[Boatanchors] antique Philco AM/SW table radio

[J Forster]

Dick KF4NS wrote:

> I hope this is not off topic. I have this Philco 42-322 which is a 6
> tube ac/dc superhet using 7 volt loctals and a 35Z3 rect/50L6 audio
> amp. I did not start learning radio until about 1950 so this one has
> me befuddled. I have been able to figure out most of the theory like
> the use of the series string filaments with the large dropping
> resistor, but I am really confused by the speaker arrangement. The
> winding attached to the back of the speaker is shown on the schematic
> as a "field coil" which is in the B+ line directly off of the cathode
> of the rectifier.

The speaker 'field' coil is serving a dual purpose. The inductance of the
coil is being used as a filter choke. The DC current through itr is making
a magnetic field for the voice coil to react with. It was a common
arrangement before Alnico became cheap and available.

> Also, the load resistance for the 50L6 is stated as
> around 2K but the output xfmr has a stated 240 ohm primary and a "less
> than .1 ohm" secondary. Then the schematic shows 2 series coils also
> in the secondary with one marked as .5 ohm. This is all very
> confusing.

Are those DC resistances or impedances?

-John