Two Amps Fail Simultaneously, I Am Suspicious

I am told what happened is they tried to switch amps to see how the other one sounded, when it was turned on the breaker in the panel tripped.

One is a Stagg. I forget the model but suffice it to say it is in the few hundred watt range, using a bridged configuration. I pop that and BOTH legs of the bridge are shorted. I had enough transistors for one leg and that's all it needs, but I ran out and haven't dry tested the other leg yet but I suspect it only needs outputs as well. How could one blow the other even if it shorted ? Ain't there current limniting or something ? It has double pairs of outputs on each leg.

(by dry test I mean cut the collector legs and see if you ast least get the DC centered)

The other amp is an Ampeg, I think a something someting 3500. It is a normal configuration with a triple set of MOSFETS. It has good outputs. The protection relay not only disconnects the outputs, but also shorts the speaker wire to ground. (I remember the discussion about the magnet on the relay, but that was a kilowatt rainge amp, this is only maybe 500 watts) Both NC and NO terminals were shorted causing the amp to run into ground.

Luckily it has current limiting but working into that short, it fooled me on the dim bulb test at first. Obviously it couldn't correct the DC offset at that low a voltage. The bulb was not completely lit. After a few tests I plugged it in directly and it did not emit any smoke. (one of these days I think I will put a meter across that bulb just to get another piece of information) Lifting the temrinal to ground got the amp working but I can't ship it that way of course.

Anyway, with this scenario, what kind of frigged up failure mode happened here ? The one thing I can tink of is that they connected the speaker to the next amp without disconnecting it from the first one. Well, they're musicians...

However they deny that. However, maybe more than one of them was doing this little hookup thing and they don't know what happened for sure.

Thing gets me is that one it blew the breaker. Two, BOTH legs of the Stagg are blown. If you short that out it would blow whatever side of the line got shorted, right ? It would not need all eight outputs.

So at this point I am wondering if we are dealing with some sort of ground fault deal. This happened at the shop which is a commercial building. You kinda think it would be wired right. (maybe I should chck ?)You know there are still those amps around that can shock you if the ground switch is set wrong. However neither of these amps have a ground switch at all.

Other thing is i have o idea what else was plugged in. These are bass amps, far as I know they were not going to record or anything so ther ewere not likely plugged into a mixing board or something like that.

Wrack your brain on that and have a beer. It is Friday.

 

[Phil Allison]

jurb...@gmail.com wrote:

Anyway, with this scenario, what kind of frigged up failure mode happened here ? The one thing I can tink of is that they connected the speaker to the next amp without disconnecting it from the first one. Well, they're musicians...

** Probably had two leads going into the same cabinet - most cabs have dual jack or Speakon sockets to allow chaining.

It's a dead sitter for dopey musicians to use both of them into each channel of a stereo amp OR in this case two different amps.

The result is almost always he same.

The driven channel or amp sees a short circuit, blows and destroys the other OR the un-driven amp blows as discussed recently and does the same.


..... Phil

 

"** Probably had two leads going into the same cabinet - most cabs have dual >jack or Speakon sockets to allow chaining.

It's a dead sitter for dopey musicians to use both of them into each channel of >a stereo amp OR in this case two different amps. "

I'd have to bet you're right. Remember these people have trouble with the concept of a LINE input versus a MIC input. Now I see that some of my (company) time is going to be spent on education. I wonder what they did before. Car dealers or something ?

Actualy I know, they had an internet business selling musical instruments and the like. One guy there, I knew his Father when I was in business. He said he was fixing black and white TVs when he was like twelve. I am sure he would know better than to do that, but was he there at the time ? It is quite possible it was the counter guy and the boss, neither one of them ever claimed to know any electronics.

Yeah, maybe not dopey per se, but unknowledgable.

I know in the Allison vernaculr the word is "fucking stupid" but you know what I mean. I am not going to go in there and call them fucing stupuid. In fact when they blow shit up I make more money. Maybe I should just leep my trap shut.

 

[Phil Allison]

jurb...@gmail.com wrote:

"** Probably had two leads going into the same cabinet - most cabs have dual >jack or Speakon sockets to allow chaining.

It's a dead sitter for dopey musicians to use both of them into each channel of >a stereo amp OR in this case two different amps. "

I'd have to bet you're right. Remember these people have trouble with the concept of a LINE input versus a MIC input. Now I see that some of my (company) time is going to be spent on education. I wonder what they did before.. Car dealers or something ?

Actualy I know, they had an internet business selling musical instruments and the like. One guy there, I knew his Father when I was in business. He said he was fixing black and white TVs when he was like twelve. I am sure he would know better than to do that, but was he there at the time ? It is quite possible it was the counter guy and the boss, neither one of them ever claimed to know any electronics.

Yeah, maybe not dopey per se, but unknowledgable.

I know in the Allison vernaculr the word is "fucking stupid" but you know what I mean. I am not going to go in there and call them fucing stupuid. In fact when they blow shit up I make more money. Maybe I should just leep my trap shut.

** Just let them know the event was their own fault and IF anything of the sort happens again, there is NO warranty.

I have repaired numerous mixers that had an amplifier leads plugged into an input and one where a Mackie 16:4:2 desk was a complete write off because the speaker lead from a 100W Marshall was *deliberately* plugged into a line input.

The silliest one was with a stereo power amp accused of making the most horrible banging noise as soon as the volume control was turned up on one channel.

The amp bench tested faultless.

It was soon back with the same complaint, so this time I really quizzed the owner. The noise heard was a steady, very loud series of bangs, about twice per second, as soon as the control was advanced beyond about 33%.

After puzzling for a time I tried a simple test - what happens if you plug the output of one channel back to the input and advance the volume ?

Answer: It becomes a low frequency *square wave* generator with full rail to rail output. Same goes for most any amp that does not invert the signal.

The dopey roadie was a adding an extra XLR lead to the back of the amp rack that was NOT needed !!!

When I explained it to the owner - his come back was: " then how come you didn't think of that last time it was here ? "

Astonished, I replied:

" Well, I didn't think anyone would do something so stupid.


.... Phil

 

[Dave Platt]

In article <1018e621-a7d1-4df0-b90d-94fd4d3134d0@googlegroups.com>,
<jurb6006@gmail.com> wrote:

Thing gets me is that one it blew the breaker. Two, BOTH legs of the Stagg are blown. If you short that out it would
blow whatever side of the line got shorted, right ? It would not need all eight outputs.

So at this point I am wondering if we are dealing with some sort of ground fault deal. This happened at the shop which
is a commercial building. You kinda think it would be wired right. (maybe I should chck ?)You know there are still
those amps around that can shock you if the ground switch is set wrong. However neither of these amps have a ground
switch at all.

Definitely worth checking for a ground fault. A reversed hot/neutral
connection might also contribute to such a failure.

Another possibility: somebody might have decided to "save time and
save carrying around a bunch of different wires" by using ordinary AC
three-wire extension cords as "speaker cables", with a cheater of some
sort at each end to adapt to the amps and speakers. If one end of
such a cheated cable ends up being plugged into an AC outlet, and the
other end gets plugged into an amp... POP/FZZZ/smoke.

Yes, there are "roadies" careless enough to do things this way.

 

>"" Well, I didn't think anyone would do something so stupid. "

Well, over here in the states I have come to expect shit like that.

Now the warranty thing, these are stock units. They play them, which is alright usually as the instruments and amps get tested.

Of course some tests cannot be passed. Lik the last time someone told me to go fuck myself. I know I can't do that. Don't even know anyone who can.