dumb question

[rickenmetal]

I know this has been talked already talked about a lot, but I am curious about it anyways.

I used to run 2 speakers to my bass amp, one 4 ohm and one 8 ohm, which would bring the total to 2.66 ohms, but my amp is rated at 4 ohms. I never played really loud and used the amp just for practice and recording. Obviously the problem in such a case would be that the output transformer would heat up and blow, but since I never had it really loud nothing happened. Is it risky if it's not up too loud?

oh, yeah, and the 8 ohm speaker was not as loud, which was perfect for the sound I was trying to achieve.

[jps]

Is your amp tube? If so, the slight mismatch of running a 2.66 Ω load on the 4 Ω tap should be okay.

[doctorwho]

I'm not the expert, but if you are running the speakers parallel, they ideally should have matched impedances, IIRC.

[rickenmetal]

Nope, solid-state, on the back 4 ohms next to the jack, in the manual something like "use only 4, 8 or 16 ohms, or risk damaging the amp and speaker".

I do not really think that matched impedances really matter, it's just that the lower impedance speaker will be much louder. In my case the 4 ohm speaker was twice as loud. I think the maximum wattage would also have to be calculated in a weird way.

I have run that 4 ohm speaker on my tube guitar amp through the 8 ohm output, it sounded funny sometimes, I think I re-tubed once after.

I guess in the future I'll simply run 2 amps if I want to use that speaker. I probably didn't screw it up because since it wasn't up very loud there wasn't enough load on the output transformer. The problem is that I wasn't sure of the impedance since I use an analog ohmmeter which isn't very accurate in the low setting, but I figured it out when I measured 8 ohm speakers as well, and then both in parallel.

I guess I could also run the two speakers in series, which would give 12 ohms, but I really don't feel like modifying my cabinet or building a series box right now.

[soundmasterg]

Most solid state amps don't have an output transformer, though some do, especially the early solid state designs from the 60's. With solid state, you get more volume out of the amp the closer you get to zero, but you run the risk of the output transistors blowing when you run a load lower than intended. Certainly at lower volumes it is better and easier on the amp than at higher volumes. With a tube amp, the match is what is important, though tube amps are more forgiving than solid state, and always have an output transformer. (unless it is an OTL amp, which are dangerous nowadays) Tube amps can almost always tolerate a mismatch one way or the other, so a 4 ohm or 16 ohm speaker on an 8 ohm tap for example. For best power and tone with both amps, it is best to match the speaker up to the output tap's impedance requirements.

Greg

[leftyguitars]

Golden rule - Tube can run under but not over, transistor can run over but not under. But really you should match correctly for optimum volume, sound and amp life.

[soundmasterg]

Running a tube over is fine, or bad, just as running a tube under is fine or bad. It depends on the application and the operating conditions. In most tube guitar amps, you are fine either way. In amps with greatly undersized output transformers, you may have a problem in some situations. Having said that, my brother ran his Silvertone 1484 for years mismatched from the 4 ohm tap into an 8 ohm load with no issues at all. It sounded more compressed and distorted and lower in volume, but it will do that when you mismatch. In some ways going under is worse because you are closer to a dead short and it is easier to get internal arcing inside the transformer between the plates. You always get the best power and tone running it at what it was made to run at. When running extreme voltages in a tube amp, such as 2000v for example, running under is more dangerous due to the arcing concerns.

Greg