Taken from aikenamps.com (some interesting stuff about attenuators):
Q: Why are resistive attenuators harder on an amp than an inductive attenuator or speaker load?
A: A resistive load is not any worse than a speaker load or reactive attenuator load. In fact, the opposite is true - a purely resistive load dissipates all the amp's power in the load, while a reactive load varies the dissipation between the load and the output tubes, depending on the phase angle of the reactive component of the load. The worst case load for an amplifier is a purely inductive load, with a phase shift of 90 degrees between the supplied voltage and current. In this case, when the voltage across the load is zero, the current is maximum, which means that the output device now has maximum voltage across it at maximum current, which results in maximum dissipation. In effect, the load gets no power while the output devices are cooking!
In addition, a reactive load has a very high impedance at the low frequency resonant point (typically up to 4 or 5 times the nominal 400Hz impedance) and a rising impedance at higher frequencies that can go up to many times the nominal impedance. These high, reactive impedances can cause very high, frequency-dependent voltages on the reflected primary impedance of the output transformer, which can cause arcing. The problem is much worse if the impedance is mismatched. The main factor in amps blowing up from attenuator use is not the fact that most attenuators are resistive, it is the fact that the amp is run full-out all the time, something that would not normally happen because the amp would be too damned loud to use that way.
Another factor in amplifier damage from attenuator use is when the amp is run into an attenuator that is not properly impedance matched to the amplifier. Contrary to some attenuator manufacturer's claims, there is no way to make an "automatic" impedance matching attenuator, or a "one-size-fit's all" product. Their products are single-impedance attenuators marketed as "safe" for all load impedances. Lower than normal impedances cause higher than normal currents in output tubes, and higher than normal impedances increase the risk of arcing in tubes, sockets, and output transformers. I believe the "resistive load is bad" argument has the same origins as some of the other common misconceptions in the amp world. Look on any amp designers or tech's workbench, and you will find a purely resistive dummy load for amp design and testing. All Aiken amplifiers are designed and tested flat out at full power into a purely resistive load for long periods of time, in addition to being tested using reactive loads and actual speakers.
Q: Why do you test your amps on a purely resistive load when they are not going to be run on a purely resistive load?I don't know anything about amp building but I do know about engineering, and when I do a test on something I will try to duplicate the exact environment in which it will be run in the real world.
A: Now, that is an excellent question! There are several reasons to use a resistive load. The first is that it is the only way to easily see the true power output of the amp. If you run into a reactive load, you can't easily see how much power the amp is putting out, because you have to take into account the frequency-dependent impedance of the reactive load, and there is a cosine of the phase angle to be taken into consideration to calculate the real power. If you run into a purely resistive load, the voltage and current are in phase, so the power you see on the scope is what is really being put out by the amp (well, you have to do some basic math with the voltage and the load resistance, but there is no phase angle or frequency-varying impedance to take into account).
The second reason is that reactive loads cause all kinds of overshoots to appear on the edges of the square waves when you drive the output stage into clipping, and you can't tell whether they are actual overshoots that you don't want, which could point to a problem in the amp, or just normal reactive load artifacts that you don't necessarily need to worry about.
The third reason is that you need a load impedance that is flat with respect to frequency in order to run frequency/phase response plots of the amp with a network analyzer to check the design of various stages or the entire amp, to see how much effect the tone controls have, what frequency the breakpoints are at, etc. A reactive load will skew the results because of the frequency-dependent impedance magnitude changes.
Once the amp is happy running into the normal resistive bench dummy load, it is then hooked up to a reactive load for the final stability testing. I like the Marshall Power Brake for this because it is designed with no limiting of the high-frequency rise on the impedance (which is probably why they have a reputation for sounding buzzy), and can really aggravate an amp that is on the edge of oscillating. You might not catch it into your bench test speaker, but it might oscillate into, say, a 4x12 cab. The PowerBrake is a pretty rough load for most amps, so it is a good test.
After the amp is checked out to make sure all is working as designed, then you test it into a speaker, using the most important pieces of test gear of all - your guitar and your ears!
Q: I have a Dr. Z AirBrake attenuator and am ordering an Ultimate Attenuator. Are these safe to use with my Sabre?
A: Be sure to set the Sabre to 8 ohms when using the Dr. Z AirBrake with either a 16, 8, or 4 ohm cabinet at any level other than bypass (where you'll need to switch back to 16, 8, or 4 ohms to match the cabinet being used). The AirBrake presents close to an 8 ohm load in most cases, but this will vary considerably depending on the cabinet being used and the attenuation setting (contrary to their marketing claims, there is no such thing as a "one-size-fits-all" attenuator that properly matches all impedances - this is in reality an 8 ohm unit being marketed as "universal"). Running the amp at 16 ohms into an 8 ohm load at high power will accelerate tube wear and could cause damage, so be sure to match the load. In addition, the attenuation steps will not be correct unless you are running an 8 ohm cabinet with the amp set to 8 ohms. If you are running a 16 ohm cabinet, they will be around half the normal steps at 8 ohms, and if you are running a 4 ohm cabinet, they will be around twice the normal steps at 8 ohms.
Beware that the Ultimate Attenuator is a 30 ohm load (or so I've been told, I've never had one in to test, but again, there is no way to "automatically" properly match all impedances as claimed). This 30 ohm load is not a proper load for the Sabre's output transformer, so use it at your own risk - your warranty will not cover any damage resulting from this upwardly mismatched load using this device, and I have had reports from Sabre customers of "flashing" inside the tube near the base when used with this device, and have seen reports of oscillations and output transformer failure from users of other brands of amps. Be sure to set the impedance selector for 16 ohms when using this device with any cabinet impedance.
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