Humdinger question.

[dragonbat13]

Is there any way to add protection to the heater wiring when using a humdinger?

I thought I had read something about it somewhere.

[trobbins]

What sort of 'protection' were you anticipating?

One fault mechanism is where B+ arcs to heater (ie. pin 2 to 3 arc on output stage socket) - and if that arc is not stopped then it can take out the power transformer and/or the output transformer - especially when the heater just uses a fixed CT to ground, and so the use of a humdinger can provide a 'poor man's fuse' if the humdinger fails 'open'.

Or perhaps you are thinking of a short from one side of a heater to chassis? If the heater uses a simple CT connection to ground, then a short will cause a large current to flow in half the heater winding, causing stress on the power transformer. If the heater winding used a humdinger, and there was a short to chassis, then that just causes one half of the humdinger resistor to pass twice the current so this certainly is more 'fault resistant' than a simple CT connection to ground. Vintage humdinger resistors were often quite 'beefy', and that may partly be so that they survive a short from a heater to ground.

[R.G.]

On protecting transformers;
Put a fuse in series with each separate winding section. If the fuse is sized properly, the transformer is effectively immortal, at least to overloads. Voltage transients can still puncture insulation and kill them, but overloads won't.
It's not practical to protect an entire transformer with a single primary fuse. A soft short on half a heater winding, for instance, can over-heat and short the heater winding and not blow a reasonably sized primary fuse. This is the reasoning that says to fuse each section of winding.
It's a lot of fuses, but then transformers are expensive.

[dragonbat13]

That's probably what I will do. Just fuse the windings.