Category Archives: Dummy load

Building a Dummy Load for testing tube amplifiers

 Since finding an old oscilloscope in a dumpster last year, I decided to build a dummy load so I could test my amplifiers. Over at DIY Audio Projects there is an inspiring building description by Gio Militano He proposes adding a 100R resistor and a speaker in parallel to the load so you can hear what you are testing at an attenuated level.
Terry Downs also has a description of how to do it if you cant find/don’t have the high power resistors in the right values.
Just for curiosity I will add the high power radio amateurs version.

I built the dummy load out of an old heat sink (85x160mm) from a butchered amp. I drilled and threaded two M4 holes in the heat sink and two 4mm holes through a piece of mild steel bar from the scrap box. I bought 4x 4ohm 50W resistors from Mouser, as I was ordering some other stuff (284-HS50-4.0F 50W 4.0 Ohm 1% Arcol Wirewound Chassis Mount). Mounted the resistors with some heat transfer silicone and the bar over all of them to keep things tight. This was way faster than marking, drilling and screwing in 8 self tapping screws.

I used scrap aluminum from work, and bent up a U shaped panel for side-front-side Holes were drilled for binding posts and speakers. Then I used self drilling chassis screws to mount the heat sink to the open end of the U shape. For top and bottom I bent up mounting flanges on each end of two squares of aluminum and  screwed on rubber feet on one. After soldering the thing up I mounted the top and bottom panel.

Here is an interior view during soldering. The speakers elements are from some cheap computer speakers they are rated 4 ohm, 2-3 watt. They are connected in series with a 100R 2W resistor to reduce speaker level, and then they are connected in parallel to the 4ohm dummy resistor. One for each channel. That way I can confirm what kind of signal I am getting without splitting my eardrums with 1K Hz sine waves.

A few final comments on the cooling. As you can see from this picture I have angled the top cover to get a air gap in the back for ventilating the inside of the power resistors. The under side cover is intentionally cut a bit short so there is an air gap there as well. There is a secondary reason for the sloped top. I don’t what to inadvertently put stuff on the top that can inhibit air flow. The angle makes things slide of. That´s design by affordance in the spirit of James Gibson.

Here is the finished dummyload ready to connect to my next triode amp project.

According to “boywonder on DIYaudio, some basic oscilloscope tests with an amplifier driving the dummy load are:

check for noise on the output (across 8R load) with input shorted. Just put the scope probe across the load R and measure noise.

check square wave response for ringing (oscillations), rounded leading edges (reduced HF resp), or sloped tops (reduced LF resp). Usually done at 10Khz from function generator with square wave output.

Check freq response by inputting sine wave and varying freq from 20hz-20Kz +/-. You are looking for the frequencies were the amplitude of the sine wave starts to drop. When the amplitude is half of the max value, that’s your -3db point. Some amps are flat way beyond 20khz…..

Check power out by inputting a sine wave and measuring voltage across the load R with the scope and an AC voltmeter. Increase input sine wave voltage until clipping on the scope is just visible, then record AC RMS volts. Power =Vsquared/R. Usually done at 1khz.

Check power supply ripple voltage by connecting scope probe to B+, set the scope for AC coupling, and zoom in for the ripple voltage. NOTE: Make sure that you are using a probe rated for the voltage, and preferably 10:1 voltage attenuation. I use a 600V 10:1 Pomona probe that I picked up on ebay of few years ago.

Be sure to calibrate the probe on the scope with the built-in sq wave generator. You want to adjust the probe to get nice clean sharp square waves out of the scope test jack.