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Can I rectify my guitar audio signal for octave effect?

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LAN_403

Austin the Ozone

Tue Jul 19 2011, 11:11AM

Registered Member #3989 Joined: Thu Jul 07 2011, 05:10PM
Location: In a van down by the river.
Posts: 52

I am trying to figure out if using a dual diode tube to full wave rectify the audio signal will create some kinda super simple octave effect for my guitar. Does that work? Like with no smoothing caps the ripple in the dc signal that I make by rectifying the ac electricity of myself playing the guitar could be used to power a small signal tube and then fed back into an amplifier. So that I will be my own hum.. but it will be an octave higher than it started. Does that work? Sorry if its a noobish question, Im excited about it.

haxor5354

Tue Jul 19 2011, 03:15PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

wouldn't you only get the top part of the sinewave if you rectricfy it?

Myke

Tue Jul 19 2011, 05:48PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

That would work but you also get a bunch of other harmonics with it. It doesn't sound particularly nice. You'd want to use an analog multiplier with the inputs together so you can have the output be sin^2(Î¸) of the signal. By the power reduction formula, it's (1-cos(2Î¸))/2.

If you can't find a way to get an analog multiplier for cheap (though it would be in your best interest), you could use 4 op-amps. Each input would go to one op-amp wired as a log amp and then they are summed by another op-amp. The last op-amp is for the antilog amp.

Austin the Ozone

Tue Jul 19 2011, 07:01PM

Registered Member #3989 Joined: Thu Jul 07 2011, 05:10PM
Location: In a van down by the river.
Posts: 52

Thanks for the replies, allthough I could buy an octave pedal, really Im just trying to learn how things work by trying to understand some useful applications and build a few projects. I am having alot of fun learning about all the applications for tubes for now, I think they are a wonderfully useful and multipurpose device! A scratch built opamp can be made from a dual triode tube I think(or maybe two of them?), tho I don't yet know how. Nor do I understand how an opamp or a logarithmic converter works even.. but that just gives me something to work towards. I appreciate the replies very much, at least the idea for the simple octave doubler would maybe work, albeit crudely it sounds like.

Austin the Ozone

Tue Jul 19 2011, 07:27PM

Registered Member #3989 Joined: Thu Jul 07 2011, 05:10PM
Location: In a van down by the river.
Posts: 52

http://www.national.com/rap/images/CCC.gif here is one I found. I wonder how four of these would work. Maybe I could use the subminiature type tubes and power it from a big wall wart still being in the realm of a guitar effect pedal..

Myke

Tue Jul 19 2011, 08:46PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

For an analog multiplier with tubes, you'd need 2 tubes for each op-amp (assuming a double triode) and at 8 tubes, I don't think it's really worth it. There are a lot of schematics for octave pedals online. They seem to use solid state technology because otherwise, I think they'd be too big. Tubes are fragile and need relatively high voltages so I don't think it's a good idea to stick your feet there. You can still have an effects pedal with vacuum tubes but maybe a distortion pedal would be more ideal.

The multiplier thing I described wouldn't introduce much distortion. All it would do is shift everything up an octave.

If you want lots of distortion, a full wave rectifier into a low pass filter should work. Look at some designs online. (I'm no audio person

)

Austin the Ozone

Wed Jul 20 2011, 03:22AM

Registered Member #3989 Joined: Thu Jul 07 2011, 05:10PM
Location: In a van down by the river.
Posts: 52

Thank you, I am only interested in the old tube stuff as a novelty, its just something Im fascinated with and am using that fascination to propel a deeper understanding of the fundamentals of phsyics and energy. I am getting a late start on all of this in life, I wish I had persued this with as much interest years ago. Better later than never. I got the idea watching you tube videos of guys drawing arcs with 6 mots humming at 300hz hooked up to three phase power at 50hz. I was just trying to figure out a way to make a frequency multiplier for my guitar somehow that way. It seems like everything that can be done solid state can be done with tubes, sometimes its just a hassle, and I dont even know why Im fascinated with them really but I just am.

Myke

Wed Jul 20 2011, 05:45AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Hehe. Yeah, I have the same fascination with vacuum tubes and have a decent collection of them. What I'm going to do? Find the reasons why to chose one over another in a certain application and use it.
I once thought of using vaccum tubes to make an analog computer to calculate paths for the lorenz attractor but then I realized that I would need 22 dual triode tubes

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