Help with OLD to NEW Varistor Rating

[flythair1]

Hello MEF Community,

I found a Magnatone 425 Bass Combo, it's late 1961 or 62. I plan on a cap replacement, three wire cord etc. and I found a "varistor" (a large 1 1/8" dia. by 1/8" thick, dark brown) component to have a burn spot on it's surface.

On the schematic the part is labled VAR-3. The voltage on the power supply side of the varistor is 440v (per schem) and 350v on the side towards the PA. The schematic seems easy to find on the net.

I am going to place an order with Mouser and I see that they sell varistors. How would I determine the one to order to replace the one in this circuit?

I've never encountered this type of part before and I'd like to ask what it does in the amp?
Thanks for your help.

Scott

[Mark Black]

That varistor is generally part of the vibrato circuit and contributes to a particular sound that Magnatones are famous for.

If you do a Google search on "Magnatone Varistor" you will find quite a few discussions, the upshot of which is that there are no currently available parts which will act like the original. So that's pretty bad news actually for a purist.

You will probably be forced to cobble together a circuit to emulate the original characteristics of that old-school varistor using other parts such as LDR's and the like.

Sorry to break it to you...

Edit: never mind aboout the vibrato stuff - looks like they use that varistor to control power tube screen grid voltages. Same stuff still applies as far as getting an exact match for that old part, but the good news is you can probably substitute a resistor and get a pretty agreeable working arrangement.

[Raybob]

You should join Magnatone-Valco@yahoogroups.com email group. Someone there might be able to help you or know of used replacements for that. Also, you might check into old schematics of Ampeg 22. It had pitch shifting vibrato and didn't use any Varistor.

[flythair1]

Thanks Mark & Ray for your replies. This amp does not have a vibrato circuit, here's a schematic, I should have included it to begin with.

Scott

[52 Bill]

Interesting, it is used as a replacement for the screen grid supply resistor. I wonder if it will sort of work like a regulator in that position?

[Jazz P Bass]

I see that the device is labeled "VAR-3", but I do not see a modern type MOV being used there.
Most MOV devices are for transient protection ie: shunt to ground.

[52 Bill]

The type of "Varistor" that Magnetone used, acted like a variable resistor depending upon how much voltage was applied to it. It was much more linear in response than the modern ones that act more like gates, on/off. Hit a certain voltage and they conduct.

If you look at the old schematics for the vibrato circuits you'll see that the lfo basically applied a small varying dc voltage to the varistor to change the signal level going through it, creating the tremolo. Add a second set of parts being driven 180 degrees out of phase with the first side and you have magic.

[flythair1]

Thanks for the comments Jazz & 52.

Is there a way to substitute a resistor or choke in place of the varistor so I can get this thing off my bench? It's a 4x12 combo. I have replaced the power cord and taken the reversing switch & cap out of the circuit. I found an original unsoldered connection on a pre-amp power supply cap and wonder if the amp could have not had noise because of that joint. The big caps are all in FP cans and none look like they have leaked. I'm temped to try it out just to see what it sounds like before a cap job. I'm torn between new FP cans and going discreete. It needs 40-40-40-30-30. I'm thinking about using a real 5Y3 instead of the 5AR4 to lower the voltage a bit as well.

For some reason there are two 1 watt 33K resistors paralleled where it only calls for a single 33K (on the schem) between points "A" & "B" in the power supply. Any ideas about why?

All comments welcome, thanks.

Scott

[Steve Conner]

They're just using the varistor to drop 110V for the screen supply. Replace it with 110V worth of zeners, of the same hefty kind that people around here like to use to adjust the B+ on their amps.

Also, check the power tubes, the varistor probably burnt because of a shorted screen grid.

If you want to replace it with another MOV, you'll want quite a low voltage one, not the usual kind for use with 120V line.

[Jazz P Bass]

VAR 3 appears to be dropping the proper voltage, according to your first post.
Why not just let it be.
The component appears to be functioning. (sans brown spot)

[Pablo]

Right, no vibrato on this one, but on the Maggies with vibrato, they use a quad of varistors to do the tone bending. As the Magnatone vibrato is so highly regarded, a modern replacement for these would be a holy grail, I should think.