Special DuMont 109 maybe available?

[wa2ise]

Originally Posted by Tom Albrecht

Looking carefully at the circuit, I believe the "polarizing relay" is NOT a vibrator, but merely a relay which makes the set work regardless of which way you plug it into a DC outlet. Probably a relay with a permanent magnet that is sensitive to polarity of current in its solenoid.

Doing that would avoid the voltage drop you get with tube rectifiers, and hopefully gives enough protection to the electroytic caps against reverse polarity. If the relay is normally open with no power applied, then maybe only when it sees power it makes the correct connections without the caps ever seeing reverse polarity. Like a cross wired DPDT switch with center off.

[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.

[Einar72]

Still want to see how DuMont does it. Anyone know the Sams # for it?