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True Vibrato varistors


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8/29/1997 12:09 AM
Tony T
True Vibrato varistors
Does anyone know what the spec's are on the Magnatone varistors used for the true vibrato (pitch shifting) sound that they get? Where can I get them?
 
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8/29/1997 7:25 AM
J Epstein

Follow this link :  
http://www.teleport.com/~vibroman/mgnatone/  
for more info on this.  
-j
 
8/29/1997 5:47 PM
Tony T

Yeah, the guy is willing to sell everything else imaginable except the varistors. He says he has a NOS limited supply. Isn't there an off the shelf type that'll work? I'd like to incorporate their design in my DIY amp without buying an old junker Magnatone and stripping. I even sent, I think it was $6, to the U.S. patent service for an copy of the original patent which is pretty cool. However no varistor detailed spec's.
 
8/29/1997 9:42 PM
R.G.

No, there aren't any off the shelf varistors that will work. Modern varistors have too sharp a knee.  
You can easily determine the right resistance values by subbing in a multi-gang pot and manually twiddling it, selecting the value by ear, or better yet watching the input and output waveforms on a scope to see the phase shift. Resistances too low or high will stop having a phase shift.
 
8/29/1997 9:14 AM
R.G.
Don't know the exact specs. I do know:  
- any variable resistance in the correct range  
will work, LDR's being a good example if you can fake the right drive to the illumination source; you could probably use a neon lamp with the tube style drive that already exists in a Magnatone if you were clever with circuits.  
- Whatzizname at the unofficial Magnatone web site will not sell them separately; claims he found the last bin of real, true Magnatone varistors. He uses them in the $2000 (!) vibrato box he sells  
- the ordinary MOV's sold for circuit protection will NOT work, the knee of their resistance change is too abrupt.  
 
The essence of the Magnatone vibrato circuit is the same as the phase shifter in a Univibe.
 
8/29/1997 5:53 PM
Tony T

I've got a copy of the original patent but it doesn't give detailed spec's on the varistors themselves. Just a few no value marked schematics and a legal pseudo-engineering description. It's kind of a cool document though, if for nothing else but historical collector's sake.
 
8/29/1997 9:38 PM
R.G.

I would very much like to get a copy of the original patent, though. Can you post the number?
 

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