Ok, let's try this schematic again.
And yes - it self oscillates at the frequency defined by the transformer primary and the resonator capacitor. You will find a lot of 12V CFL stepups (like for auto "angel eyes") that use a similar topology since it only costs a few transistors - same reason I like it.
If you're interested in a 12V to 300V solution, we can continue exploring it - but I now realize your input is probably 300V rectified mains, and you need step down to 12.6 and 6.3V for... heaters? Anything else need to work on those rails? What is your total heater current on each of those voltages, and can you stack two tube's 6.3V heaters to run everything on one single 12.6V rail? This is usually OK (user's choice) for things like 12AX7's.
And yes - it self oscillates at the frequency defined by the transformer primary and the resonator capacitor. You will find a lot of 12V CFL stepups (like for auto "angel eyes") that use a similar topology since it only costs a few transistors - same reason I like it.
If you're interested in a 12V to 300V solution, we can continue exploring it - but I now realize your input is probably 300V rectified mains, and you need step down to 12.6 and 6.3V for... heaters? Anything else need to work on those rails? What is your total heater current on each of those voltages, and can you stack two tube's 6.3V heaters to run everything on one single 12.6V rail? This is usually OK (user's choice) for things like 12AX7's.