I LIKE THE TONE OF MY VINTAGE FENDER PRINCETON Reverb combo, but the vibrato is not very deep. Even with the intensity control turned all the way up, I can barely notice it. What can be done to make this effect more intense? —John, via email

The Fender Princeton Reverb vibrato circuit, though really a tremolo circuit and not true vibrato, achieves tremolo by modulating the bias setting of the output tubes. This circuit is completely different from the Super Reverb/ Twin/Bandmaster optocoupler style tremolo that modulates signal volume. Because the Princeton circuit modulates bias, it is important to have the output tubes biased a little on the cold side to allow more “space” for the tremolo to swell the volume when it modulates idle current. Unfortunately, there is no bias adjustment pot on a stock Princeton.

You can make a bias adjustment control by replacing the 22kΩ resistor on the bias circuit board with a 25kΩ cermet element pot wired in series with a 10kΩ w-watt resistor. This will allow adjustment of resistance from 10kΩ to 25kΩ, so there will be plenty of room to vary the bias setting.

As always, the amplifier should be unplugged and the filter capacitors drained to avoid electrical shock before attempting any servicing!

Start by making a subassembly. One end of the 10kΩ resistor is soldered to the center lead (wiper) and one end lead of the 25kΩ pot. This assembly will replace the existing 22kΩ resistor that is mounted across the 25uf/50-volt capacitor on the component board near the pilot lamp. Use the free end of the 10kΩ resistor and the free end of the pot in place of the two 22kΩ resistor leads. Solder each lead and you now have an adjustment pot to set output tube bias.

If adjusting the bias still doesn’t get the intensity you want, there is a 1 Meg resistor on the main circuit board that connects between a .02uf cap and a 220kΩ resistor. Change the 1 Meg resistor to a 220kΩ value and the intensity will go up. If that still isn’t enough tremolo depth, look at the .02uf cap that is was connected to the 1 Meg resistor you just changed. Remove the capacitor and the tremolo will become even more intense.