Just posted on TGP, but it was suggested I try here too.
Being a Magnatone nut, I've mostly thought of varistors being used in Bonham's patented vibrato circuit. I've read that metal oxide varistors are used in some modern amps to provide sort of a "fail safe" for the fuse, from what I can gather.
I do also recall having seen one used in one or two of the highly regarded DeArmond/Martin amps but I don't know what they were doing there. Here's another example. The 12AX7 to the left is providing some kind of recovery function after reverb and tremolo. Between there and the phase inverter 12AX7, which is on the right, is one varistor, with no value or rating specified from what I can tell. Any idea what it's doing there?
[img470]http://www.chalnick.org/LeonsGear/varistor.jpg[/img]
Where does the connection from the junction of the varistor and the 0.022u cap go? What does the circled "5" indicate? Does it refer to a note somewhere?
Sorry, the schematic is huge. I think I'm beginning to understand this now. That lead that comes into the varistor is from the output of the tremolo circuit; so this probably has to do with amplitude modulation. I wonder how to get a handle on the value of varistor that was used...
Ok that helps a lot. The tremolo output voltage against the voltage set by the 47k-270k divider modulates the the "on" state of the varistor, blocking or allowing the signal to pass. Looking at those two voltages should tell you what range to look in, and you can always adjust the divider to suit a particular varistor.
I think you are confusing the thermistor on AC inputs (surge reduction is why they are there) with varistors which vary with current draw. In this case it modulates the tremolo, but it's a different part entirely.
FUCHSAUDIO wrote:I think you are confusing the thermistor on AC inputs (surge reduction is why they are there) with varistors which vary with current draw. In this case it modulates the tremolo, but it's a different part entirely.
Both thermistors (resistors that have negative temperature coefficients) and varistors (resistors with a nonlinear voltage current relationship "MOV's") are used to protect electronic devices.
So there's a variable resistor symbol on the schematic and the printed word Varistor is next to it. That looks like a mistake.
Hmmm.....given that schematics aren't always perfect, I don't think I would be so trusting to assume it is a Varistor and not a potentiometer.
Is there already a depth/intensity knob identified for the tremolo section?
I think I would try to find a few more schematics from the same manufacturer to see if they use any varistors in their other designs OR if they recycle this section in another amp with a more illuminating schematic.
As you have mentioned Magnatone did use varistors (but they used a different schematic symbol) , I don't think many others did back then.
470]http://www.chalnick.org/LeonsGear/varistor.jpg[/img]
