Footswitches

The circuit design on which I'm basing mine includes an overdrive (OD) feature: the input signal is amplified and shaped by a pair of preamp triodes and is either passed as a "clean" signal to the power section or diverted to a second pair of triodes, the OD section, before then getting routed to the power section. (It's important to note that the OD feature is most useful for lead/melodic playing, but not for playing chords to accompany a tune - at least not to my ear... the distortion created by the OD can make it hard to distinguish relationships between multiple notes, so it's less desirable for chords more complex than simple triads). If this amp is to be used for live playing, then I'd like to be able to easily switch between clean and OD during the performance of a piece, when reaching to a switch on the front panel of the amp would be impractical. A remote footswitch is a commonly implemented solution for this problem - all of the amps I've owned have had this feature... one I have taken for granted until having to design and build it into an amp myself.

Switches mounted on the front panel of an amp, such as those I included in my first build, typically carry and route the guitar signal this way or that through the amp. But, in the case of a remote foot switch design, we're moving the switch outside of the protection of the chassis, far from the delicate audio circuit, and adding a dozen or so feet to the length of the signal path. This would be likely to degrade the quality of the signal and introduce noise into the circuit - it's really just not done as far as I know, at least not in modern amps. Instead, we make the footswitch simply an actuator that controls an electronic switch that is inside the chassis, which keeps the signal path short and protected. To do this, we need an actuator for each switch and access to a power supply to drive them. It's not that hard to do, and it's been done a jillion times, but it IS a separate subsystem in the amp that requires it's own design and implementation effort, and wasn't something I had thought that much about before now.

In my design, even though I've got 3 switches in the circuit (Bright / Pre-amp Boost (PAB) / OD), I already have a two-button foot-switch that came with the salvaged Fender amp, so I'll just implement two of them, the PAB and OD. I also want to be able to actuate them in the usual way, with switches on the front panel, AND I'd like to have some visual indicators that tell me if each switch is on or off - LEDs will do. Here's the newly added section of the schematic that describes this additional subsystem.

Everything to the right of the lamp is newly added to support a pair of footswitches for the PAB and OD, as well as the associated panel-mounted switches.

Here are the bits and bobs needed to implement the switches: salvaged footswitch unit, relays and their mounting boards (one is assembled in the picture), power supply board with bridge rectifier and smoothing capacitor.  

Also, made the major mods to the chassis, cutting holes and installing mounts for the power transformer and tubes: