I've put together the tillman preamp/boost again on a new bit of board however the "boost" seems to be missing.
This one:
http://www.aronnelson.com/gallery/main.php/v/TheBigMan-Layouts/preamp.gif.html
With an exception that I dropped a 10u cap in parallel with R2... which could be my error... and that I went with a 56K for R4 - as that is what I had at hand this evening when bored.
The volume is not higher than input, it is having an effect on tone (somewhat subtle but there) and when measuring the voltage at the drain I'm only seeing around 2v and low across the rest. It COULD be a faulty JFET... I've confirmed that soldering is (probably) all good and that I've (probably) not missed any bridges. So in the hope I will actually learn something here - where may have I gone wrong with this simple little thing?
:-[
EDIT: Addional stuff - the boost seems to kick in momentarily when the battery is unlinked (and decays is a way I think I want to make a box do intentionally!) and yes - battery is giving 9v. ;)
you need to bias the fet to +/- 4.5v at the drain or you can also bias from the source
That makes sense based on the low drain reading - so less resistance at R3 or R2 is the answer?
Would a "one off" odd reading of nearly 9v up at the source be dismissable as "my hand slipped" or indicative of a faulty FET?
you could use a pot for 3 wired as avariable resistor, you could also bias by ear since most fets would sound best not really exactly on 4.5v (with a 9v power supply). look fior the best sound and remove the pot from the circuit and measure the resistance. then find a suitable resistor to put in R3
Thanks, will get back to the fiddling.
:D