I used the ROG Fetzer calculator (http://runoffgroove.com/fetzervalve.html) for the 1K presence cap Q1 in this:
https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20mini%20layouts/Snow%20Day%20Overdrive/Snow%20Day%20Overdrive%20schematic.png
Now, since I built the thing, I know that at some point I must have found 390nF caps at a reasonable price ... but I can't remember where that was, since I doubt I paid over a dollar each for them at Mouser (especially for MLCC), and Smallbear doesn't carry them at all. Looks like Tayda has them, so that must have been it, but they have them listed on the "greenies" page, so who knows what you end up with (mine are red poly film).
I think (unless I did the math wrong, which is always a strong possibility with me) that 470R and 330nF results in essentially the same frequency boost, but I don't know whether the FET's properties, or the ultimate value of the collector resistor (assuming a 6V bias), make a difference. The ROG article seems to indicate that it doesn't.
The product of the R and the C should be the same. So your target R will be 390 * (390/330). I get 460.9 which is darn close to 470. The gain of that stage is adjustable by the trimmer so I think you're fairly safe there, unless the optimum point is way over at one end. By increasing the resistance in the source circuit you (all other things being the same) reduced your low frequency gain by... uhh... (cough) 20 * log10(390/460.9) which is a little more than 2 dB. As far as the bias point, yes that would change too, but since you're going to dial in the sound with the trimmer anyway I'd say let 'er rip. Then you can publish your "mod" to the adulation of teenage girls everywhere (or so I'm told). ;D
Quote from: Digital Larry on April 18, 2014, 12:18:54 PM
The product of the R and the C should be the same. So your target R will be 390 * (390/330). I get 460.9 which is darn close to 470. The gain of that stage is adjustable by the trimmer so I think you're fairly safe there, unless the optimum point is way over at one end. By increasing the resistance in the source circuit you (all other things being the same) reduced your low frequency gain by... uhh... (cough) 20 * log10(390/460.9) which is a little more than 2 dB. As far as the bias point, yes that would change too, but since you're going to dial in the sound with the trimmer anyway I'd say let 'er rip. Then you can publish your "mod" to the adulation of teenage girls everywhere (or so I'm told). ;D
Thanks, Larry! :)