Carbon Composition Resistors

[Geoff Bourquin]

Back in RF class in college we were told to use nothing but composition resistors in RF circuits, like RF amps, oscillators, etc. The inductance of the spiral of metal or carbon film resistors plus the capacitance end-to-end, and between the spirals in the resistor made them too likely to resonate all by themselves, with highly unpredictable results.

My feeble memory tells me that 1/2 watt carbons were good for 300V, and 2W were 600 or 700. (I wouldn't bet more than about 50 cents on my memory). Also, there are some reallly L O N G carbons out there, and I suspect they are rated at higher safe voltage. I have made chains of carbons when I wanted to work with higher voltages (like making my own focus dividers on a TV. {act of desperation to get it running}).

I think those blue RCA Flameproof jobs were metal film. I probably still have some in the shop. I should file one down and see.

Old carbon resistors may be way out of value, so be sure to check them. There was an article in QST magazine a few years ago about that, and I think the author baked them in an oven at low (140?) temp for a while to drive moisture out of them and try to return them to their origional values. If I remember right, he was only partly sucessful.

Thats my rambling for the evening.

Oh yeah! Dont you just love the smell of those things when they flame out? Takes me back to my childhood, when the house always smelled like burned carbon resistors. Mom hated that!