Part fix-it, part theory on stand-alone reverb build

[luthierwnc]

Hi all and happy spring!

I've been here before with this project but this a new issue.  The schematic and layout diagram are attached.  This is a reverb unit using a capacitor-driven tank as per Ampeg.  I got it nearly done but for a nasty little buzz which happened when the reverb was engaged or not.  The original schematic and tank call for amp grounding for both the input and output jacks which I did.  Those are grounded on an aluminum chassis right where you see them in the lower right of the layout diagram.  The other signal grounds are on a bus wire behind the pots.

I've got a little gizmo that has a 0.1, 630v cap triple shrink-wrapped on one end and an alligator clip on the other -- very handy for isolating unwanted noises.  When I put it on the grid of the 12AX7 recovery triode it eliminates the buzz with no effect on the sound in either reverb or bypass mode.  I tried it with smaller caps and the bigger one worked best.  Everything else works fine.
 
A practical man would hang a suppression cap on the pin and call it Miller time.  Me; I'd like to know what happened.  On some schematics the voltage divider at the tank output/grid/ground is different but that wouldn't (shouldn't?) affect the dry sound.  Any ideas?  Thanks, Skip
   

[separateness]

Does it buzz with the send and return cables to reverb tank unplugged?

[Williamblake]

I would connect the tank chassis to chassis and the coils to the circuit only. But from reading so many threads this in (my) theory is good grounding practice but rarely the cause of the buzz.