High voltage DC offset servo

I'm working on a project and I need some assistance.

I have an audio tube amplifier output stage that I need to have adjustable operating points while maintaining a DC null at the output.

[![enter image description here][1]][1] As you can see it's a white cathode follower with a DC offset servo to null the output. I need to be able to adjust the load line but if I adjust the operating point I must also adjust the servo offset.

R12 is the variable resistor to set the CVS voltage but in order to maintain 0vDC on the output of the amplifier and keep the current of the tubes balanced I need to decrease the negative supply voltage of the servo when I increase the resistance of R12 and I need to increase the supply voltage of the servo when I decrease the resistance of R12, up to -300v. Sadly opamps with the required supply voltage and output swing don't exist unless you want to pay \$500

So I have 2 problems, I need to have some sort of servo that will adjust the supply voltage of the DC offset servo as I change the resistance of R12 and I need to figure out an alternative way to an opamp for use as a DC offset servo since no opamp worth paying for is going to handle the needed voltages.

• Tubes have quite a good swing! Can you use another one to boost the output of your op-amp up to the required voltage, at least for the -ve rail adjust? – Neil_UK Feb 9 '16 at 11:53
• Tubes can't output a negative voltage and the min/max swing range needs to be pretty much +15v (or more) to -150v. – coinmaster Feb 9 '16 at 13:41
• if you say so. Are you thinking outside the box enough? Don't forget I said boost the voltage up, to provide the -ve rail! – Neil_UK Feb 9 '16 at 13:44
• I'm not sure what you mean. The output voltages of a tube can only be positive and positive voltages on the grid bias or -ve will obviously not work. In order to produce -150v on the output of the opamp I need a negative supply of -300v to the opamp, which will kill it. – coinmaster Feb 9 '16 at 13:51
• I mean the opamp and the tube grid/cathode at the PSU -ve rail, the tube anode providing the -ve rail, or at least the -150 to +15v, to where it's needed. Obviously you would have to level shift control signals down to the opamp if they came from a groundy area, locally regulate your amp +ve supply to PSU -ve +25v. Perhaps grid to PSU -ve and cathode follow the output of the opamp? – Neil_UK Feb 9 '16 at 13:58