Why not use cathode resistors --130 ohm--(no separate supply required)(bypass them -100uf)

If using bias supply, then...
Should require a negative DC voltage decoupled & fed to 6BQ5 grids (cathodes grounded)

Bias Voltage: 23 v for 250 v plate/screen, or 28v for 300v plate/screen


Reference: RCA Receiving Tube Manual