Fender Bassman 100 speaker cabinet puzzle...

[Ray T]

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

 

[Stumbo]

How were they wired in their old cab?.

The 3 ohm speakers are probably 4 ohm and the 13 ohm are probably 16 ohm.

Wire the two 4 ohm in series to get 8 ohms and the two 16 ohm in parallel to get 8 ohm. Together, 4 ohms total.

See below for the technical answer.

 

[Old Garage-Bander]

Multimeters don't measure impedance and the impedance rating for a speaker is nominal, not instantaneous or necessarily exact.

The meter is reading the resistance of the wire in the voice coil.
Resistance is a DC measurement. Impedance is an AC measurement and depends on the frequency.

What that all means is that when you measure a speaker voice coil with a meter it won't give you the impedance.
But the good news is that it will be close enough to figure out what's what.

The 3 Ohm DC measurement equates to 4 Ohms impedance.
13 Ohms Resistance indicates a 16 Ohm speaker.

 

[Ray T]

Thank you Stumbo...and another question: if the cabinet claims to be 4 ohm, is it desirable to make that the actual resistance to match it with the amp ? Or if the amp output is say 4 ohm, and I match it with an 8 ohm cabinet, the worst that can happen is that it might be a bit less efficient (quieter ?) As long as the speaker cab impedance is equal to or greater than the amp's ...that's ok ? The back of the 135 Bassman amp just specifies that any speaker/ext. speaker must be a minimum of 4 ohms. So my plan is to make the cabinet 4 ohms...to match the amp's min. impedance.

 

[Oren Hudson]

For maximum effort, matching cab and head ohms is desirable.

 

[Ray T]

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 ?

 

[Wasnex]

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 ?]

Being as you have two 4 ohm and two 16 ohm speakers you don't want that configuration. That diagram is designed to use four speakers with the same impedance. The total load will be equal to the load of a single speaker. Four 16 ohm speakers will give a 16 ohm load, four 8 ohm speakers will give an 8 ohm load, four 4 ohm speakers will give a 4 ohm load.

Instead follow these directions:
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.

 

[Ray T]

Thank you wasnex, I followed your guide and the multimeter showed a shade over 4 ohms....it all works well and sounds great...much appreciated !