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Vintage threads from the first ten years

Varistor

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6/22/1999 8:49 PM
SteveR	Varistor Does anyone know where I can get the varistors Magnatone used in their vibrato circuits? Or anyone know where I can get the specs so I can order them from Mouser or other components vendor? thanks Steve

6/22/1999 10:06 PM

Unless things have changed, you're out of luck.

There are no current production varistors that can sub for that part. The Magnatone used soft-threshold varistors that changed resistance slowly with applied voltage. The modern varistors have mutated to sharp-threshold devices that change resistance abruptly. A fellow who now runs the Zachworld web site contends that he has all of the remainining NOS Magna varistors. He used to sell them, but now states that he will use them only for his own amp repair business.

It ...might... be possible to sub in a neon-bulb/LDR arrangement like the Fender tremolo part to take the place of the Magna varistors, but that's a long shot, and you'd need to retrofit the whole amp.

There may be sources of these things I don't know about, but I'm pretty well connected to sources for wierd stuff world wide.

6/22/1999 11:51 PM

>>

It ...might... be possible to sub in a neon-bulb/LDR arrangement like the Fender tremolo part to take the place of the Magna varistors, but that's a long shot, and you'd need to retrofit the whole amp

<<

Click...vzszszszszszs....splunk...

Ok, I'll bite on that one.

I'm actually thinking of *building* a vib unit or putting one into a hifi rebuild I'm working on.

Could you explain this a bit more or point to a reference or specific Fender schem that I could study?

I really want to try to recreate the authentic vibrato as opposed to tremolo as is discussed on the Vibroworld page.

I've looked at a few of the Magnatone schematics and I haven't been able to really figure how the pitch shift is occuring.

Thanks for any help you can provide

SteveR

srathman@metacreations.com

6/23/1999 3:35 AM

>>Could you explain this a bit more or point to a reference or

>> specific Fender schem that I could study?

The Fender "vibrato" is actually a tremolo done several ways , but

many of them use a tube section to make a phase shift oscillator, and this drives another tube section which has a neon bulb driven by the plate current. The light output of a neon is a function of the current, and this light is coupled to an LDR. The LDR is set up to modulate the signal amplitude.

>>I've looked at a few of the Magnatone schematics and I haven't

>> been able to really figure how the pitch shift is occuring.

It threw me for a while too. It's pretty simple once you peel the

layers a bit. The Magnatone vibrato is a phase shifter, very similar

to the Univibe in theory. The signal tubes act like phase splitters,

and the out-of-phase signals are recombined through a capacitor

and the variable resistance of the varistors. The varistors are

arranged so they are driven by the LFO, but there are pairs driven

out-of-phase so the drive voltage of the varistors tends to cancel

out. They went to a lot of trouble to get a one-notch phase shifter in

there. I used the basic idea in my all-tube version of the univibe,

which worked OK, but needs more work before it's ready for

anyone else to build.

Any way of getting a variable resistance there where the varistors are will work, and the neon/LDR is more natural to the circuit than a solid state circuit to vary the light on an LDR>

6/23/1999 11:27 AM
GFR	The VOX AC-30 has a simpler vibrato than the Magnatones, but it is vibrato not tremolo. It is also based on phase shifts and cancellation. The out of phase waves from the LFO modulate the grid voltage of a pair of triodes. If the triodes and other components are not well matched there may be some residual tremolo left.

6/23/1999 12:45 PM
R.G.	So so some earlier Fenders, in a way. They do what amounts to a crossover, splitting lows and highs, and then amplitude modulate them with the two sides of a phase-split LFO. This creates the sense of a one-notch phaser right around the crossover frequency while preserving the change in loudess as the highs/lows get louder and softer. Very similar.

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