Back in the days before we had direct recording outs and stereo effects loops, if a guitarist wanted to take his favorite amp’s tone and make it LOUDER, it was done by something called amp slaving. This is accomplished by taking the signal of one amp and boosting it via the power section of another amp. This method allow you to use the primary amp as you tone-generator, and the other amps to provide additional volume for larger venues. Using a more modern amp as the slave, you could take the out of the primary amp into the effects return of the secondary amp and bypass the preamp section of the slaved amp altogether. This maintains the “pure” tone of the primary amp, which is more often than not the goal of amp-slaving, and it’s a cool way of essentially combining the preamp section of one amp with the power section of another. For example, love your Marshall’s preamp response but wish you could hear it through a KT88 or 6L6 power amp instead?
Either way, you’re able to boost your volume appropriately when running an amp with more wattage as the slave. And you can use slaving for simple volume boosting or as a way of inserting insert effects into separate chains. These setups can be as basic or elaborate as you like. But first, there are a couple of things you have to consider as precautionary measures before ever attempting it.
Please note – and this is very important – if you’re considering doing this with an old-school amp without any type of direct out or effects loop, you can’t just bypass the amp’s speaker output without potentially seriously damaging – if not destroying – your amp’s output transformer! It needs to ‘see’ a speaker load, so you’ll need to employ a cab, the internal speaker of a combo, or better yet a load resistor box of some sort. There are many types on the market at various price points, like the THD Hotplate, the Radial JDX or the Palmer Speaker Simulator that will mimic a speaker load (at the correct ohmage) and spare your amp’s output transformer any serious damage. These units are also beneficial for silent recording, but that’s a topic all its own. Once you have that vital safety measure covered however, particularly if you’re using a tube amp – you get to experience one of the other main benefits of slaving: you can CRANK the tone-shaping amp’s power section to get maximum power stage distortion, the kind only a dimed amp can produce, considered by some to be the “Holy Grail” of rock guitar tone.
Slaving also allows more flexibility in that scenario. One could hypothetically have their 100-watt Marshall cranked with a speaker load, and slave, say, a 50-watt power amp to achieve power distortion and yet still maintain a manageable volume in a smaller venue too. That having been said, it’s usually about “MOAR VOLUME” and the ability to run more cabinets in various stage locations as well. After all, stories of EVH slaving his own “grail” Marshall into H&H power amps are not just legendary, they’re still in his touring rig to this day, for example powering cabs used as monitors for his brother Al, and for side fills if needed, etc. And since the power amps are only reproducing the “master” amp’s tone feed, the sound is consistent anywhere on stage.
If you do have an FX send and run stereo effects after the tone-shaping stage, you can have wet/dry/wet cabinet setups for sounds that are positively massive. This can be accomplished with a stereo effect (assuming it has a ‘dry’ output), a single stereo power amp and the cabinets. In this scenario the dry signal would go back to the tone shaping amp’s FX return and drive the center speaker, and the left and right outputs of the effect would go to each respective channel on the stereo power amp to provide the full effect, and drive more cabinets. The effect, particularly with time-based effects like chorusing, flanging and delays, can be quite dramatic, but pretty much everything sounds huge. If you have the capability and you feel like schlepping the extra gear around, it’s quite cool and useful, especially if your favorite amp can’t cut it by itself, and if you happen to have something else with a little more horsepower, so to speak.