[morzh]

Nick,

if your other pot that gave you volume no longer works, it is then posibly not the pot, unless the other pot is bad also.

First test: disconnect the pot fully, connect the 1st Audio input (cap 101) that went to the pot's wiper to the 2nd Det output (cap 74) that went to the pot's top pin.
That should give you full unattenuated volume.

See if this works.

PS. I am not sure why it should be positive on the grids. From the sch it should be negative to the chassis. Grids are pulled to the B-, and cathodes to Chassis, and chassis is above the B-, so Cathodes (filaments) have to be negative .