Michael Black wrote:

As for 12v tubes, you most certainly saw converter using them. SOmeone
was asking about 12v Nuvistors a while back, and the next to last edition
of the ARRL VHF Manual, late sixties or so, has a converter using one.

We are not talking about normal radio tubes that have 12V heaters, we are
talking about special tubes that have 12V heaters, and a 30V maximum plate
voltage rating. These special tubes were designed to use normal car battery
voltages as their plate voltages.

-Chuck