Any tube output works like tube output.
I have 12x4 working unit upstairs [IIRC] someone changed the caps and let me pick it up. I put IEC=Ground chassis and Connectors on it, fired it up and it hums. Has AC supply though, layered-cobweb-of-hanging-garden circuit design.
If it's a 'car' unit, that implies is 12vdc supply intention, running from a DC-Cell..seems like it'd allow very minimal B+ filtering, which should be taken into account if alternate PS is used.
If using AC/DC conversion for the power supply, IEC it. Test that the chassis/earth is actually like bolt/lock-eyelet connected to actual Earth ground.
Check for non-short then connect DC to the supply rails [observe/obey polarity], use a ~HD battery [to supply the needed current].
Measure the current draw, will be much higher at tube start up, that'll take a 'gnaw' out of a wimpy powr suply every time too, you might not even get away with it many times.
Then if it's working fine for a few good whiles [ie..how to lo-tech-pre-test a tube amp, try it while it's self-heating completely, check, check to see no redplating], and then it either sounds great and/or you then Have to mess with it anyway...
Perhaps attain a more suitable power supply when the battery holds up the DC voltage for only so many whiles.
Since I mention PS:
TDSL the tubes, [don't overvoltage the capacitors] but consider the tubes are able to run within their parameters [AC/DC conversion rate, and PS loading/voltage drop etc.] before choosing an operating voltage.
Then you will have to find 'the' speaker, and probably need or have a good one anyway so it's cool. Try selections if you can access pulls from old consoles, perhaps auto-speaker, the tone/effeciency may vary greatly.