Connect a combo amp to a 4x12 cab.. Can it be done?

[eschertron]

If you're going to do it that way, you might want to recess the plate back into the amp a bit so that if the amp tips on it's back or gets slammed into anything you don't break the plug. (Not trying to be a critical a$$ eschertron.).

That's a good critique! I hadn't thought about protecting the jack with the plug inserted, but that's a good point. I will probably re-engineer the bracket to allow the recessed features.

[Paleo Pete]

I did this with a Gorilla practice amp years ago, and as enzo has already noted, it will work fine with any number of speakers. I run my Champ through a Kustom 2x12 cab all the time. I simply made up another speaker cable. I've run it a lot through a 1x12 cabinet too, same cable. Don't have a 4x12 but I have no doubt that would work, and it will drive it.

For the Gorilla, and the reason I posted here, I chased down the speaker wires inside the amp and added a shut off jack. That's a type of jack that does nothing normally, but if you plug in a cable it shuts off the circuit running to the speakers, and only uses the jack you plugged in. (I'm not sure what the proper name for it is) It worked great, with nothing plugged in it used the internal speaker as usual, when I plugged in an external cabinet it shut off the internal speaker and only used the external one.

Then I mounted the shut off jack on the back of the amp chassis, and later added a footswitch for the gain channel, also mounted on the chassis. That took a bit of ugly scratching on the back of the chassis to remind me which was which...but it worked great. The wiring is fairly simple, I can't remember it without having the jack in my hand, (it's been 20 years) but I'd be willing to bet these guys could easily tell you how to wire it so you can simply plug in the external cabinet and not worry about it. I'm sure they know what jack I'm referring to, it has a blade connection that is pushed away from one contact when plugged in, breaking the original circuit and using only the one plugged in. I used the same type for the footswitch so unplugged it still used the built in push button.

[Logan12197]

1 month later and the project is complete

I did the amp to cab a few weeks ago and just added another female TRS to the speaker in my amp to use just that speaker when I need my amp to be portable.

I think throughout the process I made just about every mistake I could have, but it was a learning experience and I would feel comfortable doing it again without screwing anything up. In the end, my connections are very solid and it sounds great. I borrowed my friends speaker cable to test the amp to cab, and don't have a speaker cable again yet (making my own once the wire I ordered comes in next week), but I tested the in-amp speaker with a guitar cable and it sounded good.


The finished project
http://i.imgur.com/m8qhl9q.jpg

http://i.imgur.com/LzmNlBu.jpg

Amp output
http://i.imgur.com/mv1xSu9.jpg

Speaker input
http://i.imgur.com/UVt8VIj.jpg


Thanks to everyone for your opinions and advice = )

[eschertron]

I tested the in-amp speaker with a guitar cable and it sounded good.

Just we're all on the same page: you KNOW that a guitar cable is not designed for more than a few milliwatts, right? fine for "testing" in limited amounts only. Make sure that when you make your speaker cable you have a nice heavy gauge of wire (16ga or 14ga is common for this application; road-worthyness is also a consideration). Not only does that big wire reduce heat-inflicted I-squared-R losses, it lessens the chance that your keyboard player won't sever the cable (and make your OT self-destruct) when he rolls his leslie cabs across it setting up for the next gig!

[Logan12197]

Just we're all on the same page: you KNOW that a guitar cable is not designed for more than a few milliwatts, right? fine for "testing" in limited amounts only. Make sure that when you make your speaker cable you have a nice heavy gauge of wire (16ga or 14ga is common for this application; road-worthyness is also a consideration). Not only does that big wire reduce heat-inflicted I-squared-R losses, it lessens the chance that your keyboard player won't sever the cable (and make your OT self-destruct) when he rolls his leslie cabs across it setting up for the next gig!

Yeah, I just couldn't wait a week to test the connection .. the "test" was about 5 seconds

[eschertron]

Yeah, I just couldn't wait a week to test the connection .. the "test" was about 5 seconds

Gotcha

[Steve A.]

