Guitar Amp Output Wattage Question

[JRod4928]

In a real guitar amp like say a 5150 III, the power section is always running 100%. So when running the FM3 into the FX Return, there is no volume control except for the signal strength coming from the FM3.

So my question is, if the amp is always running at full wattage, how do we know we're in danger of blowing a speaker (like a low wattage Greenback)?

Is it as simple as: don't use an amp more than 2x the lowest speaker rating (in a 2x12) at that speakers impedance?

 

[Budda]

You'd hear the speaker start to distort, would you not?

 

[Repartee41]

Turning down the volume turns down the wattage. Volume is created using power, so less volume = less power.

Volume controls aren't linear, I'm sure you could figure out the math behind it, for example a log10 pot at noon = x% wattage, but I'm not experienced enough to answer that for you. However, if you turn down the volume output of the FM3, you'll reduce the output wattage from the 5150, so your speakers won't distort/blow up. Use your ears, speakers usually give lots of warning in the form of distortion.

There was an article somewhere that said a specific loadbox could accept amps above its rating if you turned down the master, which is essentially want's asked here, I'll update if I can find the source.
Edit- here's the closest thing I could find. Scroll to the bottom to find "High gain amps such as Marshall or clones are hard on attenuators. Unfortunately, an amp’s volume dial is not a good indicator of how much power is being produced. Many amps can reach full power at 3 or 4 on the volume dial, so it is a good suggestion to get an attenuator with 4x the power handling. For example, if you have a 50w high gain amp, you should opt for the MASS 200. If you have a 100w amp, you should still go with the MASS 200 but you should not crank the amp."

 

[JRod4928]

You'd hear the speaker start to distort, would you not?

Not in my case. I have a Greenback (25W) paired with a Alnico Gold (50W). The sensitivity of the Greenback is 97 db vs. 100 for Gold. So if we had volume high enough that the greenback starts to get damaged, it probably wouldn't be loud enough to overcome the volume of the Gold.... plus, you'd likely be playing at high volume with a band, making it even more difficult to hear speaker distortion/damage before it's too late.

Even if it wasn't my specific case, I'm not sure you'd hear speaker distortion/damage fast enough (in a live situation) to catch it before it gets damaged.

 

[JRod4928]

Turning down the volume turns down the wattage. Volume is created using power, so less volume = less power.

Volume controls aren't linear, I'm sure you could figure out the math behind it, for example a log10 pot at noon = x% wattage, but I'm not experienced enough to answer that for you. However, if you turn down the volume output of the FM3, you'll reduce the output wattage from the 5150, so your speakers won't distort/blow up. Use your ears, speakers usually give lots of warning in the form of distortion.

There was an article somewhere that said a specific loadbox could accept amps above its rating if you turned down the master, which is essentially want's asked here, I'll update if I can find the source.
Edit- here's the closest thing I could find. Scroll to the bottom to find "High gain amps such as Marshall or clones are hard on attenuators. Unfortunately, an amp’s volume dial is not a good indicator of how much power is being produced. Many amps can reach full power at 3 or 4 on the volume dial, so it is a good suggestion to get an attenuator with 4x the power handling. For example, if you have a 50w high gain amp, you should opt for the MASS 200. If you have a 100w amp, you should still go with the MASS 200 but you should not crank the amp."

Interesting....