I use this dummy load all the time when working on amps. It will easily handle 100W for long periods and up to about 300W for a short time. The load includes a speaker jack which gets 1/4 of the power when plugged in. That way I can run a 100W amp flat out and listen to only 25W. My bench speaker allows me to divide that down to less than 1W.
The load has two 4 mm banana binding posts and a 1/4″ jack as input connectors making it easy to plug in a speaker cable form an amp. The red binding post is also handy for a place to put a scope probe to see the output signal.
You can set the load to match 4 ohm, 8 ohm, or 16 ohm loads with the center-off toggle switch. The jack on the right side is used to connect an 8 ohm speaker. The speaker will get 1/4 of the amp power when connected.
My original design which you will see in the pictures below also included a 1:1 isolation transformer across the input terminals for connecting a scope. This is handy when the amp has a bridge design and both sides of the speaker are driven. The little transformer I chose was woefully lacking in low frequency response so I don’t use it. In any case 99% of the amps I work on have one side of the speaker output grounded.
I found the 8 ohm 100W resistors on Amazon. These are commonly used (apparently) in LED light systems for vehicles. The one gallon paint can came from Lowes.
To make the load handle the power continuously the resistor assembly is suspended in oil. Ideally you’d use transformer oil but, being cheap, I just use cooking oil. I’ve made loads like this for RF work in ham radio as well as for low frequency loads like this and never had any trouble with the cooking oil.
It is important to note that no effort was made to make the can and its lid oil tight. So if I were to knock mine over it might leak. Just beware.
The following pictures give you an idea of how mine was assembled.
I used three pieces of aluminum angle to make the resistor mount. Be sure that the wire ends of the resistors don’t touch the bottom of the can.