Laney Cub 12 r - internal and external speaker together

[disilldo]

Hi,

 

I have a Laney Cub 12 r that I am thinking of modding to allow me to run both the internal speaker and an external 1x12 cab together but I want to make sure I am going about it the right way.

 

Basically the amp has an internal connection to the internal speaker that runs at 8ohms and is disconnected if the external speaker connection (8 - 16 ohm connection) is used. What I am thinking of doing is:

disconnecting the internal speaker connection

running a speaker cable from the external speaker connection to the internal speaker first

then continuing the run, in series, to another jack plug (this is where it gets fun...)

I am thinking of using a switched jack plug so that if there is nothing plugged in, the circuit should complete and the internal speaker would run, on its own, at 8ohms.

If I think plug in my 1x12 to the new jack plug it should run, in series, at 16ohms.

 

Does the theory make sense - would the switched jack plug work in this case, i.e. allow the internal speaker to run when there is nothing plugged into it, and then run in series when something is plugged in?

 

Thoughts are welcome - cheers.

[WRGKMC]

First off you should NOT screw with your speakers unless you know exactly what you're doing, which you obviously don't based on your post.

 

First you need to confirm form the manual what the heads maximum impedance range is. (Which I have done for you already - its 8~16 ohms only)

 

The reason the internal speaker cuts out when you plug in an extension is you cant run anything lower then 8 ohms and by connecting another speaker in parallel, you get 4 ohms which is too low for the head.

 

Next, if your external speaker is also 8 ohms then it's possible to run your internal and external speaker in series at 16 ohms. In series the impedance is additive - 8 + 8 = 16 (In parallel its subtractive 8 + 8 = 4 ohms)

 

Problem with series wiring speakers in series is the same as lights on a Christmas tree. When one bulb goes out they all go out. With two speakers in series you disconnect one and they both go out. You can wire the external speaker jack so it wires the internal and external speaker in series,

You could also make a series adaptor so you could plug two speakers into the back of the head to be in series or plug one speaker in and it will see 8 ohms. This might be the easiest for a beginner.

 

This is how you want the speakers to be wired up.

 

[ATTACH=JSON]{"data-align":"none","data-size":"full","title":"spkr_s.gif","data-attachmentid":32246262}[/ATTACH]

 

What you'd need to do the job well is some 1/4" jacks in a box. Sorry for the crummy drawing - I was never very good drawing or writing with a mouse but this at least gives your an idea of how two speakers can be connected with jacks to a single speaker output jack. Having the jacks wired in a small metal box will keep everything tidy and reliable. You could use a 3rd female jack and a separate cord to the amp if you want. Just be sure the cords all use speaker wire, not guitar cords. Guitar cords will fry from the inside out with higher speaker voltages so you want double stranded speaker wires all the way through.

 

[ATTACH=JSON]{"data-align":"none","data-size":"medium","data-attachmentid":32246263}[/ATTACH]

 

 

 

 

 

You also need to realize - each speaker is only going to get half the wattage of the total. If the amp is 15W then each speaker sees 7.5W for a total of 15W. Your amp may have more bass response using two speakers but its not going to be any louder then it was using a single speaker (Unless the speaker being added also has a higher SPL level) Typically you should get a couple of DB boost if you have matching speakers in the same box due to cabinet resonance. In separate cabs you will likely break even for loudness. The only thing you will get is the combination of tone between the two cabs which is wider then a single speaker and maybe a wider area of sound dispersal with two cones instead of one. Whether that's actually an improvement depends on the speakers and your own personal likes/dislikes.

[onelife]

You could (carefully) modify the wiring of your existing external speaker jack so that, instead of disconnecting the internal speaker, it connects the external speaker in series (and in phase) with the internal speaker.

 

 

btw, it seems you do know something about speakers, impedance and your amplifier - based on your post.

[disilldo]

Thanks for the replies folks.

 

Onelife - I would need to check but for some reason I thought the internal speaker connection on the amp is 8ohm only and only the external connection is 8 - 16 ohm - but thinking about that now, its a bit daft since they run off the same circuit.

 

WRGKMC - you're talking about a series box - I've considered that and its definitely a solution. The only modification I am talking about making to you drawing is for one of the jacks to be a switched jack, so that if there is no plug in there the current would pass through to the ground therefore completing the circuit. This would mean that the internal speaker would be "always on" and the external would be optional but in series. Logically in my head it works but honestly I don't even know if I want to take the risk of blowing my amp...

 

[WRGKMC]

^^ If you were going to rewire the amp itself then you'd need a switched jack to allow one speaker to be run.

 

With an external box , you could use a switched jack, but its pretty senseless. If you were only going to run one speaker, just unplug the box and plug the one speaker directly into the amp. Makes No sense in having a series box connected if you don't have two speakers to connect in series.

[onelife]

The amplifier already has a switching jack in it that disconnects the internal speaker - that circuit could be modified slightly to facilitate the series connection.

 

https://www.thetubestore.com/lib/thetubestore/schematics/Laney/Laney-Cub-12R-Schematic.pdf

[disilldo]

What you're saying is true except I wasn't originally suggesting a series box at all, I was thinking of a cable running to the internal speaker from the speaker out, then connecting this on to a switching jack (which would be attached to the back of the chassis) which would mean that if nothing was plugged in, the internal speaker would work, and if something was plugged in - both would work.

 

Effectively a series box without the box.

 

Sounds like, in theory, my idea would work though right?