DIY Fender Princeton DSP 65 footswitch

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It’s been some years since I became the proud owner of a used Fender Princeton 65 DSP, a transitor guitar amplifier with one of the best quality/price ratios. Its best qualities are:

  • crazy sound volume: a sonic pressure almost comparable with that of a valve-based counterpart;
  • ultra-clean on clean channel, aggresive enough on distort channel, particularly inclined to Blues, Country and Jazz;
  • simple but effective embedded digital effects: reverb, reverb + delay, delay, chorus, chorus + delay, chorus + reverb, flanger, Vibratone;
  • pre out and power in plugs, usable as send / return or slaving to external amp.

I’ve found this cool toy in a “used” section of a well-known shop in Milan, for 260 euros. Unfortunately, the amp didn’t include the necessary footswitch, so during these years I had to either use a single channel during the hole song (integrating the sound with TubeScreamer and other pedals) or switch manually.

A positive note: the amp was sold along with manual and schematics (which can anyway be downloaded from the Fender website), but being not proficient at all in electronics I did not think about building a footswitch by myself. I tried buying a Marshall-type two-button footswitch from the local store, but it did not work. Many shop owners told me that Fender footswitches are peculiar, in fact they employ a mono cable instead of stereo (or multiple mono), leveraging some fancy electronics to handle the signal… As someone once said, “it is never too late!”. So I wrote to a couple of friends of mine, electronics experts and musicians:

After some email exchange, here is their advice:

The investigation

Max Farnea

I’m trying to understand. It looks like amplifier sends an alternate signal to the switch, so probably switches are discriminated based on the half-wave that they let pass through, thanks to the diodes. I think that both regular signal diodes 1N4148 or recifier diodes 1N4001 will do. The LEDs in the pedal polarize according to the corresponding diode, but be aware that LED not only emit light but also cause small tension drops. If amplifier circuit is sensible to tension drop (I don’t think so) it should suffice to mount two diodes in series for each diode in the schema. The tension drop of the second diode is the same of the LED (which are diodes themselves). Do you have just switches in your pedal, and no other components? Maybe the jack is a stereo one, but the amplifier just ‘listens’ to signal polarization.

For the wiring part… the black or white belt corresponds to the veritcal bar in the diode symbol found in the schematics. The remaining part of the circuit (which is very essential anyway) should be copied as-is. If you want some hint you should tell me where are you starting from, i.e. what do you see when you open up the cover.

Alex Petrini

The switch is realized with a mono cable and the circuit inside the amplifier select channel and effect on/off based on half-waves: positive, negative, both or none. The diodes in the footswitch just block one or both of the half-waves, so play a fundamental role in the pedal.

I sent you a drawing that should depict how to wire the components, that should be very helpful. It should be clear enough, but in case you’re unsure let your friend have a look at it before grabbing the solder: six eyes are better then four.

In any case I would not eliminate the LEDs, you know better than me how useful they are on stage! They complicate the assembly phase, but it’s worth it. If you prefer not to mount them, follow the advice of your savvy friend and replace them with the same non-led diodes you’re going to use in the footswitch.

The schematics says that non-led diodes are 1n4448, equivalent to 1n914 or 1n4148, check your local store. Be aware that they usually have a different color than the ones depicted in the picture, you should keep the black belt as reference.

About the switches: I supposed thath the central pin is the common one; when soldering check with a tester that current flows or not, based on switch status.

That’s all, if you have any problems I’m here.

Have a good week-end and good luck!

Princeton DSP 65 footswitch schematics

The result

I decided to modify the Marshall-style footswitch, using the following procedure.

Materials and tools:

  • a two-button footswitch;
  • electrical wires coming from a defunct computer internal speaker;
  • 2 red LEDs, found in a corner of my storage room, already soldered on 1N4003 diodes (!);
  • tin-lead solder, soldering iron, a clipper;
  • small metal or plastic belts.

The steps involved are:

  • join the ring and the sleeve of the stereo jack, transforming it to a mono plug;
  • solder everything as in Alex’ schematic (see above);
  • drilled two holes in the footswitch case, for the LEDs (with the screwdriver!);
  • test everything!

Photo-reportage

The “Laboratory”:

The Lab

Test center:

Test Equipment

The circuit, carrying the worst solderings ever made:

DIY Princeton DSP 65 footswitch

The box, seen from above::

DIY Princeton DSP 65 footswitch

A bit of reordering:

DIY Princeton DSP 65 footswitch

In action!

DIY Princeton DSP 65 footswitch