Cathode plus fixed bias arrangement

[LarBal]

I've got a Magnatone Custom M10 here whose 7189 outputs share a 100r Rk. There is also a neg voltage supply at the grids via 270K . How do I go about figuring what sort of neg voltage to put on the grids.
I've got some modern 7189 tubes and have already jumpered pins 6+9 and 1+2 on the sockets for the original 7189a

Schematic says -17VDC . Close enough for rocknroll?
magnatone_M10.pdf

[stokes]

Better to measure the plate current rather than just rely on the bias voltage setting.