------------** Fair enough - the OP has been very vague. ** But not *power* triodes - like the 300B.
They like 700 to 800V.** Only IF you use toriodals.
The magnetising current with cheap E-cores will spoil that quite a bit.
Look, I admit have done it myself for a low current, tube pre-amp with a pair of 12AX7 / ECCL83s.
I admit I have done many things myself I would never recommend on an internet forum while ever I use my REAL name.
There is so much risk of *misinterpretation* of a seemingly good idea.
One reason being, 500VDC is dangerous, concieveably fatal for anyone with little experience.
BTW I think you are one of the better, more serious posters here.