Southbay Ampworks wrote:My first amp client bought a dual 6V6 amp from me. My amp builder (at that time) rated it at 10.8 watts RMS at 2 on the volume knob. 15 watts at 5, and around 19 on 8, although the volume dial went to 10..
First things first, fire the amp builder. ANY builder/designer of an amplifier that would spec an amp that way has no business building amps. Even if he was making any logical sense at all, spec'ing power based on preamp gain setting without regard to input signal level and EQ, he still used a linear pot as volume pot instead of a log or audio taper. Gain settings have nothing to do with output power.
Southbay Ampworks wrote: The client had me recone an old G12-65 to 30 watt specs for him from the local Celestion dealer.
13 months later (and one month out of warranty) the speaker fried.
Not all amps are created equal in terms of volume, drive, gain, etc. This client found that an amp with a single 12AX7, two JJ 6V6's and one 5AR4 rectifier could kill a 30 watt speaker in 13 months..
Did you dissect the VC to see what was the mechanism of fault? Was it heat or mechanical failure?
Possibly the speaker was not really capable of handling 30watts of sustained power.
Backtracking: Speakers power is based on heat dissipation and expressed in RMS power which is dependent on the wave form used in the measurement. In actual use, a voice coil can subjected to RMS power in excess of what the amp is rated at. An amp is measured using a sine wave which has a very predictable relationship to peak voltage. An amp in a distortion mode can have a much higher duty cycle than a sine wave so it can generate more heat in the VC than a sine wave of the same peak voltage. A True RMS measurement calculates the heating power of a wave form as related to steady state potential( DC) of the same numerical value. 1 volt AC RMS applied to a purely resistive heating element, will heat water to the same level as 1 volt DC.
An amp with voltage limiting of say, 1 volt peak will generate different heating potential depending on the duty cycle of the waveform. This can range from short high speed rise time pulses generating .001 watts at the same 1 volt peak voltage as a fully saturated Square wave generating 1 watt of heat into the same load.
So amps are rated via sine waves, and speakers are rated in total heat dissipation which to make any sense in real world applications, would require rethinking how amps and speakers should be rated. Until then, the whole issue of matching speaker ratings to amp ratings an apple to oranges relationship.
A square wave amp measurement rating would make more sense that could be related more realistically to dissipation ratings of speakers, that would represent a realistic worst case value. Worst case is the rating needed for heavily distorted harmonically rich wave forms as seen in all guitar amps when pushed.
Given all of the above, no rule of thumb quite fits. It depends on style of play, thermal limits of the speaker voice coil and and duty cycle of the waveform.
When we design powered speakers, none of these ratings are worried about, neither is impedance since it is a closed system. A combo guitar amp should have had all the variables considered by the engineer and power ratings don't mean much. Just SPL out before sounding like crap. If the acoustic power of a combo is 10 watts( a high value of sound energy), it makes no difference whether it is 50 watts RMS generated by the amp feeding an efficient speaker/cabinet system or 1,000 watt amp feeding a high power, low efficiency speaker. The SPL could very well be the same.
High power drivers are usually low effeceincy due to higher mass of the cone and VC wire diameter, but most important is the wider magnet pole piece gap needed for clearance of a less precisely controlled postion of the more massive VC.
In other words, large amps do not necessarily mean high SPL. To make a speaker survive the mega amps available, speakers keep getting less and less efficient, which in turn, makes their minimum power requirements to get anything out of them keep rising.
Southbay Ampworks wrote: He ran that amp at 5-6 on the volume knob and didn't use pedals. So some amps vary in their power output. You might want to have your amp measured for it's true power output at the volume you run it at. The 60 watt speaker might be fine... or not.
This might give a false sense of security, depends how the power is measured and the mechanical/thermal characteristics of the speaker. Again, the settings of the knobs mean nothing to any of this, a hot pickup or some eq settings can easily generate full power at low settings.