Guitar cables typically have a center conductor between 18GA and 24GA. I was checking the AWG charts and a cable with an 18GA center conductor should be able to handle 2.3 amps at 120 volts which works out to 276 watts. Following the same formula a cable with a 20GA center conductor should be able to handle 180 watts and with a 22GA center conductor it should be able to handle 110 watts.

Here is a link to the chart I used for those computations with me assuming that current ratings were determined using 120VAC (let me know if that is not correct!)

American Wire Gauge (AWG) Cable Conductor Size Chart / Table

I've used 6' guitar cables with 18-20GA center conductors to connect speakers for 40+ years with no noticeable problems. Of course this was not with a 100W Marshall head at full volume. A speaker cable is preferable, having two balanced conductors with more insulation between them and much more durable to boot. Because of that you would not want to use a guitar cable on stage- you are asking for trouble.

But in a more protected environment I have never had problems using a 6' guitar cable to connect a speaker. On the other hand when I used a speaker cable instead of a guitar cable I was tearing out my hair for an hour or two trying to figure out why the amp I was rewiring got so noisy (I thought I had inadvertently screwed up something on the pcb.)

Helpful hint: put colored heat shrink tubing on speaker cable plugs so you don't use them as a guitar cord by mistake. You also might want to color code your speaker cables with heat shrink tubing if you are running more than one cab.

Steve Ahola

P.S. I have bought a few really nice 6' speaker cables and use them- at least when I can find them.

[Steve Conner]

Well, the voltage going to the speaker isn't 120V. It depends on the power rating of the amp and the speaker impedance. For example, 100W into 8 ohms corresponds to 28V RMS at 3 amps, a little too much for an 18AWG wire.

The conclusion is still the same, you can get away with using a guitar cord as a speaker cable for test purposes at limited power, but it's better to use a real speaker cable. I like to make mine out of bright orange line cord with Neutrik ends.

I used to jam with a bassist who would turn up with a "guitar cord" clearly labelled "Van Damme Twinax Speaker Cable". It was completely unshielded and did hum somewhat. He said it didn't matter as it was only bass.

[Jazz P Bass]

Yeah a guitar cable will 'work'.
At some point in the power curve it will simply get warm, progressing to hot.

[g1]

It's not so much the center conductor I worry about (using shielded cable as speaker cable), it's the shield. Not too many instrument cables have that nice braided shield (that is tougher to work with). More likely it's made of really fine non-braided wire that is very often all broken up in older cables. It's still in contact where it's broken, so it shields and carries guitar signals but you don't want to be running any current through it.

[Steve A.]

Well, the voltage going to the speaker isn't 120V.

My assumption was that the AWG chart was based on the common line voltage of 120V. So is it the current or the wattage that is critical in a speaker lead? (I was thinking wattage but you are suggesting that it is current.)

It's not so much the center conductor I worry about (using shielded cable as speaker cable), it's the shield. Not too many instrument cables have that nice braided shield (that is tougher to work with). More likely it's made of really fine non-braided wire that is very often all broken up in older cables. It's still in contact where it's broken, so it shields and carries guitar signals but you don't want to be running any current through it.

Good point. I used to make my own cables with a heavy duty shielded cable and those were what I usually used for speaker cables. When I replace broken plugs on modern cables I often find that the shield is not braided. I am always running my chair over guitar cables and while they still work for guitar I can see that they might not be very good for a speaker lead.

If the shield were to break then you would have an open circuit which tube amps don't like; if it were to short out it would be a short circuit which ss amps don't like. Probably best not to tempt fate...

Thanks!

Steve

[Tom Phillips]

My assumption was that the AWG chart was based on the common line voltage of 120V. So is it the current or the wattage that is critical in a speaker lead? (I was thinking wattage but you are suggesting that it is current.)...

Current flow regardless of voltage determines the wire gage required. It is the insulation around the wire that is influenced by the voltage potential between the wire and other things around it. Or, for distribution lines up on the power poles, the air space between the wire and other things.