deicide67 wrote:1) is the bias voltage of .7v more important than the -58v being dead on?
Absolutely. The 700 mV drop across the 10 ohm resistor gives a measurement of the idle current through the tube. I=V/R, so 700 millivolts divided by 10 ohms equals 70 mA idle current.
The -58v bias voltage is just the means to set the idle current where you want it. If the idle current is too high, then you need to apply a greater negative bias voltage. The electrons are cooked off the cathode by the hot filament. They want to fly through the vacuum to the plate because they are attracted to the large positive voltage there. If left to their own, you'd have an enormous current flowing through the tube (at least until it melted). Putting a negative voltage on the grid "scares" the electrons back towards the cathode and doesn't let so many of them get to the plate.
2)Do you typically balance the two until they have the same bias voltage with the 500K pot and then use the 10K to get them to the .7v level regardless of the voltage present at points G & F?
When starting with a brand new set of tubes, I'll adjust the 500k multi-turn trimpot on the Poseidon board until it is right in the middle. This assumes the KT88 are reasonably matched. Then I turn the 10K pot fully counterclockwise, which on a Mark III would set the maximum negative bias voltage. At this point you could power it up with the rectifier pulled out and verify with a meter that there was roughly -65 volts DC at pin 5 or 6 of the power tube sockets.
With the tubes and the rectifier in, power it up and monitor the voltage drop across the 10 ohm sense resistor. Fiddle the 500K trim pot until both sides show an equal voltage across their 10 ohm resistors (from the bias test point to ground). You can even stick one meter probe in each bias test point and measure the differential between them. You want the differential to be zero - the power tubes are balanced at this point.
After than, I'll slowly turn the main 10k bias pot clockwise and check each side to see that it goes to 70 mA (700 mV). It may be necessary to go back to the 500k trimpot again and re-balance them as you bring the idle current up.
Keep in mind the 70 mA specification is for KT88 with about 500 volts at the plate. 70 mA times 500 volts equals 35 watts, which ought to be within the dissipation limits for the KT88. If your plate voltage is substantially higher you will need to reduce the idle current to keep the dissipation within limits.