. Should I be checking for something that came loose?

 

[Doc B]

If someone else built it for free, look for a loose connection around the tube sockets. If someone else built it for money, ask them to look for a loose connection around the tube sockets.