• Converting a split-phase to a long-tailed pair phase inverter

Fender Princeton and Princeton Reverb amps have a split-phase phase inverter that has a gain of 1. A long-tailed pair phase inverter can supply quite a bit of gain, but requires another tube stage. If you're willing to lose the tremolo, the tremolo tube stage can be used to build a long-tailed pair phase inverter. There's the option of adding a presence control at the same time. Gerald Weber wrote a column on doing this mod in a 1996 issue of Vintage Guitar.A note up front--I don't like the sound of this conversion in a Princeton Reverb. It gives it too much gain and it becomes real dirty at all settings, and converts an amp with several tones into a single-tone amp. John Stokes has a tweak that gets some more volume out of the amp without sacrificing tone. My notes on his tweak are in the Phase Inverter section.

Here's my sketch of the stock Princeton Reverb amp layout. Not all details are shown, but there's enough here to orient yourself. (Click on and print the images to see them larger.)

It's necessary to have a schematic of the Princeton and a schematic of the long-tailed pair to really understand this.

• 1. Remove the wires on the wiper and end lug of the tremolo intensity pot, solder and heat-shrink the joint.

  2. Unsolder the 2 wires that connect to the end of the 56K resistor, solder and heat-shrink them together.

  3. Unhook the ends of the 2 red wires that attach to the 220K resistor, solder and heat-shrink them.

  4. Remove all the parts in red.

  5. Unhook the wires with a red X on them.