mcmichael: SMC (S); Duplex four

There is an early version applying manual gain control by means of filament variation in the RF stage.

This was state of the art for battery triodes in the late 1920's.

But now the RF triode was replaced by a superior screen grid valve. Such a valve is behaving different to filament variations, in particular at low emission for low gain, where the setting is rather unstable.

Obviously led this drawback to a redesign of the volume control by leaving the filament on full supply while a variable grid bias was introduced. This results in an easier and smoother control function.

The available documentation describes the late version only.

The early version was on my workbench for repair. It had a low resistance rheostat (original, physically identical with the high resistance potentiometer in the late version) but some "repair" attempts of previous owners left a chaos: senseless resoldered wires, wrongly connected on-off-switch etc.
He must have desperately decided to sell the spoiled thing after replacing two original Audios by defect ones (different brands; all 4 windings open), putting four valves with open filament. The next owner (in Germany) could not do much better and sold it again. 
Now the buyer asked me for help.
I can proudly report that the radio is back to life now (after spending about 3 days to navigate through the wiring jungle and the wrong map (schematic)...

 

To thank the Author because you find the post helpful or well done.