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Vintage threads from the first ten years

tube vibratos

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2/5/2000 2:20 PM
andy fuchs	tube vibratos Anybody built any Magnetone or similar tubed vibratos with 'modern' parts ? I mean with a modern optocoupler or fet/led type device instead of the varistors...

2/5/2000 3:12 PM

Haven't built one, but those things are great. A friend picked up an old Magnatone for $40, and the vibrato is KILLER. I like the little pitch shift thing it does. Nothing seems to sound just like it. For a REALLY funky sound, we ran an old Maestro Phaser through it. (Take a Dramamine before doing this). I'd like to see if one could put a tube vibrato like that in a stompbox, and make it work well through an amp.

2/5/2000 11:00 PM
andy fuchs	There were alos AMpegs (I can't recall model) that had similar way-cool vibrato. vibroworld has tons of schematics, but there has to be a modern way to execute the varistor portion of the circuit. Those things are like hens teeth to find ! Definate way-cool vibe !

2/6/2000 2:01 AM
R.G.	LED/LDR does it fine.

2/12/2000 2:03 AM

Re: tube vibratos etc..

The thing that confuses me is how the varistors do the job they do. I understand the oscillator, the way the shift phase by taking signals from plate or cathodes, and the different cap values to change the frquency response. I get all of this. The part I don't get is how the varistors well.....vary ! I understand how the Fender or Ampeg LDR/Light Source or even the photo resistors that Mesa uses, but I don't quite "GET" how the Magnetone swings the signals from the varistors. Anybody ?

2/12/2000 7:55 PM

Varistors are Voltage-vARiable resISTORS. As the voltage across them changes, the resistance they show to much smaller signals changes. The more voltage across them, the lower the resistance they show to small signals.

In the Magnatones, they are set up so the ends of two varistors are tied together and the small signal is added there. The other two ends are driven by the signal from a phase inverter with the low frequency oscillator, so the two ends go up and down together. The signal is tapped off the LFO driven ends by two capacitors. That way the LFO signal cancells out in the signal paths, leaving only the small signal as modified by the resistance of the varistors.

It's a neat trick to cancel the 50-70 volts of LFO signal needed to get a big resistance swing on the varistors. It's much simpler to use a bulb/LDR like the univibe. There's a lot of tube circuitry eaten up in the varistor drive in a Magnatone.

2/6/2000 6:27 AM

Re: tube vibratos

The Magnatone circuit implemented with LDRs becomes essentially a tube based Univibe that is set on "vibrato".

For further explanation, read the Technology of Phasers at RG Keen's GEO.

To answer your original question, I have built a tube based Univibe, and it sounds great! In fact, I wrote an article on it entitled 'The Tube-O-Vibe' that will appear in Vacuum Tube Valley, issue 14. If you are not familiar with VTV, it is an tube audio magazine, probably the best one out there. VTV requires you to send the unit for listening tests before agreeing to publish, so apparently they thought it sounded good also. Unfortunately, issue 14 will not be out until probably June 2000. As I am being paid for this article I cannot disclose my schematic for free on the internet, but I would encourage anyone interested to purchase the magazine when it comes out.

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