Varistors for Voltage Spikes, How to size ?

[HaroldBrooks]

I found out about using Varistors to stop voltage spikes from Rob Robinette's wonderful Website. He suggests TDK 625v Varistors for the DC plate / Output transformer protection, and that's what I've used. https://www.mouser.com/ProductDetail...jRPStIMZVHlPc=

But I decided against using that high a voltage Varistor for the AC line protection, and went with a lower 470v unit recommended by Digikey. https://www.digikey.com/product-deta...1434-ND/593833

Here's my question : I realize AC peaks at above 125vac and you need something more than that voltage, or risk burning out the Varistor prematurely, but what voltage is appropriate for AC to protect your transformer ? How about DC at the output tube plates, is 625v to far away from the approximate 350vdc volt DC on most of my amps, or is it just about right ?

I would like to understand how to properly size Varistors, and not have too low or too high a voltage rating, or for that matter too few or many Joules ratings.

Also, how likely is it for a transformer to be damaged by a very fast voltage spike, up to the point of a 470v Varistor clamping kicking in ?

Thanks for any help !!!

[Zozobra]

This and the other parts will have the info you need, I think.

https://www.premierguitar.com/articl...mp-mods-pt-3-1

[HaroldBrooks]

Thanks, that's it. I had the belief that the Varistors I was talked into buying by Digikey might be too high in voltage for AC (300V AC, 470VDC), now I am fairly sure they are. Oh well, I guess they are better than nothing at all !

[trobbins]

There is some detailed discussion on MOV protection for output transformers in the link below - perhaps a bit technical, but hopefully provides some more insight for you:

https://dalmura.com.au/static/Output...protection.pdf

[HaroldBrooks]

Fantastic ! Thank you, I learned a lot from that one paper.

Perhaps the Digikey 470vdc / 300vac Varistors are ok after all. It seems transformers can handle quite a bit of high voltage. Heat and current draw will kill them but voltages have to be extreme, as in kilo volts range (think significant surges and spikes), so if the varistor clamps at or around 300vac it should do the job. Too low a voltage would invite the Varistor to be engaged more, and perhaps wear out earlier, so in retrospect I think the guy at Digikey was correct in his recommendation to me.

Lot's of other good stuff in that paper. I just scanned it, but saved a copy so I can read it late at night (only time with peace and quite).