Bobtavia diodes

[bograt]

I'm new to all of this, but I have made a working Tweak Fuzz so I'm off and running.

Having searched the site for "Bobtavia" and looking for Diodes, I can't find an answer to why there are 4 diodes arranged the way they are.

I've tried drawing the signal pattern from the transformer, but don't really know whats going on.

I've bought all the parts, and am ready to go, but "if"  it doesn't work I'd like to figure out why - not knowing what this part of the cct does bothers me!

Thanks

[jmusser]

The diodes set up a full wave rectifier off the transformer to give the up octave effect, flipping the bottom half of the wave form on top of the reference. If they're in series like that, I believe it doubles how big the wave form is on top of the reference, when compared to what just one 1N4148 would do normally. In doing that, I would think it would have to make more of a squared off wave form, since it only has so much room to make a wave form in, and the rest has to get clipped off. I believe it also aids in how much overall output you get from the effect also. I may be off by a football field on this, if so, someone correct me so I don't further confuse this poor guy. It's how I understand it though. The Bobtavia was my very first effects pedal, and is still near and dear to my heart. I'm still trying to get a handle on exactly what's going on too. One thing I don't really understand, is why the wave form needs to be inverted before it's rectified. The op amp versions invert the wave form, and the transformer also inverts the wave form 180 degrees from what went in on the primary. Maybe the wave form would naturally fold under to be a down octave otherwise, I don't know. This is how I've interpreted the stuff I've read, and may have convinced my self of something that's totally false!

[Monster Metal Magician]

Find the Schematic for the Ring Modulator soundFX circuit used in the "1950's" Science Fiction Hit " Forbidden Planet ".  Hopefully this will shed some light.  As far as the Bobtavia goes, build a "TYCOBRAHE" instead.

With a purposefull grimace and a terrible sound, he pulls the city's high tension wires down... "Monster Metal Magician"   at--Castle Blackmore.....
:twisted:

[jmusser]

I have no idea what MMM was meaning by the ring mod thing, but the Tycho is a great circuit. The Bobtavia and the Tychobrahe have two totally different sounds, so it really depends what you're going for. The Bob is a little easier to build than the Tycho for your second build. Listen to Doug Hammond's sound sample of the Bobtavia you can find off the schematics page on this site, and then listen to the one for the Tycho on General Guitar Gadgets. Both are great clips, and both are totally different. The Bob is a high pitched paint ripper, where the Tycho is more of a gutteral Hendrix sort of sound.

[bograt]

Thanks for the replies.
I did see a thread here about the second diode in a series providing more clipping, so I'm thinking along those lines.
I haven't looked at the Tycobrahe schematic yet, and you're right, I'd like another easy one as a second effort
I've got enough stuff around to do quite a few effects, so I'll almost certainly build both.
Regards.

[jmusser]

If you go to the General Guitar Gadgets site, click on "cross reference", then click on "schematics", go to the bottom, and my EZ Build Schematics are there. There's one for the Bobtavia, and also one for Guss Smalley's Simple octave Up. The SOU I built recently, uses the same transformer as the Bobtavia, and I like it better then the Bobtavia. The Bobtavia doesn't have the massive fuzz that the SOU does. Both are great circuits, but for even a few less components, I believe the SOU is better. I have not built the NeOctavia yet, but I have heard a whole lot of good things about it. I don't know which transformer it uses from memory, but it's a simple circuit on the same theme as the Bob and SOU. There are plenty of up octaves around to choose from. They all have a certain something that makes them unique. One that you may want to look at after this, is Tim Escobedo's Octup Blender. It mixes up octave in with the standard guitar signal to taste, or, you can leave out the straight signal all together, and you get fundamental square wave (with a hint of down octave) to up octave passed the 10th fret in a very smooth transistion.

[Monster Metal Magician]

Bograt,
The Bobtavia & Tycobrahe octave pedals are both good DIY builds, the bobtavia is an easier build.  Check out GeneralGuitarGadgets site, you can't go wrong usings their Layout, schematic, & wiring diagram for the TYCOBHAHE -ala JDsleep.  Download the sound sample of the TYCO, to me it sounds like Japanesse movie monsters doing battle.  Radio Shack has all the parts for the Bobtavia, the tough find for the TYCO is the 42tm011 interstage transformer, i think its made by mouser.  Enjoy
Monster Metal Magician....   Why buy? when you can do the DIY!!!