MOV

[Koop]

Please excuse this newbie question. What is the purpose of the Metal Oxide Varistor on the on - off switch of the Express? I don't have one, if it's absolutley necessary where can I find one?

[sliberty]

Not required. Not part of signal chain. It supposedly provides some surge protection, but I never really understood how.

[M Fowler]

The MOV takes the brunt of an AC spike rather then damage to AC wiring in your amp.

[geetarpicker]

If I recall correctly the stock value for the MOV in a wreck is 130 volt. The MOV is tied across the AC line at the power switch. IF the incoming voltage exceeds 130 volts, the MOV will go dead short which in turn will take out the fuse totally shutting down the amp at that point. The idea is the MOV will strike if there is a voltage spike and/or if the voltage coming in is simply past 130 volts. Years ago I had an unfortunate situation where the stage power (USA gig of course) had been incorrectly wired and it was all 240 volts coming out of the 120v outlets. At that gig I lost two Crown power amps who's output stages were then toast and were never the same even after repair. The sound board and other outboard gear had blown fuses but was later fixed 100%. Luckily we figured out the issue before we had turned on the bass or vintage Marshall guitar rigs. Point is an MOV would have saved every piece if they had them. The only downside of an MOV is it only works once, and then you need to put in a new one along with a new fuse. As said before it has no effect on the sound, it's just a safety feature safety of the amp that is.

[Koop]

Thanks for the responses. I understood a MOV to be usually a part of surge protection, but since there's so much voodoo around KF designs I wanted to be sure I wasn't missing something here.

[M Fowler]

Here is some more reading material.

[Koop]

Thanks Mark, interesting stuff.

[M Fowler]

No problem, being over protective doesn't hurt or cost much.

Mark

[surfsup]

If I recall correctly the stock value for the MOV in a wreck is 130 volt. The MOV is tied across the AC line at the power switch. IF the incoming voltage exceeds 130 volts, the MOV will go dead short which in turn will take out the fuse totally shutting down the amp at that point. The idea is the MOV will strike if there is a voltage spike and/or if the voltage coming in is simply past 130 volts.

I dont understand this. The voltage is ac, so a 125vac supply will peak at 175volts-ish. So shouldnt the mov be 200v, or higher?

[leadfootdriver]

I ditched the diode string, and put MOV's on the output transformer leads. They're the 2 mustard colored beads in the pic.

[IMG768]http://i258.photobucket.com/albums/hh26 ... G_1194.jpg[/img]

[IMG768]http://i258.photobucket.com/albums/hh26 ... G_1181.jpg[/img]

[LeftyStrat]

Here is some more reading material.

We should get a mod to sticky those. I have them saved, but it's good advice for everyone.

[surfsup]

If I recall correctly the stock value for the MOV in a wreck is 130 volt. The MOV is tied across the AC line at the power switch. IF the incoming voltage exceeds 130 volts, the MOV will go dead short which in turn will take out the fuse totally shutting down the amp at that point. The idea is the MOV will strike if there is a voltage spike and/or if the voltage coming in is simply past 130 volts.

I dont understand this. The voltage is ac, so a 125vac supply will peak at 175volts-ish. So shouldnt the mov be 200v, or higher?

So datasheets show rms and dc values...

[M Fowler]

Here is some more reading material.

We should get a mod to sticky those. I have them saved, but it's good advice for everyone.

I'll add them to the sticky "Reading Material on Steroids"

[Jana]

More info:

http://www.youtube.com/watch?v=-Lza3NVH6Ig

http://www.youtube.com/watch?v=gn2El8kskhk

[hitchcaster]

If I recall correctly the stock value for the MOV in a wreck is 130 volt. The MOV is tied across the AC line at the power switch. IF the incoming voltage exceeds 130 volts, the MOV will go dead short which in turn will take out the fuse totally shutting down the amp at that point. The idea is the MOV will strike if there is a voltage spike and/or if the voltage coming in is simply past 130 volts. Years ago I had an unfortunate situation where the stage power (USA gig of course) had been incorrectly wired and it was all 240 volts coming out of the 120v outlets. At that gig I lost two Crown power amps who's output stages were then toast and were never the same even after repair. The sound board and other outboard gear had blown fuses but was later fixed 100%. Luckily we figured out the issue before we had turned on the bass or vintage Marshall guitar rigs. Point is an MOV would have saved every piece if they had them. The only downside of an MOV is it only works once, and then you need to put in a new one along with a new fuse. As said before it has no effect on the sound, it's just a safety feature safety of the amp that is.

F#@*&!!!! jackass move! sometimes those outdoor stage guys are just retards... thats such a bummer... KF was smart knowing that he can't control all the variables.. what were the power amps for?
to OP, if your building your own amp you can be more safe and maybe take voltage readings at suspect gigs... leave the MOV out, its not a tone secret, and might even sound a hair better without (haven't tested it). maybe get a nice power filter with regulation that tells you the voltage..ive got a furman PF pro and its not the best but its way better then nothing... i had a club where once things starting going the power would dip down to like 90v and sound like shit and put my tubes thru cathode stripping hell i added the PF pro and it helped a little before i ran a 150 foot extension from somewhere else for myself and let the rest suffer don't worry about the MOV its the least of your worries.. worry about lead dress