VibroChamp with odd hum issue

[Randall]

I have a 1970 Vibrochamp in that I recapped about 1.5 years ago with a CE Dist multi can. It was making a modest hum with nothing plugged in. Pulled the PI, still made the hum. Swapped out the 6V6 and then 5Y3, no change. I then clipped a fresh 22uF on the terminal the feeds the preamp node while the amp was powered up. It made a pop of course, and stopped the hum. Aha, I thought a wonky can. But, after removing the clipped in cap the hum remains gone. I have powered down and drained the can with 5 watt resistor, then powered back up, hum remains gone. Also, the preamp when turned up is quite noisy, I suspect plate load resistors.

What do you think is going on here? I have not experienced a cap be bad and then OK. Could there be a connection between noisy plate resistors and the plate node cap? I'm perplexed.

http://fenderguru.com/wp-content/upl...-schematic.jpg

[Helmholtz]

Clipping a discharged cap to a charged cap will cause a surge current of several tens of amps. This current mainly flows between the two caps involved and maybe ground. Its peak is only limited by the ESRs and the contact resistances. It may either destroy a cap or fuse a bad contact.

[Randall]

My bad. But it did something, because now the hum is gone, at least for now. Turns out the plate resistors were not the source of the preamp noise, it was the tube. Noisy as hell.

I still don't why it was humming and now it's not after I clipped in hot. Perhaps it did something to the can?

[Helmholtz]

I still don't why it was humming and now it's not after I clipped in hot. Perhaps it did something to the can?

As indicated above, the high surge current may have fused/point welded/repaired a bad cap internal or external (ground shell) contact. Always a good idea to clean and retension the ground shell.

[Randall]

Well, I did find a sputtering noisy V1. I wonder if it was putting heater voltage on the preamp node? It's quiet now.

[Helmholtz]

Well, I did find a sputtering noisy V1. I wonder if it was putting heater voltage on the preamp node?

Hardly possible.