I am thinking of placing an inline fuse on some of my valve radios on the mains line. Is this a desirable practice? If so, would you use a 1 or 2 amp fuse? Slow or fast blowing fuse? Regards

Most radios chew between 40 and 60 watts, one or two maybe around 80 if there's lots of valves or a turntable included. You would need to look at the plate on the back of those receivers that have one or invest in a tong tester to measure the current flow through the mains lead.

Note that if using a tong tester you can't simply wrap the tongs around the mains cord or you will get a reading of nought. You need to wrap them (safely and taking care) around active or neutral only and this is best done with the chassis on the bench in a secure location so that single conductors can be accessed.

You can also use the AMPS range on a standard multimeter although this requires extra care and is somewhat fiddly. I wouldn't recommend doing it this way unless you feel competent with working around 240v mains wiring.

Using the figures above, most radios chew about 250 to 300mA. With this result I would recommend a 1A fuse. There will be enough headroom to stop it glowing whilst in normal service but low enough to blow quickly when there is a short circuit on the primary side of the tranformer. If there is a short circuit on the secondary then the fuse will still blow but not as quickly. When a fuse blows and there is no longer any wire inside the glass then that is a short circuit. If the 'blow' only takes out the middle portion of the wire then that is an overload situation - not as bad but still important to fix up.

Don't worry yourself with 'slow blow' fuses. They cost more yet don't work as well in this situation. Slowblowers are more for protecting electronic components in amplifiers. Standard fuses are more fail-safe.

Do I install fuses as a matter of routine? No. I don't see the need but if you feel safer with a fuse installed then I can't see why you shouldn't do it.

Thanks Brad. I will use a fast blow 1A fuse. Does anyone put them after the secondary of the mains transformer and before the filter caps to protect the transformer from blowing?

To be honest I am not sure. The current rating on the high tension secondary is lower because of the higher voltages however the low tension secondary has a higher current flow. Some radios have two low tension secondaries, mainly those with an 80/5Y3 rectifier, so if you wanted to protect the secondaries that is upto three more fuses.

Slow blow fuses are for protecting transformers, motors and devices that have a high inrush current.
Quick acting fuses are for protecting silicon devices, transistors and such. They are supposed to blow on an overload/short before more expensive silicon devices get burnt.
Some tong testers will give you the average current of the first 5 milliseconds of switch on, and display this as the peak value.