[Joeztech]

I recently looked at the schematics of a 41-290 to help a friend and in the process noticed the difference between the Williamson type output circuit used in it versus the output schematic of the 41-280. The 41-280 does not use the Williamson design, yet still achieves a push-pull output stage. At first glance I could not figure out how it worked. Thanks to another amateur radio friend the explanation became understandable. You have to look at the phase relationships as waveforms move through the tubes. I took the liberty of adding waveforms to the schematics so people could see the phase relationships. In the 41-280 the screen grid of the top output tube is not bypassed until it gets past resistor 56, a 3900 ohm resistor. This voltage which is in phase with the signal at the plate is coupled to the grid of the bottom tube's grid thereby making its output opposite in phase to the tube above it, thus achieving a push-pull circuit. In effect the output tubes become a phase inversion circuit as well as an output gain stage. A whole tube is eliminated in the process, thus reducing costs.

Joe