Eh Hot Tubes direct/effect out blend

[SonicVI]

I have an original EH Hot Tubes that I se with bass.  I don't ever use the direct out so I thought maybe I could put a pot in the direct out jack hole and wire it up to blend between the direct and effect outs.  I tried it out using a 500K pot and it worked and sounds amazing, but I'm getting a really bad buzzing from the effect out even when there's no instrument plugged in. It goes away when the pot is all the way to the direct side.  Anybody have an idea of what the problem is and a possible cure?  Here's a link to the schematic.

http://www.schematicheaven.com/effects/eh_hottubes_dist.pdf

[SonicVI]

bump

[Toney]

It isn't quite that simple.
You need this 
http://aronnelson.com/gallery/Toneys-Album/Buff_n_Blend
Search for the Schem

[SonicVI]

I've done that before, but I thought maybe since the direct out is going through the first opamp already it would be almost the same thing.