Tube based guitar/bass boost pedal
Based on the excellent Mitsumin Valvecaster with some minor mods.
Drawn with gEDA schematic editor.
If input is from bass or guitar this works as a basic boost pedal only. If a low impedance input is used of ~5v p-p it distorts quite a lot. A small 2 x jfet preamp circuit could do that, or a second tube stage exactly the same as the first.
Unfortunately, tubes at such low plate voltages are not particularly clean and don't respond too well to control.