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    Resistor wattage rating

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    Captain Coconut

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    Resistor wattage rating

    Post by Captain Coconut on Tue Jan 09, 2018 2:03 pm

    Looking at a schematic/manual of the ST-35 (or any tube amp), there is a list of resistors with a number given for power rating. The ST-35 use resistors with a 1/2W rating primarily, with a few 1W sprinkled in. How much beyond these figures can you go?

    Higher wattage means larger size and more expense. Will larger sizes be more prone to noise pickup? If not, and you’re not concerned about expense, will a 2W grid stopper, for example, be a bad choice where a 1/2W will do?
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    Peter W.

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    Re: Resistor wattage rating

    Post by Peter W. on Tue Jan 09, 2018 2:55 pm

    The very short answer is: It depends.

    Resistors have two functions - one obvious, one not-so-much:

    Function 1: Act as a current limiter, voltage-divider, or voltage dropper under specific conditions, depending on 'where' in the circuit. This is mostly a function of resistance, with current rating being chosen for the minimum 'safe' amount and tolerance dependent on the specific need.

    Function 2: Act as a fuse. In which case, the tolerance and current rating will be chosen with equal care.

    The bean-counters will not tolerate expenditures beyond what is absolutely necessary (dependent on several parameters not discussed here). $0.00001 does not seem like much, until multiplied many thousands of times across many thousands of parts.

    So, in direct answer to your question, you will need to determine:
    a) The consequences if the resistor opens.
    b) The consequences if the resistor *does not* open.

    Writing for myself, I tend to use, exclusively, 5% resistors-or-better, and I tend to take the time to further match across channels. But as to wattage, I am far more careful. To use a solid-state example, too high a wattage on some resistors in the venerable Dynaco ST120, and a spike will destroy the driver and output transistors. SOMETIMES, however, the resistor will fail first. However, in most tube applications, higher wattage *is* better. On the FM3, one resistor as-supplied is 2-watts, and runs at about 1.9999-> watts. Going to 5 watts is a very good thing.

    Ignore the bean-counters. Look at what the resistor is doing, and size accordingly.
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    sKiZo

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    Re: Resistor wattage rating

    Post by sKiZo on Tue Jan 09, 2018 3:45 pm

    +1 on that. In many cases, resistors are PLANNED to act as fuses to protect more delicate (and valuable) components down the chain. Never ever up the wattage until you've done the homework and are willing to take the risk.

    PS - kind of on topic ... many older rigs use "fusible" resistors that DO run hot and can go brittle over time. OK to replace those with similar rating metal resistors. DO remember to kink the legs so the resistor sits high to keep the heat away from the board.
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    Peter W.

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    Re: Resistor wattage rating

    Post by Peter W. on Tue Jan 09, 2018 4:01 pm

    sKiZo wrote:+1 on that. In many cases, resistors are PLANNED to act as fuses to protect more delicate (and valuable) components down the chain. Never ever up the wattage until you've done the homework and are willing to take the risk.

    PS - kind of on topic ... many older rigs use "fusible" resistors that DO run hot and can go brittle over time. OK to replace those with similar rating metal resistors. DO remember to kink the legs so the resistor sits high to keep the heat away from the board.

    Oh yes!

    Sometimes (quite rarely) they can be noisy.

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