Re: Mongoose / Giant Hogweed queries.

Posted by nocentelli on Nov 01, 2015; 3:00pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Mongoose-Giant-Hogweed-queries-tp25642p25646.html

If you just changed R5 and the voltage on the non-inverting input (pin3) was 5.8v and it sounded good, altering R4 will shift the voltage again, and may or may not sound good. Measure the voltage on pin 3 again with them both at 33k, decide if you like the sound (the exact voltage might not matter, drolo suggests it is because of assymetrical clipping caused by biasing above 4.5v). You could briefly replace the left hand 47k with a 100k (leave the right hand as it is) and dial in the best sound, then replace with a resistor of the measured value of the pot.  


With the giant hogweed, david rolo has split the direct DC-coupling of the transistor boost collector into the lm308 (by using a coupling cap which blocks DC bias and a subsequent buffer stage), so the two 47k resistors on the hogweed schematic bias only the transistor: the lm308 separately receives non-inverting input bias via that 200k resistor from the "stability pot" within the power section on the bottom right of the schematic. This pot allows you to dial in a variable amount of gating by freakin with the bias. If you think it's worth the bother, you could replicate this section to give the mongoose variable bias for the 308 separately from the 47k pair:

http://www.diystompboxes.com/smfforum/index.php?topic=108767.msg992490#msg992490