Weirdest Deluxe Reverb issue

[Randall]

1973 Deluxe Reverb comes in with a LOUD obnoxious hum. Poke and prod. Replace 6V6s and rectifier tube. Pull V1 and V2, no change. Pulling the PI does make it stop. Filter caps are 20 years old, so I lift and measure, they look good, but no change. So I clip in a fresh one in each position, no change. It is then I pull all the preamp tubes to install them one by one to see what I can glean from that, and lo and behold the LOUD hum is gone! WTF? Three more hours of letting it run and trying everything to make the noise come back, but it does not. More poking, prodding, tapping, wiggling, but it is silent.

What in the world was the source of this loud hum? Best I can come up with is a reverb or trem tube pin and socket connection that corrected by removing and reinserting the tube, but that is just an obtuse guess. I hate sending stuff out when I don't know what the cause and fix is. Bizarre.

[pdf64]

Yes, may have been a bad connection to V4 pin 7 grid, leaving it open circuit.
It may be worth paying a bit of attention to the tube pins and socket contacts, if you haven’t already.

[drewl]

Those 70's Fenders always have loose tube sockets

Usually from decades of tubes going in and out!

[Randall]

I did spray out the sockets, but wouldn't the loud hum follow the reverb control if it was V4? It did not matter that the control was turned full off and the tank was disconnected.

[Chuck H]

Well I suppose it wasn't V4 then. But it could have been other things too. As long as you cleaned the sockets and then wiggled the tubes with the amp in operation to check for noises (and found none) I'd guess your original assessment of incidentally correcting an intermittent pin contact is correct.

[pdf64]

I did spray out the sockets, but wouldn't the loud hum follow the reverb control if it was V4? It did not matter that the control was turned full off and the tank was disconnected.

Not if the bad connection was in the 6,7,8 section, particularly the grid pin7.