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FDP Forum / Amp Mods, Repairs, and Projects / Connecting Speaker cabinet to Fender Blues Junior and have both speakers working...

artist1952

Scotland

Apr 15th, 2019 04:02 AM   Edit   Profile  

Hi, I have just bought a new Fender Blues Junior combo and would like to connect my Fender Deluxe speaker cabinet (with 12" speaker) to the Blues Junior. However, I need to unplug the jack plug for the speaker in the Blues Junior to plug in the speaker cabinet. That obviously means that the speaker in the Blues Junior won't work along with the cabinet. As I would like both speakers to work can anyone please let me know how I can do this? Many thanks, Will

Peegoo
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Just beyond Mars

there's a world of fools
Apr 15th, 2019 08:36 AM   Edit   Profile  

You'll have an impedance mismatch if you do hook 'em both up, but so long as you don't run the amp on 10 you'll be okay.

In situations like this I keep things simple. The BJ is an open-back combo, so the simple way to do this is to make up a short speaker cable with a 1/4" plug on one end and two insulated alligator clips on the other.

You leave the BJ speaker plugged into its jack, and clip the alligator clips to the speaker's terminals. Plug the other end into your extension speaker's jack.

This places both speakers in parallel and halves the impedance.

Two very important things:

1. Mind the polarity so both speakers are in phase.

2. Don't use a shielded instrument cable for this. Lamp cord (18 ga. two conductor 'zip cord') works really well for this sort of stuff.

ejm

usa

Apr 15th, 2019 09:25 AM   Edit   Profile  

What Peegoo said, with some caveats and more homework.

First of all, what is the impedance of the speaker in the BJr?
Impedance of speaker in ext cabinet?

Second, instead of connecting the speakers in parallel, you could also connect them in series.
This migh lower the available power out, but you'd also be increasing the amount of air that is being moved by adding more speaker area, so I'm not sure if you's notice a difference or not.

And of course, if you're averse to adding special Y cables and such, you could rig up another system to connect the speakers.
I'm thinking of adding a connector plate of some kind to the back of the amp. With nothing plugged in you'd have the BJr speaker connected.
Depending on how you wire up the plate, plugging in another cable could give you both speakers in parallel, both in series, or the ext speaker only.

And........
If you used an external attenuator with two speaker outs, problem solved without using any Y cables.

Lots of possibilities, as you can see.
The hard part is figuring out what you want to do.


artist1952

Scotland

Apr 15th, 2019 11:45 AM   Edit   Profile  

Many thanks Peegoo and ejm for all your help. I'll give it all some thought, thank you. :-)

(This message was last edited by artist1952 at 01:46 PM, Apr 15th, 2019)

Peegoo
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Just beyond Mars

there's a world of fools
Apr 15th, 2019 08:14 PM   Edit   Profile  

If you're going to run the amp regularly with the extension cabinet, it's good practice to match the impedance of the speakers with what the output transformer is spec'd to handle.

Running an amp with an impedance mismatch makes the amp really inefficient...sort of like driving your car in 1st gear all the time; the engine and transmission run too hot from revving, and gas mileage sucks.

An impedance mismatch can also cause the output transformer to overheat if you dial up the volume past five or six.

If you plan on regularly running the amp with the extension cabinet, make up an adapter cable with a 1/4" plug and two 2-conductor leads coming from the plug. A short 10" lead with spade connectors for the internal speaker, and a 30" lead with a 1/4" plug for the extension cab.

You should also swap in speakers of the correct impedance: two 16-Ohm speakers if running them in parallel (8 Ohms total), or two 4-Ohm speakers if running them in series (8 Ohms total).

If you need a review of how speaker impedance is calculated when using more than a single speaker, click the link below.

Click

Te 52

Laws of Physics

strictly enforced
Apr 15th, 2019 09:36 PM   Edit   Profile  

"...If you plan on regularly running the amp with the extension cabinet, make up an adapter cable with a 1/4" plug and two 2-conductor leads coming from the plug..."

