I've usually up rated any out of spec resistors I find if I'm unsure of the voltage rating of my replacements. Especially if I'm caught short and need to use Jaycar resistors eg I replace 1/2w with 1w and 1w with 2w.

I've just been looking at an AWA 430MA that has a couple of 1/2w 100 ohms resistors between the transformer and the 6X5GT rectifier, obviously as fuses. It got me wondering if there are any other common locations in radios where the resistor also acts as a fuse and shouldn't be uprated, possibly the choke? I'm a bit concerned now and might need to go back over my previous work

Warren

It normally pays to use 1Watt resistors on B+ as many supermarket 1/2 watts cannot handle the voltage. It would be handy if they did quote Voltage.

Many AGC's use 1.75Meg. They do not have a lot of volts on them nor current through them & there is a 1.8 meg in the lower wattage types.

You do not "heavy up" Backbias resistors; It is better to let them burn with an overload, instead of the transformer.

The 100R is not primarily there as a fuse, however 6X5 is a horror.

Certain designs of them are notorious for going short circuit. There is not a lot of voltage across the 100R resistors, so again, better to have them as "fusistors".
The voltage drop (AC) across the resistors would give you an idea of how much current is going through those resistors

If you are lucky enough to have the parameters? The square of the current by the resistance, will give you the wattage the resistor has to disipate (not the wattage it is rated at)

Marc

So it seems the most important ones not to uprate are bias resistors?

The circuit shows total HTof 60ma so if I've worked that out right it's about 360mw over the 100 ohms, a small increase to 70ma would take the current up to the rating of a 1/2 watt resistor. Interesting, I wonder how much safety margin is built into resistors before they fail. I suppose the designers take all that into account.

As you can see a 10mA increase in current makes the 110R 1/2 watt resistors life a misery.

If a cap, or the rectifier shorts, you can, I am sure, appreciate the cost of a resistor is far cheaper than a transformer.

Back bias resistor/s normally carry all of the current of the valves. These are not to be confused with cathode resistors, which are also used to set a valves "self" bias.

Marc