Special DuMont 109 maybe available?

[N2IXK]

Interesting.

I've used a full-wave bridge rectifier on the input side of DC-operated equipment in the past, where there was a possibility of incorrect polarity being applied. If you connect the positive and negative output terminals to the equipment, you can apply either polarity of DC to the AC input terminals, and the diodes will "steer" the output to the proper polarity, regardless of how the input is connected. Handy on small inverters, battery chargers, etc. subjected to use by people who may not understand simple conventions like Red=Positive, Black=Negative.