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Post subject: Speaker ohm question...
Posted: Tue Jun 29, 2010 8:38 pm
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I just scored a Deluxe Reverb II. What an amp. It gives me much joy.

I noticed that under the speaker input there's a sentence that says
"Speaker 8 ohms min."
Does the min. stand for minimum? Does this mean I can try out my 16 ohm Hellatone 60 or my 410 16 ohm Orange cab? Or will this damage the amp?

Thanks!


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Posted: Tue Jun 29, 2010 8:43 pm
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You can use a 16Ω speaker (or speaker cabinet) without fear but the amp will not develop full power at that impedance. A 4Ω load should be avoided.

Arjay

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Posted: Wed Jun 30, 2010 5:31 am
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Retroverbial wrote:
You can use a 16Ω speaker (or speaker cabinet) without fear but the amp will not develop full power at that impedance. A 4Ω load should be avoided.

Arjay


Thanks, Arjay. Does that mean the amp will be behaving as if it were a 10 watt amp instead of 20? Or will it just "appear" to be?


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Posted: Wed Jun 30, 2010 7:58 am
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At 16Ω the amp would develop approximately 14 watts. But that would only be the case if the amp were dimed.

Arjay

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Posted: Wed Jun 30, 2010 8:08 am
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OK, I understand. Thanks!


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Posted: Thu Jul 08, 2010 10:15 am
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In general I was always taught just not to screw with impedances, and to match them properly...

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Posted: Fri Jul 09, 2010 8:04 am
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I see this question alot on this forum. I agree with no mismatch period. It is strange that a Deluxe Reverb II (tube amp?) has an 8 ohm minimum. Usually, that is reserved for solid state. I've read numerous times that it is better to go to 4 ohms on an 8 ohm rated tube amp than 16 ohms. For solid state, it is different. I do know for a fact that damage is done to an output transformer with no load on the amp, yet solid state can survive it. Putting a short on a solid state amp will fry it for sure, but a tube amp can survive it. To me that would mean more load on a tube amp is worse than less load. I sure would like to straighten this out.


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Posted: Fri Jul 09, 2010 8:26 am
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Lord Valve speaks:

Actually, this thread pops up every month or so. It always winds up the same way, too... the guys who know their stuff tell everyone that it's not a good idea to run a tube amp at a higher load impedance than it's looking for, and a bunch of other dudes chime in with "Oh Yeah? I ran my (whatever) into a 16-ohm cabinet for years and it was rated for 8 ohms. What about that?"

Well, this is what about it: sometimes it'll be OK. Things are different from amp to amp; playing style and signal type factor into the equation, too. A player who plays loud, with lots of treble, through an amp with high plate voltage (old Marshall, Orange, Music Man, older Ampegs, etc.) is much more likely to wind up with arced sockets (or worse) than a dude who plays jazz, with bassy tone, through a Fender amp with 430V on the plates. The type of mismatch matters, too... running an 8-ohm amp into a 16-ohm cabinet is less likely to cause problems than running a 4-ohm amp into a 16-ohm box. Running a downward mismatch will eat your tubes up a bit faster, but if your amp has a good output tranny that's probably the only thing that will happen.

Older Fenders have an extension speaker jack that's wired in parallel with the main one; if you plug an extension cab into a Twin Reverb (for instance) the load will be lower than 4 ohms, no matter what the box is rated at. If the amp was going to be damaged by running a load that was lower than the rated impedance, it stands to reason that Fender would have wired the jacks in series. They didn't, though.

A downward mismatch is usually OK, or at least it's better than the upward variety. I see a few Super Reverbs every year that have been re-speakered by their owners; they go to a lot of trouble to find out how to do a series-parallel hookup for the speakers because they "know" that the amp should be running an 8-ohm load. All of a sudden, the amp starts sounding like crap and blowing fuses. A Super Reverb wants a 2-ohm load; running it at 8 ohms pretty well guarantees you'll be making a substantial contribution to the Lord Valve Home for Lord Valve. (My favorite charity.) Of course, there will be a few "experts" who are convinced that SS amps and tube amps respond to improper loading in the same way. They're wrong, no matter how loudly they may screech.

As far as solid state amps go, it's a lot more straightforward... if you go below the rated impedance, you're going to smoke something unless the amp has really good current limiting in the output stage. Running a higher-than-rated impedance is just fine... you get less power output, of course, but the amp will run cooler and last longer. You can run a SS amp into an open circuit 'til the cows come home, and it won't do jack to it. Run a tube amp into an open, and you'll probably arc a socket (or worse) with the first note you play. It's a really common failure; I do three or four a week, year in, year out.

-Lord Valve


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Posted: Fri Jul 09, 2010 8:46 am
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Great info, guys. Very appreciated, too. I've decided not to fiddle with trying the other speakers (all 16 ohms) that I own in the DRII. In fact, after a four hour practice session last Friday night, the amp sounded great with the band and the sparkle and edgy grit provided a nice contrast to the other guitarists' thundering, rumbling Matchless DC30 and Bell Custom ToneBlaster. http://www.bellcustomguitars.com/10toneblaster.html


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