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In almost every Fender tube amp design, the first filter stage feeds the power tubes their plate voltage by way of the output transformer. The next stage controls voltage to the power tube screens. The stage after that sends power to the phase inverter tube's plates. The final stage feeds the preamp tubes.
Then there is the Princeton family. It has a simpler phase inverter design than its bigger brothers. For some reason, the power is picked up at the same stage as the preamp tubes.
You'd think the designers, if they needed less voltage at that point, would not have made the expense of including that extra stage of filtering and left that in. This made many conjecture that it might have been a last minute mod to lower the clean output of the Princeton so it wouldn't compete with the Deluxe Reverb.
Sooner or later, someone had to get the bright idea to reverse that strategy. It worked wonders for the amp, supplying greater depth, clarity, and headroom - and so the Stokes Mod was born.
Fast forward to the PRRI... Everything has been recreated faithfully including the low powered phase inverter.
This is an ultra simple mod. Printed Circuit Board (PCB) stays in the chassis !
Properly discharge power supply caps**. Remove Chassis from amp. Consult Layout Diagram, above. Locate the colored components R37, R17, R57, and R59. Clip the appropriate ends of the plate load resistors, R37 and R17 (see diagram!) well away from the resistor body, near to the PCB entry point. Bend the freed resistor leads up away from the board to insure against accidental contact with the old connection, and to facilitate soldering.
On the diagram Red Lines indicate new connections to be made. Gently lift (pry) the lead on the side of R57 and R59 that will be connected up a millimeter or so from PCB to make room to solder. Do not disconnect.
With a low watt pencil, solder 22 or 24 ga wire to make connections as in the diagram. Do not overheat the unclipped resistors, R57 or R59, lest you release them from the PCB!
For dress, your wires should make straight runs along the board as in the diagram. Finally, secure the resistors you snipped with a dot of fast drying clear silicone. Get some that's specifically for electronic repairs. It won't release corrosive fumes when curing. You are done! Install chassis.
This mod gives the amp a lot more headroom, depth and clarity. Useful for those that feel humbuckers are too much for this amp. Works especially well alongside an Allen TO20 output transformer.
Pic located here - http://www.fenderforum.com/userphotos/index.html?recid=70457
[YMMV and Fritz_D_Cat or any of his likenesses is not responsible for any damage or injury because you messed with a perfectly good amp]
** DISCHARGING THE CAPS
Since you're going to have your solder iron out, why not make a discharging tool? Buy an alligator clip lead and resistor at Radio Shack. Snip the alligator off one end. Solder the resistor, any value from 1k to 10K to the snipped end of the lead. Good idea to insulate with electrical tape or shrink sleeve from the body of the resistor to the wire insulation.
Unplug amp from wall. Remove tubes. Label them so you get them back in the right places.
Clip alligator to chassis. Turn standby switch to "on". Touch resistor lead to each pin slot of the second tube socket from the left, looking from the back, for 30 seconds, being careful NOT to be touching anything conductive.
Use one hand only, for best safety. Be sure the resistor lead you are using as a probe is making contact with the metal of the pin slot in each case. A flashlight in the other hand is useful.
If you know which socket pin to touch, great. If not, just do them all.