What type of phase inverter is this and how does it work?

[gary sanders]

Can someone make sense of this for me:

Input is a 250K pot, and then a 33k series grid resistor also. The cap coming out of the plate is a .039uf. Then it goes through a voltage divider with a 470k at the top and a 12k to ground. Then into the other grid and out through another .039uf cap. The grid leak on this 7868 is a 470k, and there isn't one on the other 7868 because it is using the divider. The plate resistors on each side are 220k's, and the cathode resistor on the first half of the PI is a 1.6k, and there is feedback injected there also. The other cathode is a 1.5k with no feedback. The PI is a 12ax7.

I had to peak inside the Conn and this just made no sense to me.

[d95err]

It sounds like a paraphase or "see-saw" phase inverter:
http://www.freewebs.com/valvewizard/paraphase.html

It is very simple. Output from the first gain stage goes to the first output tube. The output is also attenuated and fed into the second gain stage. Since each gain stage inverts the signal, the output from the second gain stage becomes an inverted copy of the first and is fed to the second output tube.

Of course, the attenuation before the second stage has to be exactly corresponding to the gain of the second stage to make the signals balance. More advanced versions of the paraphase use a feedback loop to keep the gain of the second stage exactly 1.

[tubeswell]

Of course, the attenuation before the second stage has to be exactly corresponding to the gain of the second stage to make the signals balance...

And of course they never balance perfectly, so they always have that rough'n'ready classic mojo (like what Jimi Page got with the Supro T'Bolt on all those classic zep tunes).

[gary sanders]

I drew a schematic out on it and it doesnt even look like it would work.What threw me off was that one 7868 had the 470k grid leak on the 180 out side and on the other it was there but it was actually on the grid of the PI cathode,series with the cathode resistor.Feedback?I will post a pic later.Very interesting

Read the Valve Wiz page.Dont know why I didnt look there,I have actually read a little of that before when I was building my loop buffer.Good stuff!

[Andy Le Blanc]

its a fixed paraphase, with the 470k/12k divider you've described.

you can adjust the AC balance at the divider, and the DC balance at the
plate resistor of the inverting side.

they are not a "clean" tone but thats their beauty, you get tone color.