^ I think this is the cleanest solution. Don't use instrument cable to make this adapter, use speaker cable or just lamp cord. No need for the cable to be shielded.

artist1952

Scotland

Apr 16th, 2019 08:14 AM   Edit   Profile  

Thanks again to Peegoo and thanks to Te52 for taking the time to help, it's much appreciated. You both advised the solution as follows...
"...If you plan on regularly running the amp with the extension cabinet, make up an adapter cable with a 1/4" plug and two 2-conductor leads coming from the plug..."

If I did that, would it mean I'd still be limited to how much volume I could use on the amp? And would I still have to use two 16 ohm speakers instead of the existing two 8 ohm speakers to match the amp's impedance? Sorry, I don't know too much about electronics so greatly appreciate your help!

Peegoo
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Just beyond Mars

there's a world of fools
Apr 16th, 2019 03:29 PM   Edit   Profile  

Fender amps can tolerate a 100% impedance mismatch pretty well. As long as you're not running the amp near max volume you should be okay with the two 8-Ohm speakers. It's not optimal, but it works.

If you can afford it, the best option is to get yourself a 2x12 cabinet that's rated at 8 Ohms. This way, when you need the punch of two speakers, you plug that into the speaker jack on the amp.

When all you need is the single speaker, you plug in the one that's in the amp.

There's no "correct" way to do what you want with the stuff you currently have. It will work, but it's not the best way to go with this.

RDR
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I tried to think

but nothing happened!
Apr 17th, 2019 11:28 AM   Edit   Profile  

Or get a Weber Z-Matcher. You get a volume controlled line out with it too.

Lookie here

artist1952

Scotland

Apr 18th, 2019 11:18 AM   Edit   Profile  

Many thanks again Peegoo and RDR. I'll look into the Weber-Z-Matcher. :-)


Peegoo
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Just beyond Mars

there's a world of fools
Apr 18th, 2019 02:12 PM   Edit   Profile  

Good option.

Important: even though there's an "Out of Stock" flag on that product's page, order one.

Weber makes them per order, and it takes a week or two to receive it.

K4
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Being defenseless

does not make you more safe
May 9th, 2019 05:47 PM   Edit   Profile  

Adding a parallel speaker jack is an easy process.

Just make sure you use a 8 or 16 ohm extension speaker. A 4 ohm speaker might blow the tranny

I thought BillM audio made a kit, but I cannot find it. Might call them

pdf64

UK

May 10th, 2019 04:49 AM   Edit   Profile  

If you're just playing quiet, eg tv volume, then fine, but I suggest not to mismatch on an amp whose design already pushes its power tubes hard.
The BJ is such an amp, only -10.7V bias with a HT of 329V and its screen grids at 307V.
The design is running the tubes above their limit; to significantly increase the stress on them by playing loud into a load mismatch seems to be inviting a catastrophic failure.
Pentodes in little bottles are different to beam tetrodes in comparitively much bigger bottles, they are less resiliant to overdissipation. Grid current starts to increase, that raises plate current, the tube heats up more, grid current increases further and things can easily spiral into destruction.
Just becasue it says 'Fender' on the front is immaterial, electrons can't read, the design is what it is :-)

Load mismatching doesn't make an OT run hot, that's an internet myth. The OT can't pass more current than the tubes, and tube plate current self limits (look at any plate curves).But when the tube overheats and spirals out of control (as described above), that self limiting is lost. Hence heavily redplating power tubes are what overheats an OT.

BJ schematic

pdf64

UK

May 10th, 2019 05:13 AM   Edit   Profile  

To properly achieve the goal set out in post #1, ie drive loads other than 8 ohms, I suggest to fit a suitable OT, that has several secondary taps (eg Hammond 1760F, see link), together with suitable additional jacks / switching arrangements to facilitate their use.
Digikey has the OT for $42.8 https://www.digikey.com/product-detail/en/hammond-manufacturing/1760F/HM4586-ND/2358159

Multitap OT for BJ

FDP Forum / Amp Mods, Repairs, and Projects / Connecting Speaker cabinet to Fender Blues Junior and have both speakers working...




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