Varistor Characteristics for Magnatone Vibrato Circuits

[martin manning]

This got started on the Magnatone-Valco Yahoo group, but I believe there are some people here that will be interested too. The attached document compares NOS and current production varistors for Magnatone vibrato circuits. Measurements of sample parts from Drew Ross and JSmokebreak, curve-traced by me are shown, along with data collected from manufacturer data sheets. Looks like the Metrosil 100-P/W/921 is the best bet, and this is believed to be the brand that the new production Magnatones use.

Edit: r.1 document posted with SiC varistor substitute (as developed in this thread) included.

[Colossal]

Hi Martin,

Back in early 2013, I had a discussion with HVR about their new production varistors. I sent them a redrawn schematic based on the Magnatone 214 (after the hype with Phil X's Evil Robot and resurgence in interest in the Magnatone circuit). If you recall, we discussed the load lines for the 214's power amp a while back.

Here is an excerpt from HVR's reply:

"We are also supplying a SDL125 480/6 which has similar I/V characteristics to the 250SDL 480/6, but a lower power (0.8W) and energy rating. The 480 (B) and 6 (n) are the electrical characteristic of the varistor. The formula for the I/V characteristic is V=BxI^(1/n) From your schematic it appeared that this would be a low power requirement."

I was considering arranging a group buy, but decided not to pursue it as it would have required a minimum purchase but I didn't think there would be enough interest on TAG.

[M Fowler]

I bought two varistor's off ebay last year not sure if they are correct but I'm going to use them to build a Magnatone type amp.

I collected data on the varistor's and put that in another thread. I should find the link to that and post here.

http://ampgarage.com/forum/viewtopic.ph ... e&start=15

Mark





Bob's Country Bunker: I'm the singer of the band and driver of the Winnebago!

[martin manning]

Mark, the data you posted the link to above is the "trobbins" data in my plot.

Dave, since you brought up the equation I checked the table from the data sheet and I appears they have an error- 340/6 in the log-log plot looks like it should be 390/5. The table of part numbers and the plot are not consistent (the table lists 340/5) so I knew something amiss, and I went with the part number in the table.

[martin manning]

Update... I came up with this scheme to imitate a varistor with Zeners and resistors at a cost of about a dollar each. The two in the picture are on their way to Smokebreak, who is going to install them in his Maggie 213 clone and see what they sound like as compared to NOS varistors.

[ToneMerc]

Update... I came up with this scheme to imitate a varistor with Zeners and resistors at a cost of about a dollar each. The two in the picture are on their way to Smokebreak, who is going to install them in his Maggie 213 clone and see what they sound like as compared to NOS varistors.

Isn't that slick, way to go Martin.

TM

[martin manning]

Thanks! If they don't work out for amp building, they can always be repurposed as an interesting pair of earrings.

[Cliff Schecht]

Yeah really impressive thinking there Martin! You've come up with some great stuff, hope this one works out as well. Your faux-tapped brownface pot trick worked great .

[M Fowler]

Thanks Martin will have to give this try one day.

[ubxf]

Thanks Martin, i'd like to try those in my Estey amp. What wattage rating should i use for the Zener

[martin manning]

Thanks Martin, i'd like to try those in my Estey amp. What wattage rating should i use for the Zener

Half-watt diodes are adequate, but I used 1.3W for their heavier leads. If the US Mail is up to its usual standards Smokebreak will be giving them a test drive in a few days, and he has promised to make a sound clip. It'd be great to have others try it out too.

[Diablo1]

Has anyone tried to contact the new Magnatone amp company to see if they will sell varistors? Most likely that their new varistors work in all the historic Magnatone designs too.

[JazzGuitarGimp]

That is very impressive, Martin! Bravo!

[Colossal]

That is really cool Martin.

[Structo]

Looking forward to the test results.