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.

[DrNomis_44]

Update:

I managed to get together all the bits and pieces I need to build the new 8.2 Ohm/110W Dummy Load, and will be building it this fortnight, will post some pics of it soon...stay tuned!

[fender3x]

Is this useful for SS amps as well as tube amps. My impression is that SS and Class D amps don't need dummy loads...or are they still useful for testing?

[DrNomis_44]

Is this useful for SS amps as well as tube amps. My impression is that SS and Class D amps don't need dummy loads...or are they still useful for testing?

Yep, you can use it with Tube or Solid State amps, it won't hurt a Solid State amp at all, a Dummy Load is very useful because it provides a load for the amp to work into while saving your ears, so you can drive the amp to full output and not risk going deaf, Tube Amps do not like working into no load on their speaker outs, if you try to do that it will cause something known as a "Flyback Event" when that happens a high-voltage arc can form between the Tube's Anode/Plate pin and either the Screen pin or Cathode pin due to the inductance of the output transformer's primary winding, and will cause carbonizing which can create a leakage path to ground or burn out the output transformer's primary winding, which could end up being an expensive repair.

[DrNomis_44]

Update:

Did some work on the Dummy Load, all the resistors are mounted in the case on their tagstrips, just out of curiosity, I measured the resistance after wiring up all the resistors, I measured exactly 8.2 Ohms, don't you just love it when it all works out in practice, looks like my maths was correct.

[fender3x]

It is great. I's so infrequent with me, however, that even when I come up with the correct figure, I recheck ;-)

[DrNomis_44]

Update:

Well, I've finally finished building the new 8.2 Ohm/110W Dummy Load I've been working on, and here are a few pics so you can see how I went about building it:



I mounted all of the 22 180 Ohm/5W ceramic resistors on two 8-way tagstrips, making three groups of six resistors and one group of four resistors, all the resistors were soldered in parallel with each other, each group of resistors were wired so that all of the 22 resistors were wired in parallel, making a single 8.2Ohm/110W resistor, I used my multimeter to check the total resistance and it actually-did measure spot-on at 8.2 Ohms, which is pretty much near-enough to perfect as I'm ever going to get it .2 of an Ohm is about the resistance of a standard multimeter lead anyway, so I'm happy with it.



Another top-view pic.



A pic of the front panel showing the two terminal posts, the two terminal posts have small holes drilled through them, so, to attach a scope probe, you can use two short pieces of tinned copper wire, the earth clip of the scope probe goes to the black terminal, and the tip of the scope probe goes to the red terminal, this way you can use one channel of your scope to monitor the output of the amp you're working on, and the other scope channel to monitor signals at various points in the amp's circuitry, note that the Dummy Load will work with both solid-state and tube amps up to 110W in power output, so you can connect the Dummy Load up to say a 5W single-ended tube amp, like a Fender 5E3 with no problems at all, as long as the output transformer is set-up for an 8 Ohms speaker load.

Note: If you want, or need to build a 4Ohm Dummy Load, all you need to do is double the number of 180Ohm/5W ceramic resistors needed, so 44 180 Ohm/5W resistors all wired up in parallel should give you a 4.1Ohm/220W Dummy Load, if you make two 8.2 Ohm/110W Dummy Loads and wire them in series, you will get a 16.4Ohm/110W Dummy Load, which should be fine for testing/servicing say a vintage Marshall Super Lead Plexi amplifier.