Varistor Characteristics for Magnatone Vibrato Circuits

[Diablo1]

I went back and grabbed some more varistors, FR1039, FS308, and FS927.
The FS927 is labeled as 1mA at 80 volts, so I took a shot that it'll work.

[Diablo1]

Can someone explain why some of the old varistors are rated with ohms resistance on the package, and not any voltage or current? I left some varistors on the shelf, because I had no idea about their V/I performance and if they were suitable for the Magnatone.

[martin manning]

We're they perhaps thermistors?

[Diablo1]

We're they perhaps thermistors?

Maybe they are thermistors. The reason I'm confused is that Workman parts are labeled FR or FS, and some are labeled varistors and some are labeled thermistors. For example, FS308 is labeled as a Varistor 1.0 Ma @ 110 volts. FS1205 Varistor .05 ma @ 80 volts. FS927 Varistor 1.0 ma @ 80 volts. FR1039 Varistor 1.0 ma @ 68V.

Most of the parts labeled FR, like FR191 show an ohm rating at a temperature e.g. 120 ohms @ 25C.

Do Silicon Carbide varistors also function as thermistors?

[martin manning]

No, they are different. A thermistor is a temperature dependent resistor, and a varistor is a semiconductor. Both are used to limit surge currents, but a varistor does it by shorting (to protect against a voltage surge or spike), and a negative temperature coefficient (NTC) thermistor is typically used to limit start-up current by having a high resistance when cold, which diminishes to a low value when it is hot and passing normal operating current. FR191 is a PTC thermistor used in CRT degaussing circuits.

[Diablo1]

No, they are different. A thermistor is a temperature dependent resistor, and a varistor is a semiconductor. Both are used to limit surge currents, but a varistor does it by shorting (to protect against a voltage surge or spike), and a negative temperature coefficient (NTC) thermistor is typically used to limit start-up current by having a high resistance when cold, which diminishes to a low value when it is hot and passing normal operating current. FR191 is a PTC thermistor used in CRT degaussing circuits.

Thanks for the clarification Martin. I wasn't sure if SiC also had temperature dependent properties that caused the resistance to drop with increasing temperature, and that allowed it to also function as a thermistor.

[trobbins]

http://dalmura.com.au/projects/Silicon% ... istors.pdf

The link is a good reference for old tech SiC devices like the Workman parts.

The old maggie varistor is best referred to as a ceramic part, rather than a semiconductor, as it is the granular interface that dictates performance.

That ceramic can be manufactured in an optimised form for low resistance and highish NTC, and hence act as a NTC 'thermistor' as we know it nowadays, albeit a poor performer when comparing with modern day devices.

In its day, it was pretty much the only cheap non-linear resistive device and so it was optimised for many many applications - all of which have now been superseded by better parts or methods.

[martin manning]

I guess it's better to say that varistors are voltage dependent resistors and thermistors are temperature dependent resistors. In fact they are both types of semiconductors.

[Diablo1]

http://dalmura.com.au/projects/Silicon% ... istors.pdf

The link is a good reference for old tech SiC devices like the Workman parts.

The old maggie varistor is best referred to as a ceramic part, rather than a semiconductor, as it is the granular interface that dictates performance.

That ceramic can be manufactured in an optimised form for low resistance and highish NTC, and hence act as a NTC 'thermistor' as we know it nowadays, albeit a poor performer when comparing with modern day devices.

In its day, it was pretty much the only cheap non-linear resistive device and so it was optimised for many many applications - all of which have now been superseded by better parts or methods.

So, is it possible that some of the old Workman SiC thermistors could double as a varistor, and have the correct characteristics to work in a Magnatone circuit? Or are the thermistor electric characteristics way off what is required?

[trobbins]

A varistor for a maggie has a very high resistance if measured by meter.

I inferred from the SiC article that a high NTC was achievable with low resistance devices - perhaps your 120 ohm measure part was of that type.

Unless the part is in a bag marked with a model number then the only way to tell what part you have is to do some testing.

[Diablo1]

A varistor for a maggie has a very high resistance if measured by meter.

I inferred from the SiC article that a high NTC was achievable with low resistance devices - perhaps your 120 ohm measure part was of that type.

Unless the part is in a bag marked with a model number then the only way to tell what part you have is to do some testing.

I have a few varistors that I bought from Warbler Muse, claimed to be Magnatone spec NOS. I measured the resistance as 14 meg-ohms at room temperature. I warmed one up by holding it near a light bulb, and as it warmed the resistance dropped down to around 8 meg-ohms. So, it is showing thermistor characteristics, but as you posted, the resistance range is very high compared to the Workman parts labeled as thermistors. So, it doesn't seem likely that the thermistors could be used as a Magnatone varistor.

[Kagliostro]

I've a GEM (old italian amp) that uses the same patent used by Magnatone

no schematics where disposable, so I reverse engineered the amp and

draw one by myself (see attached)

Whell, the two Varistor are very different (in the aspect) from those you can

see in a Magnatone, looking to the image, someone is able to read the

colors on the varistor and read it ? I tried to do that, but I wasn't able

to achieve a result

Thanks

K

The image is labeled as unknown component because at that time I didn't

know the Varistor vibrato circuit and didn't understand what it was

[trobbins]

K, please double check all the circuitry close to the varistors, and indicate what the 'blue' coloured numbers mean.

Some of the parts do not appear to be in the correct place - especially the vibrato modulator tube, which shows a 4N7 cap bypass on the 82k, and a 680pf capacitive top arm feed (as that should be a resistor arm, as the bottom arm of the modulator is the two 47N caps). Also the varistor midpoint is shown being dc coupled to other stages.

[Kagliostro]

I was near sure that the schematic is drawn as the circuit is, but I'll chek it again

thanks for the advice

--

the blue numbers refers to the numbers you can find on the schematic in this

.pdf file (the patent), see fig. 3

K

[Diablo1]

Also, your notation of cap values is written like they are resistors. That is confusing, as we don't know the units. For example 47K for a cap value or 10K for a cap value. How many microfarads or nanofarads or picofarads etc.?