Teach me about ext cabs

[antichef]

Strat, since you've got all this knowledgeable attention on the subject of combos and cabinets, would you consider it a hijack if I asked about the best practices of installing a circuit breaking external speaker jack into a combo that *doesn't* have one? It seems straightforward enough to me, but so do a lot of other things that I shouldn't be messing with.

 

[WhiteStrat]

Plugging a second cab into the external jack puts the cab in parallel, not series, but it does halve the impedance, as you say, provided the ext cab is the same impedance as the internal speaker. Connecting spkrs in series doubles the impedance, parallel halves it (again, assuming the same impedance for both).

I have a Blues Jr, and I don't remember it having an ext spkr jack. IIRC the internal spkr is hard wired to the amp with no jacks at all.

I had the back cover off, sampling some replacement tubes, and I noticed a little junction box with 2 1/4" jacks -- one was empty and labeled footswitch, the other went to the speaker. I was surprised it wasn't hardwired, esp since it's up behind the back cover (a half cover really), but now that I know it's there, I could reach in there and switch it without removing the cover.

So I guess it's not that there's an ext speaker jack per se, but that the internal speaker connects with a jack instead of being hardwired--so I can at least change it on the fly.

 

[ggunn]

I had the back cover off, sampling some replacement tubes, and I noticed a little junction box with 2 1/4" jacks -- one was empty and labeled footswitch, the other went to the speaker. I was surprised it wasn't hardwired, esp since it's up behind the back cover (a half cover really), but now that I know it's there, I could reach in there and switch it without removing the cover.

So I guess it's not that there's an ext speaker jack per se, but that the internal speaker connects with a jack instead of being hardwired--so I can at least change it on the fly.

Good info; I'll check it out.

 

[Guitarer]

Yeah I was confused about Guitarer's comment too. I assume that by "daisy chaining," he meant plugging the 2nd cab into the 1st cab, but then I remembered that this was a combo amp we were talking about.

Right, that's what I meant. Sorry. I forgot we were talking combo here...However, since the built in speakers plug in with a 1/4 jack, you could hook up a 2nd cabinet in place of it, then hook up the original speakers to the output of the new cabinet (What I meant by daisy chaining), therefore putting it in series.

Something that may or may not need to be noted, but should: If you have 2 or 3 speaker outputs for different ohm ratings (you said you had an impedance switch, so this probably can be ignored), you can't plug in a cabinet of the noted impedance into more then one of those outs, as that will result in improper load. Dunno if that made sense...

 

[eyema_believer]

However, since the built in speakers plug in with a 1/4 jack, you could hook up a 2nd cabinet in place of it, then hook up the original speakers to the output of the new cabinet (What I meant by daisy chaining), therefore putting it in series.

This would depend on the way the jacks are wired, but most cabinets would be wired in such a way that what is described here would result in a parallel wiring scheme, and not series.

~Shawn

 

[ggunn]

Right, that's what I meant. Sorry. I forgot we were talking combo here...However, since the built in speakers plug in with a 1/4 jack, you could hook up a 2nd cabinet in place of it, then hook up the original speakers to the output of the new cabinet (What I meant by daisy chaining), therefore putting it in series.

Nope. Parallel.

 

[famous beagle]

Nope. Parallel.

Read it again ggunn ... I think you must have misread. This would be series.

It would be going into one cabinet, then out of that cabinet into another speaker (assuming I read Guitarer's post correctly).

 

[eyema_believer]

Read it again ggunn ... I think you must have misread. This would be series.

It would be going into one cabinet, then out of that cabinet into another speaker (assuming I read Guitarer's post correctly).

In order for a setup like this to be a series connection, it would have to be wired like this: Lets assume there is only one speaker in each cab.... from the plus side of the amp to the plus side of the first speaker in the first cab, then from the negative side of the first speaker in the first cab to the positive side of the second speaker in the second cab, then from the negative side of the second speaker in the second cab to the negative side of the amp. I'm thinking this would be impossible unless you had something different than a 1/4" connection, like maybe some banana clips or something else. In any situation where both of the positives from both speakers is connected to the positive side of the amp, and both negatives from both speakers is connected to the negative side of the amps output, you have a parallel connection.

 

[iqi616]

I had the back cover off, sampling some replacement tubes, and I noticed a little junction box with 2 1/4" jacks -- one was empty and labeled footswitch, the other went to the speaker. I was surprised it wasn't hardwired, esp since it's up behind the back cover (a half cover really), but now that I know it's there, I could reach in there and switch it without removing the cover.

So I guess it's not that there's an ext speaker jack per se, but that the internal speaker connects with a jack instead of being hardwired--so I can at least change it on the fly.

Make sure that junction box is appropriately supported - you won't want to weaken its connections by repeated plugging/unplugging to an assembly that might only have been intended to be used during manufacture and servicing.

 

[WhiteStrat]

Make sure that junction box is appropriately supported - you won't want to weaken its connections by repeated plugging/unplugging to an assembly that might only have been intended to be used during manufacture and servicing.

Good point. Thanks. I'll watch out for that.

 

[ggunn]

Read it again ggunn ... I think you must have misread. This would be series.

It would be going into one cabinet, then out of that cabinet into another speaker (assuming I read Guitarer's post correctly).

If a cab has two 1/4 inch jacks, they are wired in parallel, or at least are in every cab I've ever seen. Two spkr cables from the amp to the cabs or one cable amp to cab and another cab to cab, it makes no diff; it's all parallel. Am I missing something?

 

[ggunn]

In order for a setup like this to be a series connection, it would have to be wired like this: Lets assume there is only one speaker in each cab.... from the plus side of the amp to the plus side of the first speaker in the first cab, then from the negative side of the first speaker in the first cab to the positive side of the second speaker in the second cab, then from the negative side of the second speaker in the second cab to the negative side of the amp. I'm thinking this would be impossible unless you had something different than a 1/4" connection, like maybe some banana clips or something else. In any situation where both of the positives from both speakers is connected to the positive side of the amp, and both negatives from both speakers is connected to the negative side of the amps output, you have a parallel connection.

You could wire up series jacks in a cab if they were the type that are shorted when disconnected, but I don't know of any cabs that are wired that way "stock".

 

[Guitarer]

In order for a setup like this to be a series connection, it would have to be wired like this: Lets assume there is only one speaker in each cab.... from the plus side of the amp to the plus side of the first speaker in the first cab, then from the negative side of the first speaker in the first cab to the positive side of the second speaker in the second cab, then from the negative side of the second speaker in the second cab to the negative side of the amp. I'm thinking this would be impossible unless you had something different than a 1/4" connection, like maybe some banana clips or something else. In any situation where both of the positives from both speakers is connected to the positive side of the amp, and both negatives from both speakers is connected to the negative side of the amps output, you have a parallel connection.

Thinking about it...you've gotta be right...The only way it could be series is if the circuit of the cabinet was wired with the output jack in series. Then plugging in a cable would be the only way to complete the circuit...Unless, as ggun stated, it shorted when disconnected.

I really should think things through more My bad.

 

[WhiteStrat]

Make sure that junction box is appropriately supported - you won't want to weaken its connections by repeated plugging/unplugging to an assembly that might only have been intended to be used during manufacture and servicing.

Good news. I checked tonight & the junction box with the two 1/4" jacks (one for speaker & one for footswitch) is mounted real solidly. Reaching up in there and unplugging the internal speaker and plugging in an external cab won't be a problem.