twin at 8-ohms?

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twin at 8-ohms?

Postby dkees on Mon Jun 18, 2001 1:04 pm

hey all...what will a twin into a single jbl 12 in. 8-ohm sound like compared to the regular set up??? also, what's the power handling of these spkrs? (the orange ones from 70's twins) :confused:
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Re: twin at 8-ohms?

Postby deltablues on Mon Jun 18, 2001 1:38 pm

The twin reverb expects to see a 4 ohm load. If you put a single 8 ohm speaker in there, you're not going to be running the speaker of any brand in its most efficient state. While it won't hurt the speaker or the amp since fender output transformers are designed to handle a 100% mismatch, it will run your tubes hotter and shorten their lifespan.<P>I suggest you either buy 2 12" JBLS and replace both your current speakers. <P>Orrrrrrrrr, you can do a trick that lots of people do to soften up the twin reverb's tone a little. Pull the outer two power tubes and only run one 8 ohm speaker. (In your case, the one JBL).<P>The removal of the two outer tubes lowers the volume of the amp by about 10% and it effectively changes the load expectation of the Output Transformer to 8 ohms vs. 4. That way you can run it a peak efficiency.<P>This latter trick is completely safe for your amp and doesn't add any stress to the OT.
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Re: twin at 8-ohms?

Postby sluggo47 on Mon Jun 18, 2001 9:58 pm

If you still want to A/B the 10's & 12 through your marshall head bring your JBL over and in about 10 min we can pull a speaker from my twin and hook the JBL up solo.
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Re: twin at 8-ohms?

Postby twang on Wed Jun 20, 2001 1:05 am

Deltablues, that's an interesting post. Since older Fenders don't have switchable impedances I always wondered about running an extension cab with my Twin. Guess the best way is an 8 ohm cab and two 16 ohm speakers in parallel in the amp itself. That way the amp sees 4 ohms. But mine sounds just fine with the two JBL D-120s in there. My favorite speaker for this amp.
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Re: twin at 8-ohms?

Postby deltablues on Wed Jun 20, 2001 11:27 am

I'm pretty sure the change in impedance caused when speakers are added via the extension jack on Fender amps is serial or "Additive". <P>ie, if you plug in an additional cabinet of speakers wired to create a 4ohm load. Adding that to an existing twin reverb combo increases the load on the OT to 8ohms total.<P>So if you add an extension cabinet with 2 16ohms in parallel (8ohms), you'd be increasing the total load on the OT to 12ohms.<P>Anything more than a 100% impedance mismatch is NOT recommended in Fender Amps.<P>And while we're on the topic, this mismatch rule only applies to FENDER amps. Marshalls for example aren't built with this tolerance. So doubling or halving the impedance in those types of amps can lead to smoke and blue flames...
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