One Speaker on a Twin Reverb loses power after a few moments of playing

[mogazi!]

Hi everyone, so here's the problem:

Basically the amp (SF Twin Reverb) works fine until (quite randomly after a few moments playing) one of the speaker (JBL D120Fs) loses a lot of volume.
They are connected in parallel so you would expect the other one to drop in volume too, right?
It doesn't seem a connection problem, could it be the speaker? (the speaker does still vibrate but much less than the other one, although initially they seem the same)

Any Ideas??

[Enzo]

I don't expect the other one to drop in volume if the first one is a bad speaker, do you?

They are wired in parallel, and one speaker doesn't act like the other. SOunds like a bad speaker to me, or at least a bad connection to the one speaker.

[mogazi!]

I guess it must be the speaker itself. the reason why I was expecting the other one to drop in volume too is that I thought something was wrong with the amp internally. Any suggestions on how to try and identify the problem? I'm not really familiar with speakers...

[Enzo]

Since the speakers are wired together, there would be no way for the amp to selectively affect one without affecting hte other. Since they act separately it must be the speake or connections.

Check the wiring closely. When the sound goes away, try wigggling the wires. And if you look at a speaker, there are two terminals where the wires connect. From the back side of those terminals are two small very flexible wires leading back to the cone. Try reaching in and moving those a little. It is POSSIBLE that one of those wires - called tinsel wires - have cracked, probably near the end.

But for the most part, a speaker is like a light bulb. Once it fails, you replace it. Alternatively, look up "speaker reconers" in your phone book. Or ask your local music store if they can recommend a reconer, or if they do recones themselves. "Recone" is a generic term for speaker repair. It has a specific meaning, but most everyone misuses it.

Over in the UK, you might use a different word like speaker refitter or something.

[mogazi!]

Hey Enzo, thanks for the infos.
I tried wiggling the wires and it didn't seem to do much. The weird thing though is that the next day, I turned the amp on it worked fine (for like 5 minutes...).
I only have 3 6L6 inside right now, not sure if that might have anything to do with it, but someone told me that rather than using 3 I'd be better off using two (one in each of the power sections) and disconnecting one speaker. So that's what i did for now.
I looked up reconing costs and unfortunately for this particular speaker (JBL D120F) it's very expensive so i guess I'm just going to get a new one. I'll keep it though, since it's original, maybe one day I'll have it reconed.

[Enzo]

A new speaker of some sort might be cheaper, but a new D120F won't be cheaper than reconing.

[mogazi!]

yeah i know, they're very expensive!!!!

I was planning on using my Twin Reverb with a single 12" 8ohm neodymium speaker (halving the amp power with that valve trick mentioned before) to make the transportation easier for live gigs. I know that would compromise the true Fender sound but at the moment I can't afford a smaller (decent) combo and the Twin it's not in the best shape esthetically speaking so I doubt I'd be able to sell it for much.

The speaker is probably going to be a Jensen NEO-12 100. I saw a few positive reviews (plus it weights only 2 Kilos and it's relatively cheap. Quite a relief considering that the D120F are 6 Kilos each )

If I go in the studio or just want to enjoy the original sound of the Twin, I'll pop in the D120F (the working one!). Hopefully the character of the 2 will mix nicely, if not, I'll consider reconing. But for now, since I was planning on getting that speaker anyway, the reconing can wait.
Thank you for your help!

[womantone]

May I ask why you are running 3 6l6's? Your putting a lot of strain on your amp since the current on each side is not equal. Its like the other team has that big buff guy in a tug of war match. Anyhow, sounds like a bad speaker as the sound is still getting through to the other speaker because the split occurs prior to entering the first speaker. Basically it just y splits at the terminals, with one part going to the bad speaker, the other going to the good. Just get the speaker reconed and you will be golden. I recommend orange country speakers or ted weber for the recone.

www.tedweber.com
www.speakerrepair.com

[mogazi!]

Thanks for your feedback. Today I took out the bad speaker to have a closer look... And I found a crack in the outside suspension.



Could that be the problem? Sometimes there was no sound until the speaker was turned up to a certain level (but at that time I was also using 3 tubes instead of 4). Could the voice coil be damaged?

Before reconing (JBL are the most expensive 12" speakers to recone turns out!!!!) i was wondering if there's anything I can do in terms of DIY jobs. If it doesn't work i can always recone the whole thing. I've heard of people patching tears in their cones with superglue and tea filter paper, nail polish, white glue, denim, rolling paper.... Not sure if it would be the same for the suspension. Any suggestions?

