Can Crossed Speaker Wires Cause Amp Damage?

[Knowcaster]

I bought a used Carvin Belair off Ebay a couple of weeks ago. The Belair has 2 speakers, which the former owner had replaced with one Eminence Tonker and one Eminence Legend GB128. The amp did sound a bit thin and lacked "oomph", but otherwise sounded OK. I noticed that one of the power tubes, a JJ EL84, looked as though it had gotten very hot because the red JJ ink on the glass had faded to a gray color, but sounded fine. I went ahead and replaced the power tubes anyway. The amp didn't sound much different with the new tubes. After only a couple of hours of playing, all of the sudden the amp stopped working and the jewel light went out. I found that the fuse had blown, and one of the new tubes had the red ink lettering turn gray, but I don't remember if it was the same tube socket as the original overheated tube. I replaced the fuse and tube, and in the process, I noticed that the wires between the two speakers were crossed - that is the black wire went from the positive terminal of the first speaker to the negative terminal of the second speaker, and vice versa (they are wired in parallel). I switched the wires around and the amp sounded great, fuller and a bit louder and have not had any problems since.

So anyway, my main question is could the crossed wires have caused damage to the amp, or could there be some other cause for 2 power tubes to overheat (I assume the lettering changing from red to gray is from excessive heat). I haven't played very long or loud with the wires corrected, but I guess time will tell. Maybe that's why the last owner sold it!

 

[RodeoTex]

I really doubt that crossed speaker wires would represent any extra load to the amp. A problem of speaker polarity may well cause sound problems but nothing that the amp would see.
I don't have a clue as to why the tube ink would discolor or if that is even a reliable indication of overheating.

 

[redstringuitar]

Power tubes generate alot of heat very quickly...it wouldn't take long for the ink to become discolored, but maybe the biasing is off, causing the tubes to run too hot...this needs to be checked and adjusted as you have replaced the power tubes.

 

[JohnnyCrash]

If the pair of speakers is wired incorrectly, the impedance mismatch could harm the power tubes and/or output transformer.

For instance, two 8 ohm speakers wired in series will mean a total impedance of 16 ohms. Same two 8 ohms wired in parallel and you'll have a 4 ohm impedance!

I'd likely not blame bias (four EL84's in fixed bias, right?) at this point, but instead perhaps an impedance mismatch. Google amp speaker wiring or dig around Celestion's site to find wiring diagrams. Read the speaker labels to find their impedances. Figure out what impedance you should be running at and check your impedance selector switch.

Then, take it to a repairman, tell him what happened, have him retube AND rebias it, wire the speakers properly, and give that a go. Don't try to fix it yourself.

Do NOT retube on your own. If the impedance is off you'll just kill more tubes, fuses, and possibly an expensive transformer. Fixed bias amps should always be rebiased when retubed.

Hopefully this helps and you'll be back in action in no time - if not, a repairman will get you back up and running

 

[giantslayer]

If the pair of speakers is wired incorrectly, the impedance mismatch could harm the power tubes and/or output transformer.

For instance, two 8 ohm speakers wired in series will mean a total impedance of 16 ohms. Same two 8 ohms wired in parallel and you'll have a 4 ohm impedance

+1. I think that may explain the broken amp. If you could notice one was hooked up backwards, it was probably hooked up in parallel which reduces impedence which could make it possible for the amp to try to draw more power than it can and break.

The out-of-phase thing explains the thin, weak sound because the two speakers would be producing waves 180 degrees out of phase with each other (i.e. one speaker is pushing out while the other is pulling back). Opposite polarity sound waves cancel each other out and you get no sound. Because treble is very directional, some of it gets through without touching the other out-of-phase wave and getting cancelled (at least at a closer distance). Bass and mids would mostly cancel each other out, leaving you with just a little bit of treble - your "thin" sound. Since those are two different speakers, their frequency responses aren't identical so they don't cancel each other out entirely, just mostly.


I'll bet that you had to crank the volume up quite a bit louder to get the same volume with the speaker wired out of phase. If the speakers are hooked up in such a way that the impedence is below the amp's rating, it will break again if you turn it up loud enough and play it long enough. Amps and speakers don't blow just by hooking up too little impedence. They blow because the low impedence allows too much power to flow through them, and the power flow is proportional to how high the volume is turned up and how much signal you put through it.

I would not consider your problem to be "fixed."

 

[Knowcaster]

Thanks for all of the thoughtful replies. I wanted to add a couple of other pieces of information that I did not include in my original post. I did rebias the amp when I installed the new tubes, so I don't think that was the problem. I have not checked the bias setting since I blew the tube though, I guess it could have drifted, but I set the bias right at Carvin specs so it would have to drift quite a bit to cause a power tube to go like that (unless it was bad from the get go).

Also, I don't think it was an impedance mismatch. I measured the resistance of the two speakers at the jack and it is about 4 ohms regardless of whether the polarity of the two speakers is reversed or not (and I had the selection switch set at 4 ohms). The speakers were not wired in series, but were wired in parallel, its just that they were out of polarity with one another. I was wondering if being out of polarity could cause a problem, and I think the answer I got was that the amp can't really tell polarity, just impedance, so that this was not likely to have been the problem.

In any event, I will recheck the bias and keep a close eye on the tubes. If I see anything else amiss I will take in in to a tech. Thanks again.

 

[bdgregory]

if the speakers were simply out of phase (ie incorrect polarity) it should not cause any damage or problem other than thinness (the out of phase speakers are canceling each other out). If the OT is designed for 8ohm or 16 ohm load having them in parallel could, though from your description it sounds like you're confident the amp is expecting a 4 ohm load.

 

[Wally]

If only one tube out of that set of power tubes has 'turned'....heated up, then there is a problem other than an impedance mismatch, imho. The mismatch would stress the entier output section, not just one tube.
That amp has 4 X El-84's, right? I am going to guess that the new tube that overheated was in the same socket as the other discolored tube. IF that is so, then I suspect a bad resistor, socket or connection in that tube's circuit....some problem particular to that socket and the circuit to it.
IF that amp had only 2 EL-84's, then one could extend the suspicion to the Output Transformer as well.

 

[Knowcaster]

I am not sure if it is the same tube or not, I do know that both times it was one of the inner pair of tubes. I should have paid more attention when I removed the original tubes, but at that point I was not anticipating any problems! I will pull the chassis out and take a look for any obvious signs of damage to that tube socket or the path leading from it. If I don't find anything I guess I will need to take it to someone. Thanks!

 

[Cleeve]

If two speakers mounted in the same baffle are wired out of phase, will the total impedance be different than if they were wired in phase? I mean, if two 8 ohm speakers were wired in parallel for 4 ohms, only the wires were crossed as in the original post, would the dynamic impedance be greater than 4 ohms because the speakers didn't really move air as well, therefore doing less work, thus drawing less current?

If one speaker pushes while the other pulls, so the air load just wiggles back and forth from cone to cone right at the baffle instead of being a big pressure front that would happen if both speakers were wired right, would that mean the effective load is less to the amp?

This has nothing constructive to do with the problem of the overheated tube, I'm just wondering if the actual real world load would no longer behave as ohm's law for speakers in the same baffle when wired 'cross parallel'..

As to the tube, I'd look inside the amp, maybe something like a solder tab or resistor leg is bent or shorting to make the bias on one of the tubes jump to ground cooking the one tube.