Skip to the bottom if you are troubleshooting a failed vibrato circuit..
A varistor is a device that has a large negative resistance-voltage coefficient so that when the voltage applied to the terminals of the device increases, its resistance decreases.
Magnatone's vibrato circuit uses between two and eight varistors, depending on the amplifier model. Original schematics did not specify a component identification other than "varistor", and at the time, there were many varistors available to Magna (and later Estey) to employ in the circuit. Although circuit creator Don Bonham left Magnatone prior to the 1962 arrival of engineer Tony Price, Price recalls Bonham visiting Estey at the Torrance facility. Bonham discussed the circuit with the engineering team and shared with them that the original selection of the exact varistor was a painstakingly slow and laborious process.
Varistors first appeared in the 1930's with both Globar Corp. and Carborundum Co. laying claim to many patents surrounding the manufacture of varistors. At some point, Globar and Carborundum merged. Other varistor manufacturers were International Resistance Company and General Electric Company. These silicon carbide varistors were manufactured by mixing fine particles of silicon carbide with water and a ceramic binder, pressing the mixture in a mold to the desired shape, and then drying and firing the pressed body.
The electrical characteristics of such varistors are expressed by the relation: I=KEⁿ. Where I is the instantaneous current, E is the instantaneous voltage, K is a constant resistance (amperes @ 1V), and "ⁿ" is an exponent that determines the degree to which the varistor departs from ohmic characteristics. (of course, n must be greater than 1, and might be as high as 6). All of these characteristics resulted in a catalog of maybe 60-100 different varistors from which Bonham had to select a specific varistor to work in his invention.
Catalogs show varistors to be characterized by (1) DC watt power rating, (2) dielectric constant, (3) response time (a percentage of the voltage pulse), (4) operating voltage, (5) peak voltage, and (6) temperature coefficient.
At a couple times during the 1968 to 1972 time frame, "Old stock" components including transformers, speakers, varistors, and even completed guitars and amps were sold to the public during a couple different Estey "factory moving sale" type events. What wasn't sold, was either taken home by employees or tossed into dumpsters. A lot of the varistors, luckily, lived on and have spread out over the years through the hands of collectors, and Magnatone lovers. Unfortunately, the stashes are few and far between, and are rarely made available for public purchase.
I haven't been able to confirm the exact model varistor that was used. From time to time, you'll see varistors appear on eBay that look identical to the originals, but all the varistors from Globar and Carborundum looked similar. Therefore, if you are on the hunt for two or more of these devices, reading this page provides you no more help then to issue a warning.
Varistors rarely, if ever, fail and the troubleshooting of a failed vibrato circuit should start with ensuring the correct tubes are being used, and that the tubes are known to be good tubes. Of course, the tube sockets must be cleaned and have good pin tension. Voltage readings should be taken to ensure that resistors are all properly functioning, and that you don't have DC voltage leak across coupling caps. From there, the phase-shift oscillator circuit should be examined to make sure it is oscillating. I've never heard of varistors failing in these circuits.