Potentiometer with SPST Switch Wiring With Bypass
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A reed relay with changeover contacts provides a fail-safe method of switching using the built-in pot switch.

At the expense of some battery drain a reed relay would provide a good solution with the added benefit that in the event of a battery failure the guitar is still usable and would avoid a cringe inducing silence on stage.



Figure 2. Typical reed relay packages.



Figure 3. Reed relay datasheet from TE Electronics.

A quick web search finds that these are available in 5/6 V DC versions (so a series resistor would be required) with a 200 Ω coil. At 5 V this will cost you \$ \frac {5}{200} = 25 \ \mathrm {mA} \$. Since the current is that high you can add an LED in series with the coil and reduce the resistor value to suit or leave it out for a white LED which typically have a VF of > 3 V. (The relay coil would provide the current limiting for the LED.)



DC thump

From the comments:


  is the DC thump you are talking about the "knock" I hear when I turn off some solid state amps?


It's more usual when the amp is switched on.



Figure 4. The bass boost amplifier. Source: Circuit Basics.

The DC thump occurs because the amplifier is powered from a single rail. Since audio is alternating current it means we need to bias the output to mid-supply or 4.5 V in this design. This should be measurable on pin 5 of the LM386. We don't want to feed DC to the loudspeaker as it will cause unnecessary heating and distortion due to biasing the speaker in or out depending on polarity. The biasing of the speaker means that it has less travel in one direction before it maxes out and distorts.


  I had a bass amp which did this. My guess though is if I can use the switch/pot as the main switch in your circuit, the DC thump would be small since the switch/pot also controls the volume for the pre-amp. Since when you turn on the switch/pot, the pot is at its lowest level meaning volume is at the lowest level.


Looking at the schematic of Figure 4 we can see that this is unlikely to help. The pot is on the input whereas the thump is coming from the amp and C1 on the output.

Since you are only using this as a preamp rather than a power amplifier you could move the pot to the output, after C1, and this might solve the problem for you. It also has the benefit that any noise generated by the amplifier will be attenuated by the pot at lower settings thus giving you a decent signal to noise ratio independent of volume setting. In the original design the amplifier noise will be the same no matter what the volume setting is.