Fender Princeton Mojo kit build power supply node question

[steveo7868]

I have not built that man Fender type amps , but have built many amps for many years
I wonder why the Phase Inverter would not just tap off the first 15K or 18K dropping resistor
but it looks like you run two of those nodes in series then tap both the PI and preamp
off the last power node, could you not drive the power tubes a bit better with the higher voltages at the PI stage, Just curious simple enough to try it both ways this is a Princeton reverb blackface circuit

[sluckey]

That's the way Fender did the original. Seems stupid to me. I've always wondered if the PI was originally connected to that unused node but for some reason it was moved to the last node. We'll probably never know.

[Fresh_Start]

I dug into this a bit since I’ve built a couple BF Princeton Reverb variants and worked on an all-original 1964 PR as well.

The answer may have something to do with the cap can having 4 20uf sections. You get better filtering with 2 C-R-C sections in series than with 1 using double the capacitance. Maybe the reverb version needed the extra filtering?  The voltage drop is significant:  400 VDC down to 240. So maybe that lower voltage is optimal for the phase inverter.

But wait!  The regular Princeton AA964 circuit has 2 cap can sections tied together for the power tube plates (40uf) and 335 VDC supplying the phase inverter.  Discussion here:  https://el34world.com/Forum/index.php?topic=10252.msg93680#msg93680

In fact, moving the phase inverter supply to that higher voltage “unused” node is called the Stokes mod. The goal there is to increase the headroom of the PI.

Sorry for rambling …
Chip

[steveo7868]

Yeah I  did do that mod and one other mod I found online to make the trem a bit stonger
so far I like this one, its ready for plunkin in the living room

[jordan86]

I’m late to the game. But yes, it is the stokes mod. Puts about 50 more volts on the PI. I did it as well. It adds some headroom for sure. You can research it ad nauseum on many forums if you like.