One of the cabs I currently use is a EA VL-210.(500w@8ohms) Is there any simple way of changing the impedance to 4ohms? Some sort of "dummy load"? I see accugroove has their "accuswitch" on their cabs. Is this something I can add to my EA? It would be great to be able to switch back and forth for different rigs.

accugrove's impedance selector is top secret technology. there really isnt a way to make that a 4 ohm cabinet. Your 8 ohm 210 is either two 16 ohm tens wired in parallel, or two 4 ohm speaker wired in series.

Well, there are ways. You could wire up an 8 ohm Weber MASS inside the cab with a switch, and wire it to give you either 8 ohms (just the speakers) or 4 ohms (speakers and MASS) total. But using the MASS would mean that the rig wouldn't be any louder at 4 ohms than at 8 ohms, since the MASS would be getting half the power.

Exactly. The additional power would simply be dissipated as heat from the resistor, not directed to the speakers. So, yes you can get twice the water from the pipe, but you'll spill half on the ground. I do know a few guitarists who have rigged devices like this because they preferred the way that their tube amp sounded with a particular load and were not really looking for additional volume, but rather looking to change the behavior of the tubes and thus the tone. With a SS bass amp, it has no value.

HMMM.... Well, I guess I'll just be running at 2.6Ohms?? EA VL-210(500w/8ohms) + Ampeg BXT210M(400w/4ohms) In this case, are both cabs getting equal power, or is one of them sucking up more juice? DB750(mono) 975@2, 750@4

Well, you certainly could do it with a transformer, but the downside is that the transformer would have an effect on the sound and some amount of power would be wasted in the transformer. Unlike the Weber mass solution, a transformer would result in a cab getting louder at 4 ohms.

For the sake of argument, the 4 ohm Ampeg cabinet would receive twice as much power as the 8 ohm EA. If the EA is 3 dB more efficient, they'd produce the same "volume". Since the cabinets are not identical, only a sound check will let you know what they're going to sound like together.

The Ampeg is 99db the EA I believe is 101db (both 1w@1m) Would this then be "pretty close"? I get so confused with all this stuff, but I'm very interested. Is there a post or a book, or page, that explains reletively simply about all kinds of speaker lingo and amplifier lingo?

Kudos on that thread!! Thanks for laying it out there, I have a better basic understanding of the terms and such now.

If I'm doing the math right, and assuming the amp puts out 800 watts into 2.7 ohms... SPL = Sensitivity dB + 10 log (Power in - 1) dB For the Ampeg 99 dB + 10 log 532 = 126.3 dB For the EA 101 dB + 10 log 266 = 125.2 dB So yes, they'd be very, very close. If I'm doing the math wrong, PLEASE correct me!!! I've offered this up in two threads today and would need to fix it...