My bench power supplies all have built-in current limiters but sometimes it's necessary to limit the current delivered from the mains itself. I often receive items like old valve guitar amps that repeatedly blow fuses for example and a current limiter will help diagnose issues as well as keep me safe. You can't easily buy stuff like this anymore so I decide to make my own DIY test equipment current limiter.
There are some pieces of equipment which might have been readily available a few decades ago but today, they're simply not as common as they used to be. Indeed, this is something I've been meaning to build for a while, now.
In line with the look and feel of the guitar amps that my current limiter will be driving, I gave this bit of DIY test equipment, a slightly 'retro' touch.
Functionally, it's quite simple and works on an established principle of using an incandescent light bulb (or two) to limit the current into an attached load, like an amp. With no short-circuit in the load, the light bulb (or light bulbs) won't come on. 
This current limiter has two 40W light bulbs allowing me to select two current ratings; about 165mA or about 330mA. Of course UK mains voltage is 240V. If larger currents are required, all I need to do is drop in higher power-rated light bulbs.
When testing 115V / 120V equipment, the current ratings would naturally be doubled.
The current limiter also has a bypass switch which allows me to conveniently bypass the limiter function without the necessity to unplug anything. A vintage style neon indicator reminds me when the unit is bypassed.
Using the limiter set to 'low', it would appear that the Randall shown below, clearly has a fault. As you can see, the amp is still on standby. Following a brief visual inspection which showed nothing obviously amiss with most of the amp, the power side did look as if it had sustained some fluid spillage in the past. There was corrosion and it just didn't look good.
My DIY test equipment current limiter isn't the only piece of gear that I've made for use in my lab. Amongst other things, one of my oldest items is a 2-channe,l heavy-duty 8Ω dummy load. As we all know, valve amps need to be connected to a suitable load, when powered up. Not doing so, risks damage and possibly destruction of the output transformer so when working with valves, a dummy load is absolutely essential.
Built before a time when speaker simulators and 'power soaks' came to be, my 2 x 8Ω load has lasted me well for several decades and continues to be used on a very regular basis.
It's a bit special. As you can see. it's rack-mounted. This was to save space at the time as all my heavy test equipment was in a 16U rack. As I'd often be out on the road, the rack solution was also very practical, if not bloody heavy!
My ol' faithful dummy load has recently been updated with a 600Ω : 600Ω audio transformer on the backend of each channel. Via the front-panel switches, I have the option to either have two conventional dummy loads or a pair of fully isolated loads which are perfect for the new generation of D-class amps with so-called 'balanced' speaker outputs which are isolated from mains earth. This keeps my oscilloscope very happy! 😎
The potentiometer simply provides a little control over the signal level going into my 'scope. As well as being perfectly suitable for attaching to 2-channel amps like the Marshall 9100 or 9200, it's easy to wire the two loads in series for 16Ω or parallel for 4Ω.