How to change component values when changing tube B+ voltage
A modest increase in R2 should suffice to re-bias the tube stage for 12V at the anode, P1 should be OK. Either recalculate using the load line method, or increase R2 until Va=12V, which will probably be around 200 ohms.

The output stage will still "work" as-is, but 125mA is quite a high current, so dissipating twice the power here will probably require heatsinking both FET and regulator for reliable operation.

Naturally you can't run the heater off 24V! So you still need to arrange a 12.6V or 6.3V supply for that. 

One alternative would be to run both the heater and existing output stage off a regulated 12.6V supply (avoiding the increased power dissipation, though I would suggest heatsinking anyway), and a relatively low current 24V anode (B+) supply maybe using a voltage doubler then a regulator. 

However this would mean you could no longer DC couple from V1 anode to Q1 grid cough sorry, gate. I would suggest AC coupling to Q1g, and providing the DC bias with a 1 Megohm "grid leak" resistor to a 6V supply (just a voltage divider with decoupling).







simulate this circuit &ndash; Schematic created using CircuitLab