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Gang, is there a rule of thumb I can go by to select resistor types for tube amps?

Decades ago, it seemed there was carbon and wirewound, 20% tolerance was standard, and 5% was usually considered overkill.

Now I find myself buying 1% (cause they're cheap) and going with the so-called "metal film" for virtually all 1/2 watt resistors no matter where in the amp they are called for. And "metal oxide" for 2 watt resistors.

I see Gregg the Geek has started selling ST70 boards with kits and he supplies metal oxide and carbon film types (I assume optimized for their particular location).

1. How does one decide where a metal oxide or carbon film or carbon composition or metal film resistor goes??

2. How do metal oxide and metal film differ?

3. How do carbon film and carbon composition differ?

Thanks for the input!

Modern resistor types, of equal power and resistance ratings, are interchangeable. In days of yore, carbon composition resistors often were a bit unstable in value, increasing their resistance value over years. Wire wound resistors were sometimes preferred as grid stoppers in old radios because their inductance may have helped prevent oscillations.

I don't believe that there is any audible difference among resistor types.

When selecting power resistors, the power rating should be at least twice the steady state power dissipated - for a safety margin.

I have a gorgeous reply to this (I had to email Tom), but can't post more than a few lines of it without the server dropping the connection!

Shannon, please help? :-(

Cheers!

I'll use carbon composition for the grid stoppers, if for no other reasons than superstitious ones. Plate loads and grid leaks should probably be metal film for low noise. If you're putting resistors on the screens of power tubes, make sure they are something flameproof, probably metal oxide.

Other than that, I don't have strong feelings one way or another.

Carbon comp is supposed to sound "warmer' than carbon film. Carbon film should not be used in applications which pass alot of current, such as, screen resistors. The insulated coating on carbon film resistors is not fire retardant (unless specifically noted to be fire retardant). Carbon film resistors can burst into flame in certain applications when they really heat up.