here's something i churned out the other day. i have been thinking about modeling the Supro Thunderbolt with FETs, but then thought- i just got in these cool new Tesla GE's, why not use those? so that's what it is, the preamp section of the legendary amp used by Page and supposedly Hendrix, done up NPN germanium style. i had to eliminate a few resistors towards the end to bring up the volume and lowered the value of an emitter resistor to up the gain a bit. also, i made the resistor on the input variable so you can adjust the input impedance for guitars with different pickups.
the real gem of this circuit is the tone pot- totally counter-clockwise you get a nice full range tone with plenty of mids. as you go clockwise it goes to a very muffled tone, and as you go even further, it goes to a very thin, cutting tone. there are some very cool sweet spots, and a lot of versatility. i like it better than a regular muff style control, but it does cut plenty of signal- hence the two gain stages after it.
it's a cool sounding circuit. nicely voiced and does a good overdrive type sound. if you don't have the trannys, you could change out the 33K Rc's to trimmers and adjust to taste. the tesla trannies are extremely consistent, so no trimmers needed. the layout below uses some large axial caps and is yet unverified, but hopefully i'll get it done sometime soon.
XXXUPDATEXXX the large parts PCB is being retooled due to infestation of mutant roaches. stay tuned! XXXUPDATEXXX
XXXXUPDATEXXXX i was made aware that there could be DC voltage on the tone pot. you may need another capacitor after it to get rid of crackle that may be present. a .047uF cap off of lug 2 of the pot should remedy this. i'll amend the schem and layout after i do some more experimenting. XXXXUPDATEXXXX
XXXXXXXXXXXXXUPDATEXXXXXXXXXXXXXX I built up the Thundertone with the old mojo parts layout and it works with no tone crackle!I will be revamping it for a tighter layout. XXXXXXXXXXXXXUPDATEXXXXXXXXXXXXXX
take it easy!
here is a new PCB using common size/value parts.