Speaker Issue - Random Shaking/Excursions on "Idle"

[chaosMK]

I recently picked up a cool beaten up old school guitar cab for a price that I couldn't turn down.

I hooked up my D-180 and noticed right away after it powered up that the speakers were just bouncing around in there with some kind of slow random big excursions. Not really producing any volume. I also noticed that the tube power section of my amp was "flickering." I unplugged right away.

It's not the amp or speaker cable- I had to rush off to a rehearsal and it worked fine.

I've taken the cab apart. All the wires are connected. I noticed an old screw stuck to one of the magnets. Aside from that, visual inspection yields nothing else out of the ordinary.

I'd fool around with it more but all of my amps are awesome and I don't want to risk 'em at all.

Anyone have any thoughts on this?!

[fdeck]

It's got to be the amp, or something the speakers are doing to the amp. A tube amp can't put out DC under normal conditions, but it could be breaking into some kind of high frequency oscillation.

[T-Bird]

Hi.

Very slow motorboating (=lf oscillation) caused by an incorrect load impedance would be my long distance diagnose.

No healthy "modern" tube amp should do that though, even with an incorrect load, there is probably something wrong with the amp = tech time.

The speakers aren't wired in series by any chance, are they?
IME very bad for a tube amp if one of them decides to call it quits all of a sudden.

Regards
Sam

[1958Bassman]

Originally Posted by chaosMK

I recently picked up a cool beaten up old school guitar cab for a price that I couldn't turn down.

I hooked up my D-180 and noticed right away after it powered up that the speakers were just bouncing around in there with some kind of slow random big excursions. Not really producing any volume. I also noticed that the tube power section of my amp was "flickering." I unplugged right away.

It's not the amp or speaker cable- I had to rush off to a rehearsal and it worked fine.

I've taken the cab apart. All the wires are connected. I noticed an old screw stuck to one of the magnets. Aside from that, visual inspection yields nothing else out of the ordinary.

I'd fool around with it more but all of my amps are awesome and I don't want to risk 'em at all.

Anyone have any thoughts on this?!

If the tube is flickering, have the amp checked out- something is wrong. Might be simple, like a dirty/loose tube socket terminal or something more devious,like some kind of intermittent component failure.

Try it again and if it repeats, go back to standby for a few seconds and flip the switch to STDBY OFF.

[BassmanPaul]

Driver cones cannot move by themselves they have to be driven. If the cones are moving visibly when no signal is being passed, the amp has a problem. Yes it's tech time.

[chaosMK]

Thanks for the thoughts everyone. It's not the amp! It operates 100% without a hitch into every other speaker I have. I am pretty sure the only thing that will screw it up is if I keep plugging it into this cab!

I did another test... took the amp off standby for a couple seconds and got the same thing. Speakers randomly oscillating with slight noise, my entire power section fickering. After that I plugged the head into my ENGL 4x12 cab and everything worked fine, sounded dreamy.

Could this be a symptom of a bad wiring job perhaps? Here is my rendition of a wiring diagram for this cab-


All speakers are 8-Ohms. The positive wire is connected to the "inner" part of the speaker jack, negative on the "outer." The jack itself looks a little old and grungy. (also note: my amp only has 8-ohm and 4-ohm taps, if that matters).

[wcriley]

Ummm....
Those speakers are all wired in series...total = 32 ohms.

[DaZombieWoof]

What you have drawn is 4 speakers in series. If so, with 8 ohm speakers you have a 32 ohm load. Series parallel would give you 8 ohm load.

[will33]

Yep. That results in something like a 400% upward mismatch on your output transformer. And that's assuming it has an 8ohm tap. If your amp doesn't have problems now, it certainly will if you play it like that.

Verify all speakers are good and change the wiring to series/parallel.

[chaosMK]

Thanks, you guys are awesome. I guess I'll rewire this beast! Down the line I'd like to get a mono/stereo jack for it.

This is the cab... looks like it's been on a few world tours and a sandstorm of epic proportions that scoured the toltex/vinyl off of it.

[Bassmec]

Originally Posted by will33

Yep. That results in something like a 400% upward mismatch on your output transformer. And that's assuming it has an 8ohm tap. If your amp doesn't have problems now, it certainly will if you play it like that.

Verify all speakers are good and change the wiring to series/parallel.

400% the worst direction. Yup that's the internationally recommended technique to destroy a set of tubes and an output transformer in less than a gig.
Funny when you can't even read a page of information on the Internet about owning a tube amp without reading about, what not to do in relation to nominal impedance and transformer taps.