Build an 8.2 Ohm/110W Dummy Load for testing, or servicing guitar amps.

[DrNomis_44]

Hey Everyone,

Here's a very simple project I'm going to be working on soon, which I thought other forum members may find handy, as the title of this thread says, this build project is for a simple, but very handy device called a Dummy Load.

So, what is a dummy load, and why is it so handy to have?


A dummy load is basically a load you can plug into the speaker out socket of a guitar amp so you can perform tests on it without risking damage to your hearing if you need to crank the amp up to full power output, like for example if you're doing service work on an old tube/valve amplifier and you need to set the biasing of the power tubes so that they operate correctly, or, if you're checking the power tube behaviour at full power output to see if nothing abnormal happens.

Occasionally, when we are doing servicing/testing work on a guitar amplifier we need to feed-in an audio signal of some kind and if we crank the amp up with a normal speaker load connected to it, things can get very loud, a Dummy Load also allows us to connect an oscilloscope up to the output of a guitar amplifier so we can actually see how the amplifier behaves, this is actually a useful thing to do because if the amplifier has a tendency to oscillate at high frequencies, we may not actually be able to hear it because the frequency is too high for our ears to perceive it, but we can definitely see it on the oscilloscope screen since most oscilloscopes have a frequency bandwidth of at least DC to 5MHz or more.


The Dummy Load I will be building soon simply consists of 22 standard 180 Ohm/5W Ceramic Power Resistors all connected in parallel with each other, giving us a total load of 8.2 Ohms with a power rating of 110W maximum.

I'm going to be mounting all the resistors on tagstrips which will be housed in a ventilated case of suitable size, the Dummy Load will produce quite a bit of heat as a consequence of absorbing and dissipating the power output of the amplifier connected to it so it's important to have good ventilation, of course I could build the dummy load in an oil-filled case to help dissipate the heat.

I seem to remember Marcel suggesting using a standard electric kettle heater element as a dummy load as an alternative, I tried looking through what my local Bunnings store had in stock, but couldn't find any kettle heater elements.

I'm going to be including a write-up in this thread documenting my build process in case other forum members want to build an 8.2 Ohm 110W Dummy Load for themselves, so as always.....stay tuned.....


Here's a schematic I drew for the Dummy Load, as you can see it is very simple to build:



You could also add a separately powered cooling fan to it, maybe something like a standard 12V DC computer case fan and a switch to switch the fan on or off, something I may actually seriously consider doing in my build.


The speaker jack will be a standard stereo Cliff Jack as used in a lot of guitar amps such as the Marshall Super Lead Plexi.