Amp blowing speaker cabs??

[RDK]

I have an old peavey jtm. Playing through 2 8ohm cabs. Both cabs are literally brand new. They just stopped working after sound check. Any possibility my head could have caused this? Old un grounded wiring in venue perhaps? Any suggestions?

[Chuck H]

What is a "peavey jtm"????

It may depend on the nature of amp and cabinets. I don't know the amp your referencing and I can't find any info on it either. So I don't know the wattage, if it's tube or transistor or what the proper impedance load is. And all I know about the cabinets is that they're 8 ohms each. I don't know the wattage or how they're hooked up.

Any other info about the failure would be helpful too. Was there a sound? Was everything in use and working when it quit?

Have you tested the cabs with another amp to be certain it's the cabs that are broken? Did you test the amp with a different cab?

[RDK]

Oops. Sorry. Meant VTM. Anyway. There is slight sound. Not much volume at all. 2 cabinets ran out of the VTM head. There are 2 speaker outputs on back of head and each cab was ran directly to one if these outputs. Each cab is rated at 200-300 watts. Did sound check. Everything was fine. Came back, cabs were dead. Tried different guitars, cables, and head.......still nothing.

[Chuck H]

That's very suspicious. I can't think of a way to blow up 500 watts of speakers with that amp. Short of DC directly on the speakers. But in that case there would have been an unmistakable report and most likely smoke from the amp.

Did you try different speaker cables or open the backs of the cabs to check for faults in the jacks or wiring? If someone were rough with the cables that could be the case. I'd almost sooner believe that both heads you tried were dead than both speaker cabinets have all their speakers blown. The faint sound you hear may not be the speakers, but the output transformer physically vibrating. I've heard them do this when no audible load is attached.

Try this: Plug an instrument cable into one of the cabs. Or cable other than the speaker cables you normally use. Now touch a 9V battery to the other end (+ to tip, - to sleeve). Does this make the speakers go "pop"?

[R.G.]

Chuck is right on the things to try out and especially on trying the battery test on the speakers. If they don't make sound to a momentary battery touch on the cable, there is something wrong inside the speaker. As Chuck said, try the amp on other speakers, too.

Could be a manufacturing defect if they are same brand and model.

[Enzo]

And really tough for that tube amp to put DC on the speakers...

[Chuck H]

And really tough for that tube amp to put DC on the speakers...

That's why I said "unmistakable report and most likely smoke from the amp."

It would mean some unlikely catastrophic OT failure or perhaps a dead short in the wiring. You would know if that happened. Probably before you even wondered about the speakers.

[Jessica1988]

I have learned a lot of practical things, thank you!

[tedmich]

Thats an old and odd amp! The little DIP switches that change preamp tube cathode bypass caps (largely) are unique and often overlooked


(kidding, the practical stuff is posted higher up)

[Tom Phillips]

One additional possibility is high frequency oscillation from the amp head. The energy sent to the speakers would be above your range of hearing. The speaker can't dissipate the energy by moving the cone so it all gets converted to heat and can damage the voice coils. It doesn't take much continuous power to damage even your "200W" rated cabs. I have seen this type of failure with Marshall Major heads which can oscillate at RF frequencies. Because of this unlikely possibility I personally would want to bench check the head before connecting it to another known good speaker cabinet.

And watch out for that 5th DIP switch selection in the photo posted by tedmitch.

[woodyc]

One additional possibility is high frequency oscillation from the amp head. The energy sent to the speakers would be above your range of hearing. The speaker can't dissipate the energy by moving the cone so it all gets converted to heat and can damage the voice coils. It doesn't take much continuous power to damage even your "200W" rated cabs. I have seen this type of failure with Marshall Major heads which can oscillate at RF frequencies...

It couldn't happen with just an oscillation in the head. Damaging a speaker with RF would require a series resonance involving the voice coil inductance. I don't see how it could happen but it would be interesting to know the ballpark frequency if you recall it.

[Enzo]

Feed a speaker 100 watts of 50kHz or 100kHz and see how it likes it. It isn;t about the freq itself damaging the speaker, it is about the speaker being unable to do anything with all the energy, so it turns it into heat. The voice coil acts as a resistor.

Less common in tube amps, but I have replaced my share of burnt up speakers when powr amps went RF on us.

[woodyc]

Feed a speaker 100 watts of 50kHz or 100kHz and see how it likes it. It isn;t about the freq itself damaging the speaker, it is about the speaker being unable to do anything with all the energy, so it turns it into heat. The voice coil acts as a resistor.

Less common in tube amps, but I have replaced my share of burnt up speakers when powr amps went RF on us.

Here's the full reasoning behind my comment. If I'm wrong I humbly defer to your speaker-blowing expertise:

Lets take a G12T75 for example with a resistance of about 14 Ohms and inductance of about 1mH. If 100kHz is what we're calling RF, the voice coil reactance is 628 Ohms.

So lets say we hook this speaker up to a 100W tube head. To get 100w dissipated in the 14 Ohm resistance would require a current of only 2.67A rms. (P=I*I*R) And that would probably be enough to warp the coil.

But to get that current with our reactance of 628 Ohms would require 1.68kV rms at the speaker terminals. (Ohms law) In other words, the power factor is around .02. If we presume the output transformer is lossless at 100kHz, and the turns ratio is 11, then we'd need a minimum of 50kV peak-to-peak at the tube plates to blow the speaker. Unless … (see my previous comment about the improbable resonance)

[Enzo]

A voice coils can act like a resistor, but isn;t one.

ANy energy going into a speaker that doesn't become motion becomes heat. If your speaker can handle 100 watts of audio, consider this. 14 ohms? about 37v across 14 ohms is 100 watts, if I did it right. 2.6A or so, so I think we agree.

So in this thought experiment, put 37vDC on that voice coil, leave it there, and tell me what happens. I bet it doesn;t just sit there dissipating 100 watts. A speaker relies on the wattage turning into mechanical energy, and that moving voice coil also cools itself with its own breeze.

As a resistor, it can't steadily just dissipate heat. That is why a 250 watt speaker voice coil is LOTS smaller than a 250 watt resistor dummy load. SO making it turn ALL that 100 watts into heat will melt it. That 50kHz or 100kHz can;t move the voice coil an iota. It just sits there becoming a space heater. It is a 100 watt voice coil, not a 100 watt resistor.

And while it has inductance, it is not pure inductance. The resistance is still part of the equation.

I freely admit this is NOT my area of expertise, I am a troubleshooter, not a theorist. SO I too am open to a better explanation of this stuff, if I am all wet on this.

[Steve Conner]

I agree, I don't think a voice coil will get cooked by RF any time soon. It is likely a very lossy inductance because of the laminated iron pole piece nearby, so the answer probably comes out a good deal less than 50kV, but I bet it is still more than any RF voltage the amp could crank out.

Enzo is right, DC will surely burn the voice coil because of the lack of cooling from air motion. I once set fire to a speaker like this while trying to measure its BL product.

Direct radiating speakers (as opposed to horn loaded ones) are a few percent efficient at most, so it's no great stretch to assume that all of the energy is turned to heat. But the energy accepted isn't always what the amplifier output voltage, the speaker nominal impedance and Ohm's law would have you expect. It is less because of the impedance peaks from mechanical resonance and voice coil inductance.