RE: DR Z Airbrake comments by its designer, Ken Fischer 
Holiday Greetings from Ken Fischer, DR Z & Komet Amplification! 

I just received this discussion thread regarding power attenuators via fax. 
Since I do not have a computer or Internet access, l have asked PinkStrat [Dean] 
to add some commentary on the Airbrake attenuator(s). First, I feel it's important 
to add that the Airbrake attenuator has been on the market for 17 years...longer 
than any other attenuator in production that I am aware of. When the Airbrake first 
became available, you had to own a Trainwreck amp in order to purchase one. Since 
I serviced all of my own amplifiers myself, I have used [and continue to use] the 
Airbrake attenuator myself. If there were any problems with this attenuator harming 
amplifiers, transformers, tubes or speakers, I would have become aware of these issues 
and would have therefore modified, redesigned or discontinued building the Airbrake attenuators. 

The Airbrake was originally [and still is designed] as a 100 watt device. However, when using 
a power attenuator, some of the power is dissipated in the attenuator and some is dissipated 
by the speakers. Also, due to the internal electronic design of the Airbrake, as you select a 
higher level of attenuation the wattage rating of the unit also increases up to a maximum of 200 watts. 
This makes the Airbrake attentuator perfectly SAFE for high powered 100 watt amplifiers, such 
as the 1959 Marshall & DR103 HiWatt heads. On these high powered British amps, due to their unique 
transformer designs, when using a single 16 ohm cabinet I recommend setting the impedance selector 
to 8 ohms when using the first two levels of attenuation. We also make this recommendation for any amp 
using EL-84 type output tubes. The reason for this is simple; when used this way, this will result 
in a reduction of the current flowing through the output tubes at FULL POWER. 

Technical observations: 

There was mention in this discussion thread of other attenuators being constant impedance devices. 
This would actually make these attenuators harder on the amplifier than if used with just a speaker 
load. Loudspeakers are NOT constant impedance devices. Anybody who has done frequency versus impedance 
studies on various speakers (in various cabinet configurations) will find that speaker impedance varies 
enormously. First with frequency and wave shapes and additionally with multiple frequencies and complex 
wave shapes as generated by a guitar amp under distortion. For example, a 16 ohm standard 4-12 Marshall 
cabinet can be as low as 10 ohms as the frequency approaches DC & over 30 ohms at some of the higher 
frequency harmonics generated by the guitar under distortion. 

I've personally done these studies at Ampeg in the 60s & at Trainwreck during the 80s. FYI--another 
feature of guitar loudspeakers is when the cones are set in motion by a signal, the speaker also generates 
its own voltage as the mechanical suspension pulls the cone back to its neutral position (at rest). 
The speakers also generate "overshoot voltages" and other various voltages during operation. These voltages 
(known as "kickback" voltages) are reflected back to the primary of the output transformer to the power tubes. 
In addition, if the amp is equipped with a feedback loop (which most amps have), these "kickback" 
voltages are also applied to the feedback loop via the speakers and output transformer secondary. 
If you take a speaker and cut the cone out & then glue the voice coil into the gap [so that it will not move] 
these kickback voltages are no longer present. Also, there is no cone inertia for the output stage to overcome 
(which affects the way the output stage responds). If you do a frequency versus impedance graph of this deconed, 
frozen voice coil "speaker," you will have a radically altered graph compared to when the speaker was functional. 
In effect, you no longer have a speaker as a load but an inductive resistor... As for how the Airbrake attenuator 
handles both 4, 8 & 16 ohm loads-- it's magic! 

Rock On' 
Ken