Experimented With a Speaker and Blew a Fuse

[blown240]

I was given this big 12" car sub and thought I would see how it sounded with my bass going thru it.  Its a 4 ohm dual Voice coil supposedly rated at 1200 watts (yeah right).  I wired it in series and it measured out 7.9 ohms.   So I put it in a makeshift box that I have had for a while and plugged my Fender 300PS into it.  In the box and thru the speaker cable it measured 9.5 ohms.

With the preamp volume and output volume at 4 it sounded pretty good, actually way better than I expected.  So I decided to crank it to see what would happen.  With the preamp volume at 7 and output volume at 10 it still sounded real good, and man was it LOUD.  That worked for about a minute and I blew a fuse.

This is the first time this head has blown its fuse.  Do you think its a coincidence because I cranked it, or do you think it could be the speaker?  

[tubeswell]

A Hi Fi speaker is not a geetar speaker, and is definitely not a bass guitar speaker. Change the speaker, change the fuse and (carefully) take some DC voltages

[blown240]

I understand that it isnt the right type of speaker, but is it possible that the speaker caused the fuse to blow?  I was pleasantly surprised at how it sounded at lower volumes for home practice.

[bigsbybender]

The only way I could see the speaker causing a fuse to blow would be through "Flyback". This is a condition where the inertia of the speaker onto the speaker's voice coil turns from motor to generator. This would create a voltage that "flies back" and is sent back through your amp. Flyback usually only really causes damage the output tubes which takes the brunt of it. Serious damage from flyback is not terribly common in guitar amps. Some modern amps use a circuit comprised of diodes to protect the tubes, however millions of tube amps never had such protection and have worked for decades.

I'm not saying this is what happened inside your amp....but it is one of the few ways speakers can have a damaging effect on an amp.

Another issue could be running the amp without speaker load. If you blew the speaker and the amp went unloaded, this could cause some trouble in the amp. Usually the output transformer gets the brunt of the assault in this case.

Get a fuse and put that amp on a current limiter when plugged into known good speakers. This will check for any major internal damage to your amp.

j.

[jeff]

 Its a 4 ohm dual Voice coil ......  I wired it in series and it measured out 7.9 ohms.  

Never heard of that. Could this be your problem? Anyone?

[blown240]

Dual Voice Coil is super common in the car stereo world.  Basically acts like two 4ohm speakers and you can wire them in series to get 8 ohms, or parallel to get 2 ohms.  There are car stereo amps that are stable to 1ohm, but thats for another forum...

[PRR]

That is a VERY low-efficiency speaker with very poor treble response.

It seems possible you were playing at much higher power and mich higher distortion than you would be able to stand with a proper instrument speaker. i.e. you were cranked more than ever before, more than Fender ever expected a 300 to be worked.

Yes, speakers like that will stand nominal 600 Watts steady for hours and may be used with 1,200 Watt amplifiers on heavily clipped music. Take it apart. Note that the voice coil is 3X to 10X longer than a stage-amp speaker's coil, most of it out in the air shedding heat. When woofing there is a strong air-current out the back of the magnet. What little coil is in the iron gap conducts through that humongious magnet structure's large area. The coil is bound with extra high temperature epoxy. It can run hot enough that the resistance of copper *doubles* (which incidentally reduces the actual power drawn). The outside of the coil may be Teflon-treated so when the coil expands from heat the rubbing noise is mild.

[blown240]

That makes sense.  I often wondered why PA and bass speakers had such small magnets.  Now I know.

Well until I can get another 12, I will just keep the volume low.  Even though I did see that Carvin has new 15s for $69.  Maybe its time for a 1x15 cab.

[bobmegantz]

what would happen if the two coils were connected backwards (that is, bucking)?

[jeff]

 

what would happen if the two coils were connected backwards (that is, bucking)?

Interesting.

Sometimes those thumbscrew terminal screw all the way off and could have been put on backwards. So maybe by connecting them IN phase according to the color code, you really have connected them OUT of phase.

Do the battery trick to test the phase of each coil and see if they were really wired in phase.

If it is backwards then I imagine the coils are fighting each other. May not be why you blew the fuse but something to check.