Tonefiend Forum

Good question. I've contemplated this before. The main issue is whether you are just looking for something basic that will work for walking a bass line or whether you want chords to convincingly track an octave down. If the former, there are some old schematics around that will sort of work but tend to be a little warbly/envelopy. If you want full tracking, then you really need a digital processor. Then there's the issue of whether you want to switch to only the lower signal or whether you want them both blended.

A decent analog down is the old boss OC2. There are a lot of schematics around for that and plenty of threads on other forums. Just google boss oc2 schematics and you'll find plenty of great info. Here's one, but I can't vouch for its quality https://www.sabrotone.com/wp-content/uploads/2011/10/BossOctaverOC2.gif. Or, you can just buy a used one on ebay for about $60 and drop the circuit in your guitar. It sounds like you want the settings pegged, so you can just set it and leave the knobs off and/or just replace the pots with some resistors. It will be smaller and probably cost you about the same as a couple rounds of experimentation (not to mention the time factor). I don't have them on hand, but I know there were a bunch of diy projects (probably anderton and a couple others) in the 70's for different textures of octave down circuits. The OC2 sounds much better than any of those to my ears though. Another good option might be the Pearl OC 07 circuit https://www.freeinfosociety.com/media/images/4093.jpg. But again, it's pretty large for dropping in a guitar.

I'm sure others here are more knowledgeable about some of the modern digital alternatives, but I really dig the EH Pog/Mini Pog/Hog processors. You could theoretically pull the circuit out of a mini pog and drop it inside, but I personally would rather just leave that in pedal form.

side note: the analog octavers seem to track better after some compression. I'm not suggesting that you need to add that to your circuit, but you might want to do some experimentation if your breadboard octaver tests aren't sounding up to your expectation.

let us know when you finally do it. I'll be interested to hear the outcome. I'm planning a couple steel framed guitars which will have a big removable block for effects so this is one of options I'll be considering when I finally do it.

For now, I'd suggest either Joe's germanium booster or a little fuzz circuit like the one he just posted. Depending on the style of playing and/or other effects down the line, a little compressor or maybe even a switchable buffer could be cool.