12au7 as cathodyne P.I.

[Edgar Sterling]

Hi Everybody
I've built something close to a Princeton Reverb with a few twists that are new to me, but I know somebody else has tried. One of the minor changes to the Fender schematic involves the cathodyne phase inverter: I wired it so that it will take a 12dw7 in that position. That means it has the 12ax7 half as the stage preceding the P.I. (which is the same as stock), and a 12au7 as the actual phase inverter. I currently have it built with stock Princeton Reverb values, so I'm wondering what kind of changes, if any, I need to make to the 12au7 section for optimal performance, specifically cathode and plate resistors values. Any suggestions?

[soundmasterg]

In a typical gain stage the 12AU7 likes to have a lower value plate resistor and a higher value cathode resistor...say 22k or 47k for the plate and a 3k3 or 4k7 for the cathode. It all depends on the supply voltage and the rest of the circuit though. In the cathodyne, the 12AU7 can swing more with lower value resistors so again, change to 47k or 22k or thereabouts and see what kind of swing you get versus the standard 100k or 56k. I would highly suggest to add a large grid stopper to the grid of the 12AU7 half and to check out Merlin's site/book for more info. Valve Wizard - How to design valve guitar amplifiers

Greg

[lookah]

Hi Edgar, I'm trying to do the same with my PR. Any feedback on those suggested resistor mods? Did you try them?

Thanks

[Bruce / Mission Amps]

I agree that the natural impedance of the 12AU7 is lower and needs lower plate and cathode resistors in a cathodyne phase inverter to get full potential but the real weak spot of this is that the 12AU7 triode has a mu of 17.
A good example of this is an amp I built for a harp player where the low gain preamp stages used two 12AU7s with 22K and 27K plate load resistors but 470 ohm cathode resistors and they all ran at less then 50vdc!! CRAZY...but worked well.
Also, remember that the cathodyne has no gain and the circuit is relying on the "gain" and output of the previous stage to produce the voltage swing needed to over come the bias voltage of the power tubes.
That's why it doesn't work well with a 5E3 or any tweed type amp that uses both elements of the same tube to be the driver and the phase inverter.
You'd be better off with a 12DW7 and wire the 12AU7 section of the tube as the cathodyne triode.

Sorry.. I'm watching the elections and not paying a lot of attention to what I'm typing! ha ha

Oops... I see this was covered already in another section...

[lookah]

Hi Bruce, I had a question regarding your comment "You'd be better off with a 12DW7 and wire the 12AU7 section of the tube as the cathodyne triode."

I'm going to rewire V4 of my SF Princeton Reverb to accept a JJ ECC 832 (12DW7) so that the PI is getting the 12AU7. However, as you and others have implied, it seems that some of the resistors could also be changed to optimize the mod. Can you offer insight on this? Which resistors to change and with what values? Are there measurements I should take to get the right resistor values specifically for my amp? Thanks!