I have a new Ashdown head, and am planning on converting an old combination amp that I have can use it as a cabinet as well. Here's how you do it.
You start by purchasing one of these Marshall style jack switches for about $6 on Ebay.
This switch will be installed between the amp in the combo amp and the speaker. In my case, I've drilled/routed a hole in the side of the cabinet to install a jack plate, but you can also make a little bracket to install it. However you wish to mount the switch will be fine. I choose the side up high, since I didn't want the wires hang down in an open back cabinet, and it will make for easy access.
In terms of wiring, it's very simple.
Looking at the above photo - the tabs on the left side (where my thumb is in the photo) are the wires that go to the speaker. The wires on the right side go to the amp in the combo amp. The two tabs nearest the front where the cable plugs in are ground (-) and the two tabs at the back of the switch where the cable tip will end up are hot (+).
Once installed, you can plug a speaker cable from the head into the switch. The cable will break the contacts, isolating the speaker and running signal to it from the cable. Pull the cable out from the switch, and you can then use the combo amp as you normally would.
So - you now have a hybrid combo amp/cabinet.
(You will probably have to solder extensions onto the wires from the amp to the switch, unless you get really lucky. And best to use a good 16 gauge speaker wire.)
1. It's possible that you may be able to unmount the amp unit from the speaker case and drill a hole in it to mount the switch where your other inputs are on the back of the amp. A word of caution though, be very careful when you open any amp. Even when not plugged in, some of the components actually store enough charge to permanently stop your heart - which is not an exaggeration. So I don't recommend this unless you are fairly comfortable working around these components.
2. The Ohm load of the speaker should match the Ohm output of your head. Speakers are usually either 4, 8 or 16ohm. In theory, you can go one step safely for most amps. So an 8ohm head could be safely used with all three speakers. But it's still best if you can match the speaker load. Keep this in mind if you are looking for an old amp to modify. Some of the new solid state heads allow multiple loads, so you can use any. But tube heads tend to be a bit more particular. From what I gather, if the speaker load is too high (a 16ohm speaker with an 8ohm head) the tubes will tend to run hot and may potentially be damaged. If the speaker load is too low (a 4ohm cabinet with an 8ohm head) it can put strain on the transformer - which if it fries may take other components with it. So always best to match the speaker load whenever possible.
If the amp has 2 speakers of either 4ohm or 16ohm, chances are that it will be an 8ohm total load. If it has 2 speakers of 8ohms, then it will be either a 4ohm or 16ohm load, depending on how it is wired. If in doubt, check with a meter.
3. Never run an amp or head without a speaker load. For the reason stated above, no load on an amp can cause extensive damage to your amp. When using the modified cabinet, I recommend not plugging it in as a safety from someone accidentally flipping the switch on with the cable plugged into the jack.
4. It goes without saying, that this will totally void the warranty on a new amp.
5. Obviously, you don't want to run a 100 watt tube head full blast through a 60 watt speaker. But chances are you are not going to be cranking a 100 watt head in your bedroom for long without the cops showing up at the door.
This makes a great little tool for a workspace. If you want to test a guitar, you can plug it into the combo amp as normal. You want to test a head, you can also plug it in - so very versatile and takes up little space.
(Should also mention that guitar cables and speaker cables are a different species. Both the positive and negative wires in a speaker cable tend to be heavier than the wires in a guitar cable. There is a lot more current passing through them between the amp and the speaker than there is between the guitar and the amp. So make sure you always use a proper speaker cable to run from the head to the cabinet.)
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