Princeton Reverb

[Bruce / Mission Amps]

A real long shot... but have you replaced the reverb pot yet?? It could be that it is not grounded well internally.
It still sounds more like a bad filter cap or ground loop ground issue so, I would try a grounded 22uF-500v cap right at the feed point of the reverb OT.... the Hi-V B+ side, not the tube side.
Make sure the ground of the cap is either right at the same place the Hi-V red/yellow center tap is grounded or the same place the circuitry of the reverb section is grounded at the brass plate.

[Euthymia]

What Bruce says: a 22uF/450V cap across the one at the reverb transformer/screen supply. If that cap has gone senile, you might very well get hum in the 'verb circuit. Even though same node supplies the power tube screens, that ripple might be getting nulled out.

[Valvehead]

Bruce - IT WORKED !

I had to use a 47uf but that killed the hum !

The 47uf isnt going to cause any problems vs a 22uf ?

[Euthymia]

Bruce - IT WORKED !

I had to use a 47uf but that killed the hum !

The 47uf isnt going to cause any problems vs a 22uf ?

Not problems, but it may change the sound and touch response of the amp.

47uF will stiffen up the screen supply and everything downstream a little. If it's your amp, you could leave it in and see if you like the effect.

If it's a friend's or client's amp, I'd get a 22uF.