blowing fuses!? WTF?

[DSGS]

Help! On two gigs recently, my amp (hot rod deville), completely died. Everything shut down, pilot light died. On the first gig, replaced the fuse, which promptly blew again halfway through the next song. It did this same thing a few years back, and the sound man at that gig got it up and running. Unfortunately, he's gone and I don't know what he did. Otherwise, the tubes are fine, the input jacks are solid, and all is clean on the circuit board. Is this a rectifier problem?

[Wild Bill]

Help! On two gigs recently, my amp (hot rod deville), completely died. Everything shut down, pilot light died. On the first gig, replaced the fuse, which promptly blew again halfway through the next song. It did this same thing a few years back, and the sound man at that gig got it up and running. Unfortunately, he's gone and I don't know what he did. Otherwise, the tubes are fine, the input jacks are solid, and all is clean on the circuit board. Is this a rectifier problem?

It could be anything! The fuse protects the entire amp. Something in the amp is making the amp draw too much current. That's what blows the fuse.

We can make guesses till the cows come home but unless we're psychic we'll never figure it out over the internet. If you're not technical there's no way we can tell you how to safely troubleshoot and repair the amp.

You need a good tech!

:food-smiley-004:

[nonreverb]

Help! On two gigs recently, my amp (hot rod deville), completely died. Everything shut down, pilot light died. On the first gig, replaced the fuse, which promptly blew again halfway through the next song. It did this same thing a few years back, and the sound man at that gig got it up and running. Unfortunately, he's gone and I don't know what he did. Otherwise, the tubes are fine, the input jacks are solid, and all is clean on the circuit board. Is this a rectifier problem?

I agree with Bill although if I were a bettin' man (which I'm not) my first guess would be a power tube with an intermittant short...just a guess:smile:

[guitarman2]

I'm not sure if it has a tube rectifier, but they are notorious for blowing fuses.

[Jeff Flowerday]

I'm not sure if it has a tube rectifier, but they are notorious for blowing fuses.

Yep I've seen the tube rectifier do it many times. Unfortunately in this case the deville has a solid state rectifier if I remember correctly.

[DSGS]

Thanks a lot for the replies. It just goes to confirm what I've suspected - that it could be anything. Very frustrating as there's no amp tech near where I live.
It did this fuse blowing thing 2 years ago, and at that point the guy who fixed it said it was the rectifier (a Weber Copper Cap). He took that out of the system, and I guess hooked it back up to the stock rectifier. That one is solid state.
Anyway, 2months ago, it blew 2 fuses at a gig. I had new power tubes put in after this happened, checked the connections, and circuit board and last weekend it blew another fuse. I'm wondering if I'm wasting time with this amp now, and I have a chance to buy an old Fender Bassman 10 head that I've used a few times over the years - I can hook that up to the speakers in the DeVille and take out the new tubes I put in there and install them in the Bassman head. they have the same tube setup (2 6L6 and 3 12 AX7). Too many decisions and variables - I'd sure appreciate some opinions. Thanks again!

[konasexone]

Here's what usually happens. Tubes heat up because of a filament heater not to mention the current that runs through them. Sometimes, when a tube goes bad the filament twists against something (the cathode) and a short occurs. When the tube cools down the filament twists back and the short goes away until the tube warms up again. Try changing fuses and preamp tubes first and see if the fuses survive. If not try the same thing on the output tubes. There are other possible causes but nine times out of ten tubes are the culprits. I hate fuses.
I'm presently designing a power amp board for Peavey Classic 50's with fusible links that reset themselves when cooled and recycled. The new boards will be ready by mid summer and will allow for 6L6GC's and cathode biasing.

[DSGS]

thanks for the help. I never figured on the tubes being the problem as it's happened before, and these tubes are new. It was fine one night, and the next it stopped working right after the first song.

[konasexone]

I have seen duds in new amps. Dealers will tell you that they've had tube amps die on the showroom floor.

[nonreverb]

thanks for the help. I never figured on the tubes being the problem as it's happened before, and these tubes are new. It was fine one night, and the next it stopped working right after the first song.

If this amp is gigging regularly, sometimes the act of moving it in and out of venues, vehicles and houses can cause the tubes to get jarred/ bumbed around. With the new tubes out there and particularly cheaper varieties, I suspect that elements within can become out of alignment. Tolerances in tubes are so tight that anything becoming even slightly dislodged can have a detrimental effect. Of course as mentioned, power tubes being the ones which take all of the current are the most prone to failure and yes they can do that right out of the box too. Kinda explains the reason the tubes a short warranty period.

[Wild Bill]

If this amp is gigging regularly, sometimes the act of moving it in and out of venues, vehicles and houses can cause the tubes to get jarred/ bumbed around. With the new tubes out there and particularly cheaper varieties, I suspect that elements within can become out of alignment. Tolerances in tubes are so tight that anything becoming even slightly dislodged can have a detrimental effect. Of course as mentioned, power tubes being the ones which take all of the current are the most prone to failure and yes they can do that right out of the box too. Kinda explains the reason the tubes a short warranty period.


Quite true! What's more, a lot of guys don't give their amp a few minutes to cool down after a gig before moving it.

Tubes are glorified light bulbs, after all. Ever notice how if you knock over a lamp when it's not turned on the bulb will still work but if the lamp was on the bulb blows every time?

That's because when the insides are warm they are much more fragile. You can ship tubes and rarely worry about damage but knock over your amp while you're playing and odds are you'll lose a tube or two.

Still, while tubes are the first thing to check for a problem, Murphy's Law states that it's rarely the easy stuff...:eek:

:food-smiley-004:

[konasexone]

Its not so much moving a hot tube that's harmful. Its the temperature transient
dTemp/dt that kills it. Cooling a tube too quickly really kicks the life out of it, even the best of tubes. I know this from personal experience with my first JCM800 25 years ago when playing house parties was common, ouch $$$. Nothing like a cold Sudbury winter night to send you running to the tube store.

[nonreverb]

Its not so much moving a hot tube that's harmful. Its the temperature transient
dTemp/dt that kills it. Cooling a tube too quickly really kicks the life out of it, even the best of tubes. I know this from personal experience with my first JCM800 25 years ago when playing house parties was common, ouch $$$. Nothing like a cold Sudbury winter night to send you running to the tube store.

Don't know about that...lightbulbs outdoors can last a long time...even through winter. There has to be some type of movement to precipitate the failure. Rapid cooling doesn't help but on it's own doesn't cause the problem IMO.