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Posted: Sun May 02, 2010 1:57 am
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whoops, double posted.


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Posted: Sun May 02, 2010 5:16 am
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No457 Snowy: I think your logic and reasoning is very sound; however, Billm is a very creditable source of info. on the Blues Jr. too. So, for me, I opted to error on the safe side and not use a total 4 ohm load.

If I wanted to do so, I would either have the mod that changes the output transformer, or as you have mentioned, get the Weber impedance matching device. It's the safest bet.

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Post subject: Re: 2x 8ohm speakers for my Fender Blues Jr
Posted: Mon Apr 11, 2016 9:06 am
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Hi,

I need some hand holding here. I have a Fender Blues Junior NOS, it is 15 watts into 8ohms and the speaker in it is a Jensen C12-N 50 watts 8 ohms. I want to add a second exact speaker, separate cabinet. Can you use a Y Cable or the Power Y Box? If so, should it be series or parallel? Or, if I want to run 2 speakers, do I need to buy 2 speakers with a different ohm rating?

Mike


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Post subject: Re: 2x 8ohm speakers for my Fender Blues Jr
Posted: Mon Apr 11, 2016 9:22 am
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The safest and simplest solution is to use a pair of 16Ω speakers wired in parallel.

Arjay

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Post subject: Re: 2x 8ohm speakers for my Fender Blues Jr
Posted: Mon Apr 11, 2016 9:24 am
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If you read this entire (zombe) thread, you'll see that your question was/is answered....not without differing opinions, but that's not going to change even though 6 years have elapsed.

For absolute, unquestionable 'safety' get one of these: http://www.tedweber.com/z-matcher-100.

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Post subject: Re: 2x 8ohm speakers for my Fender Blues Jr
Posted: Wed Apr 13, 2016 6:13 am
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This is from the Billm site, and I agree.

Paul.w
July 13, 2013 at 8:18 am

Hello bill, is it possible to run a blues jr. speaker as well as an extension cab ? Thanks.


bill
July 14, 2013 at 11:45 am

Yes, the aux speaker jack puts the two speakers in parallel, for a 4 ohm load. Tube amplifiers can handle a 100 percent mismatch, so an 8 ohm amp can drive 4 or 16 ohms with only a small loss in performance. For maximum efficiency, the TO22 output transformer has 4 and 8 ohm taps, and comes with a switching jack that automatically switches to 4 ohms when you plug in the second speaker.

http://billmaudio.com/wp/?page_id=295

As Bill points out, matching the impedance of amp and speaker, is to get the most power from the amp. There are no "danger" or "safety" concerns at all, nor any risk of tube or amp damage, with an impedance mismatch of 100%. Sometimes, a mismatched impedance can sound better than when matched. I routinely use many of my amps intentionally mismatched, because it sounds better. :)

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Post subject: Re: 2x 8ohm speakers for my Fender Blues Jr
Posted: Tue Apr 19, 2016 8:36 am
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A mod I produce for extension speakers in general uses a 1/4" phone jack with a NC switch. The speaker wires going to the internal speaker are routed through the jack switch.
When an extension speaker is plugged in, it connects them in series, which is generally a safe mismatch that won't blow power tubes. It can be hard on the OT, but a couple diodes from the plates to ground can eliminate that issue.
I mount the extra jack with an angle bracket on the inside of the cabinet so the plug doesn't protrude out the back.


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Post subject: Re: 2x 8ohm speakers for my Fender Blues Jr
Posted: Wed Apr 20, 2016 5:37 am
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Good idea. I thought about ditching the FAT jack and using the hole for an external speaker jack. There is no stress on the OT, or any part of the amp, when using a mismatched impedance, the amp just doesn't develop full power, that is all. :idea:

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Post subject: Re: 2x 8ohm speakers for my Fender Blues Jr
Posted: Wed Apr 20, 2016 5:46 am
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Check out this tube chart from an RCA RT manual. It shows how the power developed in a tube peaks near "matched" impedance, and falls off when the impedance is higher or lower. The power is never fully developed when the impedance is mismatched, so there is nothing "extra" that can harm the amp when the impedance is mismatched. Impedance matching in a tube amp really is all about getting the most power from the amp and nothing more.

Note that the distortion is also at minimum with a "matched" impedance. This is also part of the sound difference when running mismatched impedance, sometimes a more pleasing sound, bluesy breakup.

Image

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