MOV in the Express

[sliberty]

I don't quite understand the theory behind the MOV's operation in an Express. If I am reading the schematic (Kelly '90) correctly, it is placed across both sides of the AC line just before the PT. So what does it do when its not blown (is this a no-continuity situation?), and what does it do if a large spike hits it and it blows (and is this a continuity situation?)?

Sorry if this is an elementary question

Steve

[paulruby]

MOV does nothing until a spike comes in. Under an over-votlage conditions, they become low resistance. I fthe spike does not have excesive energy, then nothing happens except the spike does not get to the PT. If the spike has a lot of energy, then either the fuse or the MOVs will blow.

[sliberty]

And if the MOV blows, does the amp stop working? Does it divert all AC from the PT at that point

Thanks for all your replies Paul.

[gearhead]

What's the value of the MOV? Pics look like 130v, but what current rating? Can't seem to correlate VZC to any modern MOV pn.

[paulruby]

And if the MOV blows, does the amp stop working? Does it divert all AC from the PT at that point

Thanks for all your replies Paul.

The MOV should come after the fuse so that, hopefully, the fuse blows instead of the MOV. When an MOV blows, it literally blows up.

I'm not sure why anyone would put MOVs in a tube amp unless there is a bunch of solid state stuff. A plain old tube amp doesn't care about voltage spikes.

[Dai H.]

in an old mesa/boogie catalog, they tout their amps as having spike protection (something about "a stage full of amps gets hit by lightning, and the boogies are the only ones left standing, yadayada..."), and in one pic a MOV is shown. I think it was V120LA20A(?). I have no idea how much practical protection that provides however.

[sliberty]

So, if this is true:

>>When an MOV blows, it literally blows up.

it means that when it blows, the amp can still function (unless of course the fuse blew too). So, it really only offers minor spike protection, but when a significant spike hits, it fails and then its as though it was never there.

Do I have that right?

[gearhead]

When Paul said it literally blows, I believe he means a catastrophic failure. It's gone, makes a big mess, and there is no material left to provide a conductive path. So no, the amp wouldn't function.

[Dai H.]

i thought the MOV would be sacrificial, and the amp would still work (with no more spike protection due to the MOV being blown up--IOW you would need to replace it).

[UR12]

If the MOV blows it does disintigrate. But if you have it after the fuse the current that it would draw in order to blow itself up should also blow the fuse. If you just have it accross the AC line, like in most amps, it goes pop and the amp still works. I wouldn't call it protection in a tube amp as to me it's more like a power line filter which filters out spikes or noise on the AC line. Don't fool yourself. Nothing, that I know of, in an amp will stop lightening if it hits the power line.

[sliberty]

I hate to beat this discussion to death, but....

If the MOV "vanishes" due to a major spike, the AC lines are still connected to the trannie, so the amp would work (unless the fuse also blew). The problem here is that you wouldn't know that the MOV blew up, so you would have no idea that you needed to replace it. It would have saved you from the first spike, but that s all.

Or am I still missing something?

Steve

[Elcabong]

in an old mesa/boogie catalog, they tout their amps as having spike protection (something about "a stage full of amps gets hit by lightning, and the boogies are the only ones left standing, yadayada..."), and in one pic a MOV is shown. I think it was V120LA20A(?). I have no idea how much practical protection that provides however.

It reminds me of all the fuss around powerbars. They take a $10 powerbar, put a $2 MOV in it, yadayada... and sell it $50

[gearhead]

I stand corrected (removing foot). I need to brush up on my switch pinouts again.

My question stands: what value is the MOV?

[Normster]

I stand corrected (removing foot). I need to brush up on my switch pinouts again.

My question stands: what value is the MOV?

Not sure about the value, but here's the Mouser P/N I ordered: 581-VZC130XX

[groovtubin]

I stand corrected (removing foot). I need to brush up on my switch pinouts again.

My question stands: what value is the MOV?

metal oxide varistor, jp