How does this phase inverter work?

[isaac_teller]

Just curious. Not planning on building it.
https://el34world.com/charts/Schematics/files/Valco/National_dobro_60.pdf

[66Strat]

AC is taken from the screen grid of the 6K6 immediately following the preamp and applied to the control grid of the other 6K6. The AC signal that rides on the screen grid is 180 degrees out of phase from the signal applied to its grid.

[isaac_teller]

Thank you.

[PRR]

The power pentode's screen grid can be used as a low-gain triode. Since a P-P OT can be cheaper than an SE OT of half the power (less iron), and 6K6 were coming down in price, and more tubes on the spec-sheet "is always better", radios (and G-amps) did this one cheap trick.

It never gets as much power at as low THD as proper push-pull drive. But it is one more rock in the marketing engineer's toolbox.

[66Strat]

The Magnatone 109 Melodier is another example of the self-split amplifier. It was at one time a popular circuit with harp players.
https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_109_melodier.pdf

[isaac_teller]

Found another one from the same era.


http://prewaramps.org/suproamp-in.htm

http://prewaramps.org/media/Saicschematic.JPG