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Post subject: 2x 8ohm speakers for my Fender Blues Jr
Posted: Sat May 01, 2010 12:14 pm
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Hey guys, I have built an external 2x12 cab for my fender BJ, and have currently got the stock eminence speaker in it leaving the other hole empty.

Am I able to put another 8ohm speaker in there, making the cab a 4ohm or a 16ohm cab (depending if I connect the speakers in series or parrallel).

The Fender BJ is 8ohms by the way (I think)

Many thanks for any advice!!


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Posted: Sat May 01, 2010 12:23 pm
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Welcome to the Forum.

The Blues Jr. is an 8ohm amp. You need either 2 16ohm speakers wired in Parrallel or 2 4ohm speakers wired in Series to give you the 8ohm load you need for your amp.

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Posted: Sat May 01, 2010 12:27 pm
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Ok, thanks! My only problem is, the speaker I want (celestion G12 65) only comes in 8ohms or 15ohms.


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Posted: Sat May 01, 2010 12:37 pm
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My Blues Jr is in a custom 2x12 cabinet and I'm using a Scumback M75 16ohm and a Scumback H75 16ohm.

Scumbacks ( http://www.scumbackspeakers.com/ ) are Pre-Rola Celestion voiced speakers.

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Posted: Sat May 01, 2010 1:34 pm
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2 x 15Ω in parallel makes 7.5Ω total load -- an acceptable "mis-match" for your Blues Jr. However, do not use the extension cab in conjunction with your existing 8Ω internal speaker.

HTH

Arjay


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Posted: Sat May 01, 2010 3:09 pm
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Thanks guys! What else would be an 'acceptable mismatch'? Would I get away with 4ohms or 16ohms? Or is that really pushing it?


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Posted: Sat May 01, 2010 3:48 pm
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4 Ohms is OK for the BJ. Fender used a 4 Ohm load in the "Two Tone" version of the BJ. (It used an 8 Ohm 10" and an 8 Ohm 12" speaker in the same cabinet wired parallel for 4 Ohm)


Cheers,

Snowy.


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Posted: Sat May 01, 2010 10:27 pm
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No457 Snowy wrote:
4 Ohms is OK for the BJ. Fender used a 4 Ohm load in the "Two Tone" version of the BJ. (It used an 8 Ohm 10" and an 8 Ohm 12" speaker in the same cabinet wired parallel for 4 Ohm)


Cheers,

Snowy.


I asked the question about two parallel 8 ohm speakers to Billm (noted for his many Blues jr. mods.) via his website. He responded that to use an additional ext. cab of 8 ohms with the stock BJ would be OK if done infrequently and not played too loud. This was enough to make me not do it anymore. If I recall correctly, a different transformer is needed to be totally safe, and he provides a mod that will allow for the use of 2 8 ohm speakers. Perhaps the "Two Tone" mentioned above had a different transformer than the standard BJ???

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Posted: Sat May 01, 2010 11:13 pm
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RVM lead wrote:
No457 Snowy wrote:
4 Ohms is OK for the BJ. Fender used a 4 Ohm load in the "Two Tone" version of the BJ. (It used an 8 Ohm 10" and an 8 Ohm 12" speaker in the same cabinet wired parallel for 4 Ohm)


Cheers,

Snowy.


I asked the question about two parallel 8 ohm speakers to Billm (noted for his many Blues jr. mods.) via his website. He responded that to use an additional ext. cab of 8 ohms with the stock BJ would be OK if done infrequently and not played too loud. This was enough to make me not do it anymore. If I recall correctly, a different transformer is needed to be totally safe, and he provides a mod that will allow for the use of 2 8 ohm speakers. Perhaps the "Two Tone" mentioned above had a different transformer than the standard BJ???


Hi mate, the Two-Tone was exactly the same OT as any other BJ, only the cabinet and two speakers were different. I remember reading a response from one of the guys from the Fender Custom Shop about this very subject and he confirmed the 4 Ohm load was OK.

Billm himself has confirmed that the OT was the same when he had some Two-Tones pass through his shop to be modded.

Some Googling would probably turn it up.


Cheers,

Snowy.


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Posted: Sat May 01, 2010 11:25 pm
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The Blues Jr's OT has a single secondary tap of 8Ω only.

http://www.schematicheaven.com/fenderamps/blues_jr.pdf

As well, the 6BQ5/EL84 is a hot-running bottle even under the best of circumstances. With its plate-to-plate resistance in Class AB1 configuration of 8kΩ at a max-signal grid current of 22mA it really doesn't do well below 8Ω. Premature tube failure due to thermal melt-down is the usual result, often with catastrophic consequences for the OT as well.

YMMV

Arjay


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Posted: Sat May 01, 2010 11:49 pm
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Retroverbial wrote:
The Blues Jr's OT has a single secondary tap of 8Ω only.

http://www.schematicheaven.com/fenderamps/blues_jr.pdf

As well, the 6BQ5/EL84 is a hot-running bottle even under the best of circumstances. With its plate-to-plate resistance in Class AB1 configuration of 8kΩ at a max-signal grid current of 22mA it really doesn't do well below 8Ω. Premature tube failure due to thermal melt-down is the usual result, often with catastrophic consequences for the OT as well.

YMMV

Arjay


But Fender DID release the Two-Tone Blues Junior with the EXACT specs as any other Blues Junior and it ran 2 x 8 Ohm speakers in parrallel at 4 Ohms.


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Posted: Sun May 02, 2010 12:05 am
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The gentleman who posed the original question does not have one of those amps. He inquired about a regular production Blues Jr.

As for the two-tone, twin speaker, custom version, I have no comment for the simple reason that I have no documentation to consult or cite. I am aware of the amp -- but beyond that I have no information to offer.

Arjay


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Posted: Sun May 02, 2010 12:48 am
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So if 4ohms would be bad for it, how would 16ohms be for it?


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Posted: Sun May 02, 2010 1:25 am
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You'll find a virtual gold mine of useful info on the Blues Jr here......

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

Arjay


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Posted: Sun May 02, 2010 1:43 am
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Mate you could go either way if want to use your existing 8 Ohm speaker with another 8 Ohm speaker, if you use an impedance matcher like this one from Webber, a handy thing to have around.

http://taweber.powweb.com/store/zmatch.htm

OK, just to clear up, I realise the gentleman posting the question doesn't own a Two-Tone, the only reason I mentioned the Two-Tone is that Fender set the precedent with that model, which was a standard Blues Junior in a different cabinet wired at 4 Ohms and sold with a 5 year warranty, That would indicate to me that the 4 Ohm load was deemed to be acceptable by the company who designed the amp and hence that relates to the original question.

Again, there was nothing else different electronically with that amp and a production model Blues Junior, nothing mysterious, the Output Transformer was the same animal.


Cheers,

Snowy


Last edited by No457 Snowy on Sun May 02, 2010 2:00 am, edited 1 time in total.

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