MOV advice

[Randall]

I want to install MOV protection on my amps, but I'm not sure how to choose them based on ratings. I'm in America where line voltage is approx. 120 vac. Would a 130 vac rated MOV at 100 or so joules be good or no?

Hip me to MOVs.

[M Fowler]

Here is some info.

[Randall]

Thanks M, that's great info from Mr. Keen. I'll definitely absorb that. But I'd like to know about ratings?

[galtjunk]

Thanks M, that's great info from Mr. Keen. I'll definitely absorb that. But I'd like to know about ratings?

It said to use the 140/150 volt one available at Mouser.

[M Fowler]

Yes Keen said the typical 120v-130v is marginal with today's USA voltages so increase to 140v-150v for added protection limit.

[Meat&Beer]

Man I love this place! Here's what I found on Mouser for the part #:

576-V14E150P

150VAC, 60 Joules, as Mr. Keen recommends. I just put 15 of em in my cart. 10+ for $0.48 each, I can dig it!

Bwahahahha Immortal-ness!!

[M Fowler]

The TW Express BOM shows the part number and data for the MOV.

[Firestorm]

There seems to some disagreement in the literature as to whether MOVs are degraded by repeated surges. I had always heard them referred to as "sacrificial devices," but I see claims that isn't true if they have high enough power ratings. There are differences, too, in ones rated for AC versus DC use. Maybe you don't use the same ones to protect the AC input as you would to protect the OT?

I don't have answers; just throwing out things I've read.

[M Fowler]

Looks like Keen suggest the 600v, 60J MOV to protect OT primary.

[Firestorm]

Looks like Keen suggest the 600v, 60J MOV to protect OT primary.

That's the part I don't get. He says the MOV clamps the voltage to B+. But it we had 4xEL34, say, couldn't the signal swing push the envelope, as it were?

[martin manning]

Seems like 600V MOV's across both halves of the primary wouldn't break-over until the voltage from either end to the CT exceeded 600V, at which point the whole side would be sitting at B+. To get 600V across half of the primary, however, the plate on that side would have to be at B+ plus 600V, so 1kV or so.

[Firestorm]

Seems like 600V MOV's across both halves of the primary wouldn't break-over until the voltage from either end to the CT exceeded 600V, at which point the whole side would be sitting at B+. To get 600V across half of the primary, however, the plate on that side would have to be at B+ plus 600V, so 1kV or so.

That's probably the answer: B+ plus 600V. The ends can swing huge relative to each other, but not as much relative to B+. This might be an instance where the MOV should be rated for large power.