Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: isaac_teller on November 27, 2021, 01:20:49 pm

Title: How does this phase inverter work?
Post by: isaac_teller on November 27, 2021, 01:20:49 pm

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

Title: Re: How does this phase inverter work?
Post by: 66Strat on November 27, 2021, 01:41:52 pm

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.

Title: Re: How does this phase inverter work?
Post by: isaac_teller on November 27, 2021, 02:27:51 pm

Thank you.

Title: Re: How does this phase inverter work?
Post by: PRR on November 27, 2021, 02:49:33 pm

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.

Title: Re: How does this phase inverter work?
Post by: 66Strat on November 27, 2021, 03:15:38 pm

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 (https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_109_melodier.pdf)

Title: Re: How does this phase inverter work?
Post by: isaac_teller on November 28, 2021, 12:46:23 am

Found another one from the same era.


http://prewaramps.org/suproamp-in.htm
(http://prewaramps.org/media/Saicschematic.JPG)
http://prewaramps.org/media/Saicschematic.JPG