Metal oxides are popular because they are less costly than metal films. Also because the resistivity of the bulk material is higher, less spiralling is required, and this would usually mean less inductance. Oxides can also be made physically smaller. They do not have the best temperature coefficient... around 200 ppm.
Metal films are considered to have sonic virtues by some, and in general will have slightly higher inductance. They can be made more accurately and will exhibit higher long-term stability when operated near rated power. Temperature coefficients are better with 50 ppm being about worst case.
Keep in mind, that the issue of inductance is usually blown out of proportion. There are certain applications, here and there, where low inductance is required though. Physical styles of either type (non-axial, non round) are available for special applications, these can be virtually inductance free. These types are usually flat in form.
Fusibles and flameproofs are for rare applications where overload (failure) is likely... it is generally someone like U.L. telling you where to put them. Oxide type resistors often tout their flameproof characteristics as a bonus... they are oxides after all... already burnt in a sense.
Speaking only to round-axial resistors, as long as you stay away from wire-wounds and carbon-comps, sonic effects should be transparent. Many will insist that only cryo-treated powdered rhino horn resistors will bring sonic euphoria.
Surface mounts are a whole 'nother deal.
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Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Re: Metal oxide film versus wire wound resistors
If it's just a series resistor for a Silicon diode being used to replace a Selenium rectifier in a record player, a metal oxide film resistor will be fine in that position. The maximum possible current flow (from applying peak HT with 0V across the reservoir capacitor) will only be of the order of 1A anyway, and it will not be sustained for long.
If you insist on a 360Ω (a value from the "+/- 5%" series) wirewound resistor, there is no harm in using two 180Ω (a "+/- 10%" series value, so more commonly available) resistors in series. If the resistors are of equal value, then each one will be dissipating half the total power. Otherwise, recall that the current through them is the same, so the power dissipation will be shared in proportion to the resistance. For instance, with a 120Ω and 220Ω resistor in series to give 340Ω, the power dissipated in the 120Ω resistor would be 120/340 of the total power dissipated, and 220/340 of the total power in the 220Ω resistor. Power in Watts = Voltage * current in Amperes, and resistance in Ohms = Voltage / current in Amperes. By measuring the voltage across one of the resistors, you can work out the current through them both and thence, the power dissipation in each one. But as a first estimate, with a pair of ECL86s, you're probably looking at about 40 mA maximum anode current and 5 mA screen current through the pentode sections, and no more than 3 mA through the triode sections. So that's 96 mA, call it a nice round 100 mA = 0.1A, which will develop 36V across 360Ω for a power dissipation of 3.6W.
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