I have a Kriesler Multisonic 11_104 console stereo, and a couple of 11_81 mantel radios. The Multisonic has no fuses, and I don't think the mantel radios have either. I'm about to open one up to check components, clean pots, etc. Is it a good idea to put a fuse in on the active mains wire, after the switch probably, or is there no need? I would have thought it was a wise step to take, considering these units are so old.

Yes it is wise. My tech friend that taught me always installs one before the switch.

The main protection you want is to stop the power transformer going up in smoke, should the main electrolytic short out, or the chaos after the speaker transformer blows.

A 55watt radio will draw 230mA, so maybe a 300mA fuse will do. You want the lowest value you can without nuisance blowing. You could also consider a time delay fuse vs an instant-blow type. The time-delay fuse can be a lower value, with the delay handling the initial inrush of power when switched on (so, 250maT).

Remember with wire fuses, when they are close to capacity they will be glowing, which means they will get hot. You need to make sure any fuse holder can tolerate the heat the cartridge will give off and you need to make sure that the fuse holder is not near anything that doesn't like heat.

There would be nothing wrong with making the fuse half an amp or even an amp to ensure that things don't get too hot. Fault current on the secondary would normally likely exceed it.

Personally, I do not bother installing fuses because I don't have that many radios that I run every day. In fact there's only one out of a collection of around 250. The rest are only powered up when being demonstrated, which is a few times a year at most. However, if a fuse makes one feel better about using their radios, I'd certainly encourage that little bit of extra protection.

The main issue with a fuse is inrush current. The reformer part of my switches in an extra 10μF HV cap; switch that into the live rail & the deliberate fast blow fuse is a goner.

Where there is a back bias resistor heavying it up is not a good idea: I would rather see it "sail with the wind" & burn than the transformer. I did actually put 100 ohm low watt resistors in series with the diodes (replacing a metal rectifier) Not only for surge: but to hopefully be that which burns rather than the transformer.

In several cases I seen a mains fuse do a Nero while the transformer virtually melted down (posted one on this site). The bench PSU here, (HM) is fitted with thermal types on the heater / filament windings. An earth leakage trip will not do so, if the fault in on an isolated secondary: Be warned.

Thanks for the replies. The Kriesler Multisonic 11-104 does employ "back biasing", which involves a resistor to ground on the secondary centre tap. The method I used for determining the fuse values on the clone of this circuit was to add up the total currents expected through all the valves, and the heaters, then multiply that by the voltage across the secondary of the PT to get total power. Then use that power figure to calculate the expected current on the primary side. Then make the fuse 1 1/2 to 2 times that. For my 2 x EL84 and 1 x 12AX7 circuit I ended up using a 600 ma fuse on the primary side. I expect I'll do the same kind of thing to estimate a value for the fuse in the 11-81 radios. I also put fuses on each side of the secondary, which I read somewhere gives your PT extra protection. I can't remember how I calculated those.

If the thing is back biased then all of the cathode current in most cases goes through it. Therefore it being of a known resistance and the voltage across it will allow the exact current that is passing through it to be calculated.

In most cases the voltage across it is the grid voltage of the OP tube/s

Dial lights can also short. I had one crazy arrangement in a Stromberg Carlson where there was a wire running to the dial pointer assy to run a globe. Being a moving part the probability of the globe wire failing again was high, so that setup got a fuse.

Marc

There are many potential faults in these things that will slowly kill the mains transformer or even cause a fire without blowing a fuse that's large enough to survive initial inrush current. That is why modern small mains transformers use thermal fuses. Conventional fuses blow when you don't want them to, and don't blow when you do!

You can fit a thermal fuse (say 105 degrees C) to an existing transformer. Encase it in fibreglass or silicone rubber tubing and position it in contact with the transformer windings. Get a good nights sleep that way.

RS Components sell thermal fuses, also known as microtemps at similar temperature ratings. Two dollars will buy most of them but remember, unless you shell out for a resettable one, once they go pop, they have to be replaced so don't make installation too permanent.

Beware of how you connect those thermal fuses. Soldering irons & thermal fuses are sworn enemies. No heatsink & get it wrong: New fuse.