Speaker sounds blown...but isn't

[bereanbill]

I recently aquired a Danelectro DM-10 guitar amp whose speaker had a dropped voice coil. I replaced it with a known good speaker and it still had that "rattle" that is indicative of a voice coil rubbing. I tried 3 different speakers, incuding a new Jensen with the same result. All of these speakers sound fine in other amps...any ideas?

 

[Robert McLean]

The problem may be the speaker cabinet. The baffle, or the sides, or anything inside may be cracked or loose and rattling around. Was the whole enclosure dropped or just the speaker driver ? ( I am not sure what you mean by the voice coil dropped )

Also, before concluding that the dropped speaker is OK you may want to do the reverse substitution, and try the dropped speaker into one of the other known good amps. Maybe the speaker and the cabinet are both damaged.

 

[bereanbill]

To Robert

Thanks for the reply. A dropped voice coil just means it has become malformed and is rubbing the gap. All three speakers have been in other amps with no rattle . It has not been physically dropped. The enclosure is solid...I wondered if the grille clothe may be flapping...I've heard of such a phenomena...but it appears to be drum-tight!!!

 

[Cal Weldon]

Try pushing, squeezing or weighting the cabinet to see if you can change the buzz or rattle while playing music. That will help indicate where the problem lies. If there is no change you may have a problem with the amp?

 

[kevinkr]

How about connecting an oscilloscope to the output of the amplifier and seeing if it is working ok? I bet it isn't based on steps previously performed. If you don't have a scope I would find someone who does.

 

[bereanbill]

No scope...keep in mind I am a weekend tinker... I did mod the circut a bit...just added a bypass cap on the I valve and the 6V6..could this have an impact?

 

[kevinkr]

No scope...keep in mind I am a weekend tinker... I did mod the circut a bit...just added a bypass cap on the I valve and the 6V6..could this have an impact?

Serious week-end tinkerers should have a scope, older Tek models aren't that expensive.
I would suggest you remove your mods and check the result.

Try to fix an amp without a scope is like trying to read in the dark...

 

[kheper]

Swap the output tubes for known good ones. I had a Fender bass amp (Bassman?) which made rattling/jangly sounds. It was one of the output tubes. I probably overloaded the input with alot of high gain effects, damaging the tube internally.

 

[Frank Berry]

Remove the modifications one by one.
That will help you determine which mod is causing problems.

 

[Enzo]

Tap on your tubes to see if any are microphonic. Wiggle the tubes in their sockets to see if any pins have intermittent contact. Conect the output to some other speaker cab so there is no speaker vibration hitting the amp chassis. ANy difference?

And unless you have a rubber mallet, ball up your fist and whack the top of hte amp hard. If the amp reacts with some sort of sound, there is a loose connection inside somewhere.

 

[rknize]

Perhaps you have a bad OPT and have DC on the speakers?

 

[Wavebourn]

Such rattle sound may indicate oscillations on high frequencies caused by mods.

 

[nigelwright7557]

I recently aquired a Danelectro DM-10 guitar amp whose speaker had a dropped voice coil. I replaced it with a known good speaker and it still had that "rattle" that is indicative of a voice coil rubbing. I tried 3 different speakers, incuding a new Jensen with the same result. All of these speakers sound fine in other amps...any ideas?

Is the speaker cone fouling the cabinet somewhere ?

Otherwise sounds like an amplifier problem.

 

[SpreadSpectrum]

I was experimenting with my amp and my amp was in a state where layout was pretty sloppy. I heard sounds similar to what you are describing at higher volumes. I confirmed with a scope that the amp was breaking into oscillation every time the amp clipped but would stop when the amp recovered from clipping. Looked wierd on the scope. Sounded exactly like what you are describing.

Cleaning up the layout (making leads very short) cured the problem.

 

[flysig]

Is the speaker baffle warped? If it is, the speaker frame could be getting twisted/warped when you attach it, causing the voice coil to rub.

I'd guess the other suggestions are more likely than this, but thought I'd toss this possibility out there, too.

 

[bereanbill]

I was experimenting with my amp and my amp was in a state where layout was pretty sloppy. I heard sounds similar to what you are describing at higher volumes. I confirmed with a scope that the amp was breaking into oscillation every time the amp clipped but would stop when the amp recovered from clipping. Looked wierd on the scope. Sounded exactly like what you are describing.

Cleaning up the layout (making leads very short) cured the problem.

Hey...SS..that is the most cohesive reply yet!!! I'll give it a shot. Seems wierd that Danelectro wouldn't bypass the cathodes on this am p when most others are. Maybe a lead dress issue??? THX

 

[jjman]

I would try it with the speaker wired but NOT mounted in the cab.

If it still makes the noise and the speaker is good, it's probably the oscillation mentioned. That happened to me when I removed the "anti-oscillation" cap from my '74 Fender Vibro Champ. (The cap also can reduce some high end.) So I changed the grid wire on that tube to a shielded one and the oscillation was cured.