Smoke from MOV

[showsii]

Hello again everyone.


I just posted a few weeks ago about a Rocket that I finished that blew a fuse because I had the heater wires shorted together. I fixed the heater wire problem and all was well. I checked my voltages and everything was right around where it needed to be. I put a good 5-6 hours of playing time in it over the week and everything seemed fine (it sounded great, the chassis would get REALLY hot though). I went to turn it on yesterday and the MOV blew up releasing smoke, as well as the fuse. So here are my questions..

1) Has this ever happened to anyone?

2) This means I have a short still somewhere correct?


Let me know if posting my previous voltages will help solve this problem I am having now. I will also get some up to date pictures on this thread.


Thanks for the help!

[martin manning]

I wonder if blowing the fuse a number of times previously damaged the MOV. I'd be inclined to remove it and get an updated set of voltage measurements.

[geetarpicker]

Typically the MOV in a Trainwreck amp is wired straight across the 2 AC lines coming in, just passed the fuse which is on the hot side. On my old Express amps the MOV is across the AC line mounted on the DPST power switch. Basically the MOV goes dead short IF the incoming wall voltage is passed the value of the MOV which I think is 130v on my amps being designed for USA wall voltages. SO, if the wall voltage exceeds that for any reason the MOV goes dead short and then the fuse blows basically protecting the rest of the circuit from overload. Perhaps your wall voltage is over what it should be, or that your MOV is of too low a voltage, OR something is wired incorrectly. Here my wall voltage is pretty high at about 123-125 but that is still a few volts lower than the MOVs in my amps. That said I've never had an MOV blow in my original Express amps. I don't own a Rocket, but none of my Express amps or Liverpool ever ran excessively hot, maybe just the power transformer after a long gig or maybe the B+ dropping resistor.

[showsii]

I wonder if blowing the fuse a number of times previously damaged the MOV. I'd be inclined to remove it and get an updated set of voltage measurements.

Would you suggest an updated set of voltages WITHOUT the MOV?

Thanks!

[showsii]

Typically the MOV in a Trainwreck amp is wired straight across the 2 AC lines coming in, just passed the fuse which is on the hot side. On my old Express amps the MOV is across the AC line mounted on the DPST power switch. Basically the MOV goes dead short IF the incoming wall voltage is passed the value of the MOV which I think is 130v on my amps being designed for USA wall voltages. SO, if the wall voltage exceeds that for any reason the MOV goes dead short and then the fuse blows basically protecting the rest of the circuit from overload. Perhaps your wall voltage is over what it should be, or that your MOV is of too low a voltage, OR something is wired incorrectly. Here my wall voltage is pretty high at about 123-125 but that is still a few volts lower than the MOVs in my amps. That said I've never had an MOV blow in my original Express amps. I don't own a Rocket, but none of my Express amps or Liverpool ever ran excessively hot, maybe just the power transformer after a long gig or maybe the B+ dropping resistor.

This is very helpful information! The place where the MOV blew was a different location than where I had been playing it originally. (A brand new building) The place where I originally built and tested the amp, the line voltage was around 117 Vac. I just moved into a new apartment (new as well) and replaced the MOV last night and the same thing seemed to be happening, the MOV getting very hot.

I took my wall voltages and it seems to be around 122-123 VAC. I believe my MOV is right at 120 so I think I will get a higher value MOV and see if that helps.

Objections?

Thanks!

[Cliff Schecht]

Another easy solution is to just cut the MOV out. It's probably the most unnecessary measure that Ken took in his amps. Yeah it's cheap insurance but I've never fixed (or even heard of) an amp failing because of a transient in the line. Even if this does occur, the transients are typically nearly instantaneous and will get filtered out before ever seeing a tube. I guess a transient could blow the power transformer if it was high enough in amplitude to cause something to arc but again, I've never heard or dealt with this. Has anybody here knowingly had their MOV do the job it's supposed to do?

If you would like, I have a box full of 130V MOV's that I scored for free. I would not at all mind sending you a few in the mail.

[Zippy]

I took my wall voltages and it seems to be around 122-123 VAC. I believe my MOV is right at 120 so I think I will get a higher value MOV and see if that helps.

That's the ticket!

Running an MOV rated at line voltage (rather than above it) is like installing a fuse that runs at your calculated idle current - it's a component looking to die.

If you want/need an MOV, use one rated for 130VAC.

[Aurora]

A 130V MOV may well be on the low side for 125V RMS..... when you hit the top of the sine curve at switch-on.
125V RMS =~ 354V peak....

This chart gives you the ratings for different MOV voltages.

EDIT : Sorry - forgot the chart link........
http://www.nteinc.com/Web_pgs/MOV.html

[Firestorm]

An MOV should be rated significantly higher than the wall voltage; maybe double. It's not intended to handle every little variation in line voltage, just very large spikes (lightning strikes, huge solar CMEs, nearby nuclear detonations, etc.) Whenever the voltage exceeds the MOV rating, it allows current to flow though itself which it dissipates as heat. The more often you use it the quicker it dies. And they all eventually die, you're just not likely to know it because the failure is supposed to be non-catastrophic.

[Zippy]

Thanks for the check on that.

Per the link that Aurora shared: "Care must be taken to use the upper tolerance limit of the voltage source, e.g., for a 220VAC line,a 10% high line condition should be assumed..."

A MOV rated at 130VAC was good for the days/locations where the wall was giving up ~120VAC. If you are typically reading 125+, you should opt for the next higher rated MOV.

[showsii]

Great, This is all very helpful! It can't hurt going up in voltage rating on the MOV correct? I will go snag some higher rated ones and see if that fixes my issue!

Thanks everyone!

[showsii]

Another easy solution is to just cut the MOV out. It's probably the most unnecessary measure that Ken took in his amps. Yeah it's cheap insurance but I've never fixed (or even heard of) an amp failing because of a transient in the line. Even if this does occur, the transients are typically nearly instantaneous and will get filtered out before ever seeing a tube. I guess a transient could blow the power transformer if it was high enough in amplitude to cause something to arc but again, I've never heard or dealt with this. Has anybody here knowingly had their MOV do the job it's supposed to do?

If you would like, I have a box full of 130V MOV's that I scored for free. I would not at all mind sending you a few in the mail.

Cliff,

Thanks for your help and insightful comments. I appreciate the offer on the MOVs, but I think ill just run down to my local store and grab a few that are rated a bit higher!

Thanks again

[showsii]

Well It seems that replacing my original MOV with the next higher value fixed my problem!

Thanks for everyone's input and useful information!