Help with wiring and measuring resistance on 2x12 cab

surfguy13

Hi Guys

I have this home-made cab that I bought ages ago - I bought it as I wanted the lovely old Baker speakers in it. Decided to leave the drivers in the cab as it is extremely well made with ply even though the tolex job is rubbish.

It has 3 output jacks on the rear of the cab.....a L and R on either side of a centre jack which the guy said was 15 ohms.  So I rightly/wrongly assumed the speakers were 8 ohms a piece and the jacks were wired so that you get the left speaker on its own in the L jack and right on its own in the R.  Individually I assumed the load was 8 ohms.  The speakers do have 8 chalked on the rear of the cone so I assume they are 8 ohms....there's nothing about resistance on the labels.

However,  I thought I really ought to confirm that the speakers are 8 ohms and also that the wiring to the 3 jacks is as it should be.  So......can I check resistance on the speakers to confirm they're 8?  Can this be done with a meter?

Also, from the photos below, does the wiring look correct given the layout of the jacks?  The pink wire goes to the positive terminal on the speakers and the green wire to negative.  The pos/neg from each speaker go to the two outer jacks.

Cheers

Guy

http://imageshack.us/a/img841/142/kr24.jpg

http://imageshack.us/a/img850/2458/f3bk.jpg

http://imageshack.us/a/img46/6269/y4x2.jpg

http://imageshack.us/a/img34/7594/r5dw.JPG

http://imageshack.us/a/img844/2496/vixy.jpg

http://imageshack.us/a/img854/1245/6rfo.jpg

ICBM

It's very likely to be correct given the apparently professional standard of work

But I don't like the way he's wired the switching for the mono mode using all four jack switch contacts in the 'stereo' jacks - that's very bad practice and could lead to an open circuit on the amp if any one of those contacts goes bad, which is a major risk to the output valves and transformer. Any switching in the speaker circuit is not a great idea, and jack contacts are notorious for going faulty with a bit of age, corrosion or tugged cables.

I would be inclined to use the cabinet as a 4-ohm one with two speaker cables from the amp into each of the individual speaker inputs, if your amp has a 4-ohm setting.

surfguy13

Many thanks ICBM.....again!!    Really appreciate the advice and I will act on it.  It would be a nightmare to risk output valves or even a transformer, particularly when it's highly unlikely I'd ever use the two outer jacks giving me a single speaker at 8 ohms.

As I don't have a 4 ohm out on any of my amps, they're all old, I think it makes more sense just to wire it to 16 ohms via a single jack and leave it at that.  That was my intention when I bought the cab, to remove the speakers, wire them to 16 ohms and load them into another cab. 

Would it be OK to use one of the existing stereo jack sockets or should I install a mono socket?

Cheers

Guy

ICBM

I would do exactly that - rewire it to 16 ohms mono - but keep the two outer jacks, just parallel them. This gives you more options for daisy-chaining and backup if one jack breaks. You can re-use those jacks, they'll be fine.

surfguy13

Thanks ICBM.....a plan!  I will just leave the two outer jacks in situ and wire the speakers in series to the centre jack.  Much appreciated.......

ICBM

I meant the opposite - remove middle jack, connect outer jacks in parallel, and connect to both speakers wired in series.

That gives you a 16-ohm cab, but which you can then daisy-chain to another one if you have an amp with only one 8-ohm jack.

surfguy13

ICBM said:

I meant the opposite - remove middle jack, connect outer jacks in parallel, and connect to both speakers wired in series.

That gives you a 16-ohm cab, but which you can then daisy-chain to another one if you have an amp with only one 8-ohm jack.

Oh god!  Thanks for that!!!  I suspected I might have misunderstood.  I actually wired it in series this morning using one of the outer jacks and disabled/disconnected the other two jacks. So now it is a standard 16 ohm cab using a single output.

I'm not sure I quite understand how to connect the outer jacks in parallel. Wiring the speakers in series is a doddle, even I can do that, but not quite sure how to wire the jacks in parallel and then the speakers in series.  Could I possibly ask you to explain how I wire the jacks in parallel?  I'll then re-do it all and, as you say, I'd then have the extra option of running another amp with a single 8 ohm jack.

ICBM

Just connect the two jacks together, tip to tip and sleeve to sleeve (and then to the speakers as normal), so it doesn't matter which jack is in use. That way you have a backup if one breaks, as well as being able to use the second one as a 'through' connection so the two cabs are then in parallel.

surfguy13

ICBM said:

Just connect the two jacks together, tip to tip and sleeve to sleeve (and then to the speakers as normal), so it doesn't matter which jack is in use. That way you have a backup if one breaks, as well as being able to use the second one as a 'through' connection so the two cabs are then in parallel.

Fantastic, thanks so much for that.  I shall get onto it in the morning......