Re: somebody help me out with "ohms"
Good posts. I've heard the "internet facts" about running a tube amp set at 8 ohms into a 4 ohm load being safer than running it into a 16 ohm load. I never heard anything like that in any of my electronics classes. It's usually safer to run into a larger load, within limits. You can set your tube amp at 4 or 8 ohms and run it into a 16 ohm cabinet. The thinking is that you will be able to run your amp harder but it won't be able to get to full volume. The truth is that it will still be loud, you won't notice that much of a difference. Running into a lower load than the setting on the amp, as Noth said, will put more stress on the components, in a tube amp, it can possibly damage the output tranny or the power tubes. In reality, this would most likely take running it this way for quite a while, still, it's not worth doing, you can't increase the amps output this way and it's not going to sound better.
With SS amps, you can pretty much take the load to infinity, meaning nothing plugged in which creates an infinite resistance, and it will be safe. Or use a large load to decrease the output, for example, use a 32 ohm speaker on a SS amp that is rated at 100W at 4 ohms. This would give you roughly 12.5W on the 32 ohm speaker. The majority of SS works this way.
With tubes, leaving the output open is worse than a load that is slightly lower than the rating. The power tubes are still pumping out full power into the output tranny which can't dump it anywhere. Those of you who have forgotten to plug in your cab and tried playing the amp may have noticed that the transformer starts "singing", that's from the power tubes dumping all the power into the transformer, it makes the windings start vibrating. It usually doesn't hurt it if you catch it quickly, if you keep trying to play without a load the singing will turn into smoke, then a new transformer and power tubes (and lighten your wallet). This is probably where the idea that it is safer to run lower loads that the amp is set for than higher loads. Don't buy it. In fact, none of these gives you enough benefit (if any) to bother with, so play it safe and use the correct settings, unless you have enough money to experiment and buy new equipment or pay repair bills. Remember, EVH has people lined up trying to give him amps, he doesn't care if they smoke. If something gives him what he perceives to be a 2% improvement, he can afford to do it, it doesn't make sense to the rest of us.
FWIW, on old 100W Marshalls and similar, you can pull 2 of the tubes to lower the wattage, just pull either the 2 inside tubes or the 2 outside tubes, don't pull the pair from one side or the other. Also, when you do this, you have changed the impedance of the output circuit, so you need to set the impedance on the amp to half of the cabinet's impedance. So, for an 8 ohm cab, set the amp to 4 ohms.
