The original Marvel Drive was posted with 2 layouts here. Since that time there have been two enhancements to the circuit. A presence knob was added in v2 and v3 incorporated a charge pump for selectable 18v or 9v operations. I have no idea if all of that could fit in a reasonable layout. If it could, I think it would be great to have those updates available. At a min, I suspect the presence knob is easy for someone that understands the circuit.
Doable? If there is not room for the charge pump, I think there are several layouts available for an addon board and switch.
I found the following comment but did not see feedback from anyone that tried it (if anyone did) or if there was confirmation that the real circuit uses this method.
IvIark20 July 2014 at 18:26
With many of these dirt pedals a presence control is often just a low pass filter before the volume pot. Whether that is what has been done on the Marvel Drive or not I don't know, but I expect we wouldn't be too far away by assuming that. You could add a 20K Presence pot and try this:
Volume 3 wire goes to Presence 3 instead of Vol 3
Presence 2 to Volume 3
Presence 1 > 22n > ground
getting hold of the through hole JFET's that are used in most of the builds on here is getting pretty difficult these days, and even if you can find them prices are generally very high and the quality is very hit and miss.....usually miss, often a very wide miss!
Probably the best solution is to be brave and enter the world of SMD JFET's (Plenty of good advice if you search the forum) it's a bit of a faff soldering up the adaptor boards, but pretty easy once you get the hang of it. The plus side though is that quality is consistantly high and prices, even including the adaptor boards are pretty cheap.
I've personally had good experiences buying from Paul at http://www.diyguitarpedals.com.au nice guy to deal with, postage is very reasonable and took less than 2 weeks to arrive in UK from Australia. Out of 50 each of the SMD J201 and 2N5457 I had a lot of very close matched groups (Including a few identical pairs) that would be perfect as sets for phase 45 / phase 90 etc and the quality of the adaptor boards is very good too.
Looks like I screwed up my parts order and ended up with antilog pots instead of log pots for the High Treble and Normal controls. If I wire these in for testing purposes, will the action just be in reverse or is there more going on than that with the antilog taper?
The antilog pots don't work in reverse, but the curve of the taper will be backwards. In other words, when you turn the knob from zero, it's gonna get really loud really fast and then the rest of the pot rotation won't do much.
If you swap the wires from lug 3 to 1, the pot will work backwards, but with a taper that may be easier to dial in