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Post subject: Can 4ohm Amp run 8ohm speaker safely, any tricks?
Posted: Tue May 12, 2015 12:25 pm
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My "PR449 Twin Amp" says to use only 4ohm speakers (GRRRRR).
There are like 3, 4ohm speakers in the world....(not cabs)
I want to run my amp into an ISO cab with a single 8ohm speaker.
Is there any "SAFE" way to run this?
I read that some people Pull 2 power tubes. Which ones?
Anything else I can do?


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Post subject: Re: Can 4ohm Amp run 8ohm speaker safely, any tricks?
Posted: Tue May 12, 2015 4:58 pm
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Just plug it in and use it. The amp will not develop full power, it might sound better with a slight impedance mismatch, or it might not sound as good. In any event it won't hurt your amp, or the speaker, provided that the speaker wattage is high enough to handle the amp's power output.

An impedance mismatch within a 2 to 1 ratio, high or low, for a tube amp is perfectly fine.

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Post subject: Re: Can 4ohm Amp run 8ohm speaker safely, any tricks?
Posted: Tue May 12, 2015 5:49 pm
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This is just one example of what I refer to about typical power out from a tube and impedance matching (GE 6V6GT). Any tube will have similar characteristics. As you can see (page 6 of PDF), the power output from the tube peaks at a "matched" impedance, and drops off above or below "matched" impedance, while the Plate current continues to drop as impedance increases. The screen current rises as the impedance rises, hence the 2 to 1 ratio being the safe recommendation, because the screen grid resistors will limit the screen current anyway. The distortion from the tube is also the lowest at about the same point as the peak power out. As clear as can be, the tube , and therefore the amp, is in no danger when using a slight impedance mismatch (2 to 1 ratio, high or low). The output transformer (OT) also plays an additional role in limiting the current and power out from the tubes/amp.

https://www.dropbox.com/s/0g6l85oxzq58nnf/GE-6V6GT.pdf?dl=0

Let me know if the link doesn't work.

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Post subject: Re: Can 4ohm Amp run 8ohm speaker safely, any tricks?
Posted: Wed May 13, 2015 3:08 pm
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Thank you for the replies! 8)
I did find info that confirms you can pull the 2 inside power tubes.
But, that is great news to not have to pull anything....
I did also read some articles on impedance mismatching.
Regards

Axe


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Post subject: Re: Can 4ohm Amp run 8ohm speaker safely, any tricks?
Posted: Wed May 13, 2015 9:38 pm
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Here is some more excellent info about tube amps. Scroll down about half-way for specific info about impedance matching. The entire site is full of very good info, if you have the time.

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#matchspkr

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Post subject: Re: Can 4ohm Amp run 8ohm speaker safely, any tricks?
Posted: Thu May 14, 2015 7:56 am
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Wow, what a page.
Thanks again 8)


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Post subject: Re: Can 4ohm Amp run 8ohm speaker safely, any tricks?
Posted: Thu May 14, 2015 4:05 pm
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Thanks shimmilou for that web page. Looks like lots of info.

Anyway,
I was a fan of one 12" speaker combos for a while. But I really like 2-12" speakers. I found that out.
So if you still want 4 ohms, get two 8 ohm speakers wired in parallel.


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