Are you testing into a dummy load?

yes, 50 ohm

If you have an antenna impedance bridge you may be able to measure the
effective impedance of the output stage. If the transmitter is operating
normally you may need some sort of external tuner.

I like to operate boatanchors in the way there were supposed to operate. So, if
the ART-13 was not designed to operate on the 80 / 75 MHz band with a 50-ohm
load, I'll just give up.

What I would like to know is whether all the ART-13s behave that way, or I
should instead suspect that a problem occurs on my particular ART-13.

So, I am looking for somebody who can readily fire-up an ART-13 and confirm or
deny my experience.

73

Tony I0JX