As I said earlier, the amp sounds absolutely fine when used with only two power tubes (one in each section) and a single 8 ohm speaker (the good one).

Once again thanks so much for you help and patience!

[Tom Phillips]

? I found a crack in the outside suspension. Could that be the problem?

No. That small crack will not cause the problem you are experiencing.

Could the voice coil be damaged?

Anything is possible but you are seeing very odd behavior. I'd put my money on a wiring problem as Enzo already discussed. Now that the speaker is out, make a really good visual inspection of the terminals and the wiring at the back of the terminals. Are the terminals still stock? Are the springs in those JBL terminals holding the wires tightly? Also verify that the wire ends are clean and well dressed before they are inserted into the terminal.
IMHO, I would not mess with the cone until this problem is solved since the D120F is a very valuable speaker. [/quote]

Regards,
Tom

[52 Bill]

Could that be the problem? Sometimes there was no sound until the speaker was turned up to a certain level (but at that time I was also using 3 tubes instead of 4). Could the voice coil be damaged?

No, the small crack in the cone will not cause the speaker problems that you are having. As Enzo posted eariler, the problem is either in the wires from the amp to the speaker or the wires in the speaker itself. Double check the leads going to the speaker. On JBL's they use screw terminals or push clips for the wire connection. Check to see that these are tight and the wires have not become frayed or cracked at the point of connection.

If all is ok with the amps wires, check the tinsel wires that go from the connector to the voice coil. You'll have to look into the speaker frame to inspect these. They go from the bottom of the connector to the speaker cone itself, and look like braided wires. Sometimes these can break from being flexed for 20 years. Sometimes they will just develop an open circuit at the solder joints. I find that the easiest way to test these is with the audible continuity function on a multimeter. Hook up the leads of the meter to the two terminal of the speaker. The speaker's voice coil is so low in resistance it will test as a closed circuit and the meter will beep. Then lightly push and pull on the tinsel wires and listen to the beep for continuity breaks.

Before reconing (JBL are the most expensive 12" speakers to recone turns out!!!!) i was wondering if there's anything I can do in terms of DIY jobs. If it doesn't work i can always recone the whole thing.Any suggestions?

For the crack in the edge suspension, you really just want to keep the crack from getting larger. I would use a tooth pick to apply a thin coat of silicon adhesive to stabilize the area.

[Robert M. Martinelli]

"comparative" coil measurement

Hi Mogazi,
All the above advice is sound and it' s worth following, my opinion is that you could have a defective moving coil inside the speaker; being the two speakers equal, you have an advantage, you can use the "good" one for comparative measurements.
With the amp off, you should try to measure the DC resistance of both speakers' coils, ( with the amp obviously off, mark the defective one and disconnect 'em both from the amp, open the parallel and test 'em one by one, the DC resistance of the two coils should be very close or equal...if the faulty speaker shows a smaller DC resistance than the other, this means some of the windings in the moving coil shorted, this could be the reason why the speaker works for some minutes and then fades ( the shorted moving coil heats up because more current than expected is flowing through the wire and heating up it gets trapped and stops moving until you switch the power off and it cools down ).

Hope this helps
Best regards
Bob

[mogazi!]

Hi everyone. Thanks again for your help. I'll try out all those things. Unfortunately I have to wait until my school re-opens to have access to multimeters, soldering equipment etc... But for the time being, thank you so much, I'll let you know what I find out.

[orbanatorfunk]

orban audio speaker repair

Hi everyone, so here's the problem:

Basically the amp (SF Twin Reverb) works fine until (quite randomly after a few moments playing) one of the speaker (JBL D120Fs) loses a lot of volume.
They are connected in parallel so you would expect the other one to drop in volume too, right?
It doesn't seem a connection problem, could it be the speaker? (the speaker does still vibrate but much less than the other one, although initially they seem the same)

Any Ideas??

www.orbanaudiospeakerrepair.com
located in los angeles best in the business backed by a one year warranty. Quick turn around and fair prices. Free shipping

[Enzo]

Hi orban, welcome to the forum.

Just a thought. I don't think anyone minds if you tell people you can do work they need, but you might at least say hello and share a comment or two when you do. Otherwise it comes off as spam.