Re: Mongoose / Giant Hogweed queries.

Posted by induction on Nov 01, 2015; 3:01pm
URL: http://guitar-fx-layouts.238.s1.nabble.com/Mongoose-Giant-Hogweed-queries-tp25642p25647.html

Beaker wrote

I changed R5 (47K) - the left hand one on the layout, to a 33K which got my voltage up to 5.8V, and yes it sounds much better.
So then I changed R4 (47K) - the right hand one on the layout to a 33K to match.

Have I done this right? Do I need to change both of them or not?

The Mongoose's 308 is DC coupled to the collector of the transistor that precedes it. As such, the 308's bias is determined by the biasing of the transistor, which is sensitive to the sum of R4 and R5. If you reduced R5 and got (actually overshot) your target voltage, then reducing R4 will make you overshoot even more. You can verify this by measuring it, of course. So if you're aiming for 5.5V, then no, don't change R4 to 33k. Otherwise, choose what sounds best to your ear.

David's Hogweed schematic has both 47K resistors on as well, so do they need altering too to get 5.5V?

Help, I'm getting confused!!!

The Hogweed schematic shows that the 308 is AC coupled (C19), so the transistor biasing network has no relevance to the 308. The bias on the 308 (Vb2) is set entirely by the stability pot. (Beware: that part of the schematic seems to be missing a ground connection.) Those 47k's are only used to bias the transistor (T7). If the biasing of the transistor is important (I have no idea), then you can play with those resistors as well, but they won't affect the bias of the 308, which gives the asymmetric clipping you're looking for.