Geoff Fors, WB6NVH


A number of people have noticed after buying a Western Electric solid state audio module to replace the tube audio output stage in their R-392/URR receivers that the receiver does not "work right" after this modification and wind up blaming the module. 

This note is to point out that the solid state module was not originally supplied as a "plug and play" replacement for the audio output tube. In the original design, a negative feedback line to the first audio amplifier stage was accomplished with R629, an 8.2 Meg Ohm resistor, going from pin 5 of transformer T603 to the grid of V606.  The solid state audio amplifier itself introduces a 180 Degree phase shift, which the tube stage does not, therefore creating a net positive feedback. 

On sets which were not shipped originally with a solid state audio module, it will be necessary to remove R629 from the circuit, the solid state module will then function properly, provided it is otherwise in good condition.   Sets shipped with the solid state module did not have R629 installed.   And conversely, if you have a receiver which is missing the solid state module and you want to use the tube, you will need to install R629.