Dummy Load for Amp Testing
I needed a reliable dummy load for testing amps that I design and repair. Over the years I've been collecting the parts I thought I would need. Finally it all came together along with the time needed to assemble it. I wanteded mono and stereo modes and at least 50 Watts of power dissipation. Finally, I wanted it to be accurate. The rest of the pictures can be found in the gallery: http://www.knausz.com/category/image-galleries/vacuum-tubes/dummy-load
To the left is a picture of the front panel with all the controls. Both inputs are replicated with several different types of connectors to allow easy connection to multiple instruments simultaneously. For example, in mono mode when using Input 1, I can connect the amp to the phono input which is easy with a standard guitar cable (the same cable that's used to connect an amp to a speaker cabinet). Then I can connect a multimeter to the banana plug inputs and finally a scope to the BNC input. I can also use a tee to split the BNC so I can connect my HP distortion analyzer along with the scope. With the amp disconnected, I can set the load's impedance with the meter on resistance mode and then when I turn the amp on, I can switch the meter to RMS voltage mode.
To achieve mono/stereo operation which is configurable to 4/8/16/32 ohms, I used four 60 W 15 ohm resistors. The switches allow several combinations of parallel and serial connections to achieve all of the impedances. Because of the various combinations of resistors, the power dissipation is not always the same. It is a minimum of 60 Watts in mono mode and in some cases more. The schematic can be found at the end of the article as an attachment as well as the table detailing the switch configurations and the maximum power dissipation for each mode. The large high power 4 ohm rheostat (potentiometer) wired in series with the load is used as a trim (it can optionally be shorted out of the circuit with switch 5). The power resistors are mounted to a very large heatsink. I drilled holes in the heatsink, tapped them and used conductive paste for a secure fit. I painted the aluminum chassis black to improve the heat dissipation - shiny aluminum just reflects most of the IR.
I tried to be neat with the wiring by using 90 and 45 degree angles in the wire bends and tie wraps to hold the wires together. I could have been neater, but this is just a piece of test equipment, so I didn't want to spend too much time. I may add a fan in the future to further improve the power dissipation. Note how the heatsink is mounted in a compartment in the chassis. That wasn't my original plan, but turned out to be convenient and will allow me to set up airflow very accurately in the future (if even necessary).
The important thing is that this load will save me tons of time in the future and it was fun to build!