Pentode drivers are very interesting. You can get excellent results out of them. Here is my version of a differential pair using pentodes. I want to try them out in the 45 PP design I wrote previously
People may get scared with the above diagram thinking: this is way too complex. In fact for me, it’s not. Using the PCBs I have it makes it easier and simple to build.
The differential pair is formed by V1 and V2. They have a tail CCS which sets the shared cathode current. Each valve has a gyrator PCB and a resistor to ground from the anode which sets the gain. The anode voltage can be adjusted by the gyrator but will not make a big difference on the anode current of the pentode (think of a flat anode curve). Remember the gain is defined by gm and Ra. However, if we vary the screen voltage we can adjust minor differences due to unmatched valves.
Regulating the screen is very good to keep distortion down. The additional complexity here is to create a resistor divider to adjust slightly (and independently) each screen voltage. The voltage reference from VR1 is isolated byR3 and R4 to avoid oscillating VR1 by adding additional filtering capacitance as C3 and C4. The SF boards are used only to provide MOSFETs driving the screen current to each valve.
You can simplify this circuit of course. I will try this and experiment a bit.