Fender Bassman 100 speaker cabinet puzzle ....

[hoodoo voodoo]

I’m hoping you can help me with some specs variations for a set of Celestion speakers in a Fender Bassman 100 (1972-74) series ‘pyramid style’ cabinet ?

I’m wanting to install them but the impedance of the units varies quite a bit.

Here are the details. All are Celestion Sidewinder 12’s

Two are S12-150 and measure 13 ohms with a multimeter when disconnected
One is a K12-300 and measures 3 ohms “ “ “ “
One is a K12T-200 and measures 3 ohms. “ “ “ “”

The registration plate on the cabinet states that it’s a 4 ohm cabinet.

They have been together in a similar cabinet for several years with no problems, so I’m assuming/ hoping that combining them won’t harm the Fender Bassman 135 amp ?

I’d also like to know how to wire up the cabinet. The jack socket at the top of the cabinet has a paralleled pairs of +/- leads coming out of it

I’m assuming these go to the 2 “upper speakers”, and that the lower pair of speakers are connected to the upper pair, but I don’t know if they should be series or parallel connected ?

Any advice you can offer on safe connection for both speakers and amp will be hugely appreciated !

If you can provide a simple diagram that would be even better.....unfortunately my tech skills (simple physics) are somewhat basic !

Cheers,

Ray

 

[cardinal]

So if you put both 13 ohm speakers in parallel, the overall impedance of that pair would be about 6.5 ohms. And if you put both 3 ohm speakers in series, the overall impedance of that pair is about 6 ohms. Then put the pairs in parallel, and the impedance of the cab would be around 3, so reasonably close to 4.

 

[hoodoo voodoo]

Thank you cardinal....and another question: the amp specifies that any external speaker/speaker must be 4 ohms minimum. So if that's connected to a 4 ohm cabinet all is fine ? Is there any advantage to making the cabinet total impedance 8 ohm....to give the amp an easier load to drive ?

I'm guessing it might be fractionally quieter (less efficient) if going into an 8 ohm rather than 4 ohm load, but which cabinet impedance should I aim for (4 or 8 ) ? The Fender reg. plate on the back of the cabinet states it's a 4 ohm cabinet, so maybe it should remain that way ?

 

[Valtyr]

At moderate volumes for bass guitar (I use my bassman for bass), running 8ohm cab instead of 4 into my Bassman 100 seems to lower the headroom. It sounds punchier and better to me at 4 ohm which is the recommended load.

 

[hoodoo voodoo]

Great...I'll keep it at 4 ohms then !

I'm pretty sure I've pinned down a diagram to show the series/parallel combination...but of A,B,C,D I'm not sure should be the 3 (4) ohm pair and which should be the 13 (16) ohm pair...assuming the diagram is correct ?

 

[hoodoo voodoo]

To help any future folk puzzling with the same issue, I connected up as per the following:

Connect the 16 ohm speakers in parallel. Connect the + terminals together and connect the - terminals together. This will give you one 8 ohm circuit. Connect one + and one - terminal to the respective terminals on the input jack.

Connect the two 4 ohm speakers in series. Connect the + terminal of one speaker to the - terminal of the other speaker. This will give you a second 8 ohm circuit. Connect the unused + speaker terminal to the + terminal of the input jack. Connect the unused - speaker terminal to the - terminal of the input jack.

Now you have two 8 ohm circuits in parallel for a total of 4 ohms....mine measured a shade over 4 but it all runs fine !