Members datru Posted February 2, 2015 Members Share Posted February 2, 2015 I have a tube combo amp with a 12" speaker. I might get another very small amp head, and rather than getting a separate cab, I'd like to be able to just use the speaker in the combo and switch between either head. Just in my home, not for gigging. Is this advisable? Any drawbacks? I thought I could use something like this to switch with with: http://www.guitarcenter.com/DOD-VAC270-A-B-Box-150109-i1124222.gc I know that running the amp without a speaker is dangerous, but I can't imagine doing that in this situation. Thanks. Link to comment Share on other sites More sharing options...
Members onelife Posted February 2, 2015 Members Share Posted February 2, 2015 I wouldn't advise it unless you are extremely careful and make sure you put your amps on standby BEFORE switching and leave the amp that is not in use on standby. Most of the power that goes to a speaker is in the form of electric current rather than voltage. A speaker is an inductive load and when current flows through an inductor it wants to keep going even after the power supply is removed. Opening the circuit effectively increases the resistance and when the current flowing through the speaker reacts to the high resistance it produces quite a high voltage that could cause damage to one or both of your amplifiers. Whatever you use for a switch must be capable of handling current in the range of three to five Amps and you may want to consider a more elaborate system that includes a dummy load for the amplifier that is not driving the speaker. Link to comment Share on other sites More sharing options...
Members wasgtrjones Posted February 2, 2015 Members Share Posted February 2, 2015 Radial 'Cabbone' is designed/built to specifically do exactly what you want. The A/B box is traditionally for line-level inputs... I would find a 'cabbone', or find an amp tech you really trust and ask if the A/B box would work. I'm betting the answer is no, but don't have the credentials/science to back that up. Link to comment Share on other sites More sharing options...
Members WRGKMC Posted February 2, 2015 Members Share Posted February 2, 2015 They make switchers but I think they are for switching two different cabs with one head, not two heads with one cab. Switching inputs on an amp isn't the same as switching the outputs. You're dealing with much higher current/voltages and there's a much greater chance of having something arch out and blow the head. If these are SS amps then feeding one with no load shouldn't be a problem in most cases**** It will depend on the head check the user manual or contact the manufacturer. Most of will advise you to turn the heads off and wait at least a minute for the power caps to die down before switching the speaker leads. Tube amps on the other hand should "always" be run with a load or you can wind up cooking the power tubes. It takes time for them to blow out but it will happen with most heads. Unplugging speaker leads with a head running may not give you a problem for a long time, and you think its fine to do things that way. Then the power capacitors get weak over time and are unable to smooth spikes like they did when they are new and you spike it one to many times and you see smoke. This is why I'm reluctant to advise anyone of switching cabs with the amps running. It may work find for awhile but all it takes is a cap not filtering as good as it used to and that big jump in current going from a load to open and its goodbye Bessie. I believe there may be some switches that will put a resistive dummy load on the head when you switch from cab to no load, but I honestly haven't investigated it much because I'd never do that with my own gear. It would make sense to use a high wattage ceramic resistor as a load substitute when switching, and would be an absolute necessity to maintain a load when using a Tube head. If you shut the head off when switching, then there shouldn't be any problems at all. You can even use a switch with one head and 2 different cabs, (which is what I believe the radial is designed for) I just don't like the idea of that instant spike when those speakers come up to full power. Coils resist voltage changes and it is a stress on the head. You should also realize Radial isn't going to guarantee their product won't cause a problem, especially if you use it for the wrong application. They're in the business of selling a product, not advising you on what you cant use it on. These are the main reasons why I just don't think its wise at all risking of hot head swaps like that. I did amp repairs full time as a tech and most major repairs come from taxing the power amp Power it down and just switch them and you'll be fine. The switch simply makes it easier then trying to find cables on a dark stage. You could even make one for a few bucks using a simple DPDT switch and a couple of jacks in a box. The box doesn't have to be shielded even. In fact it might be better if it was plastic so you don't connect the two heads grounds and wind up with a ground loop between the two heads. Link to comment Share on other sites More sharing options...
Members datru Posted February 2, 2015 Author Members Share Posted February 2, 2015 Great. Thanks to all. Link to comment Share on other sites More sharing options...
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