Grace Overdrive/Big Daddy Distortion

[gz300]

The schematics for the Grace Overdrive 3.0 and the Big Daddy Distortion at ROG differ by only two components and a couple of wiring connections.  ROG indicates that it may be possible to devise a switching scheme to allow switching between the two effects without duplicating components.  So far I have been unable to come up with a way to do that using a dpdt switch and I am trying to avoid using a 3pdt.  If anyone has come up with an approach or has any ideas, I would greatly appreciate any help I can get.  Thanks...

[Ben N]

Bringing back an oldy, because something occurs to me: Using the "reversibility" of the drain and source on most jfets, and the fact that the drain resistor is the same value (15k) on the Big Daddy as the source resistor on the Grace v3.0, wouldn't it be possible to use a DPDT as follows: Assigning D & S arbitrarily to the two outputs of the JFET, connect D to the 15k resistor and the coupling cap, and the other end of the resistor to one pole of the switch, which switches between 9v (Big Daddy) and ground (Grace). Connect the second pole of the switch to S, with one throw to ground (Grace) and the other to the Big Daddy source resistor/cap combination. Wahlah! Whad'ya think?

[B Tremblay]

Here's what I drew up long ago when someone emailed about combining Grace & Big Daddy:


However, the FET gain stage could certainly be improved by applying some of the Fetzer Valve article concepts to it and the added 4M7 pulldowns can probably be much lower.

[Ben N]

Hi, Brian, thanks for the response. Any thoughts about "flipping" the JFET to do it with fewer parts? BTW, thanks for the suggestion on the dropdowns-4M7 does seem kind of high. So, 1M, 2M2, I guess? Also, even if I were to go with 2 JFETs, is there any reason not to put the switch "left" of the 0.0068 cap?

[ode2no1]

Grace was one of the very first pedals i ever built! in fact, i think it was THE first. aside from the decay being a little crackly i thought it sounded great pushing my bassman 50.

[B Tremblay]

Hi, Brian, thanks for the response. Any thoughts about "flipping" the JFET to do it with fewer parts? BTW, thanks for the suggestion on the dropdowns-4M7 does seem kind of high. So, 1M, 2M2, I guess? Also, even if I were to go with 2 JFETs, is there any reason not to put the switch "left" of the 0.0068 cap?

The switch needs to be after the input stages' output caps.  If you moved the switch over and had them share a common 6n8 cap, there would be a switch pop due to the DC voltage present at each stage's output.  1M is fine for the pulldowns.

[sean k]

You could switch 1 jfet but you'd need a 3pdt to do it which is more exspensive than doubling up a few parts.

Have the drain and source both with 15k resistors, to b+ and ground respectively, and .0068 caps out from drain and source with resistors to ground off the switch (as it is now) but then switch b+ to the drain on the second pole and a 2.2k (maybe 2.4k or 2.7k as it'll now be in parallel with the 15k) and the 2.2uf on the third pole and there you go.

By switching the b+ to the drain it'll just not see the 15k already from b+ to the drain and by putting the 2.4-2.7k resistor across the 15k it'll parallel up to 2.2k and with 15k resistors always to b+ and ground theres a leakage path for any charge buildup on caps in the vicinity. The only problem I can see is whether the untended .0068 and high Meg resistor to ground would create a rolloff thats affecting its use. In both cases it a high pass set at, well, 5n at 1M is 32Hz so 5Hz at 5M is about 6Hz so it'll shunt loads of signal to ground...

So on the signal out of the drain the .0068 on the source will act as a boost by pulling all the AC so you won't need the 2.2u and maybe the same will happen on the drain when used a source follower.

What do the people who really know what they're talking about think?

[Ben N]

Just in the interest of facilitating comment/input on this question, I took the liberty of putting the three approaches in this thread into a single graphic (thanks, Brian). I'm pretty sure Brian's and Sean's approaches would work, but I'd still like to know if mine would as well, mainly because I think the idea of "flipping" the jfet within the circuit is pretty cool, although I could see there being a popping issue. (I haven't yet unpacked my breadboard after a move...)