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Thread: 5E3 Bias resistor wattage

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    Member E biddy's Avatar
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    5E3 Bias resistor wattage

    I've seen a few recommendations to use a 10W resistor instead of the 5W on the schematic. Why?

    If you have approximately 20V dropped across a 250ohm resistor that is 80mA. Then the power would be 1.6W. Double it for safety and you are at 3.2W. Isn't 10W overkill?

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    Supporting Member eschertron's Avatar
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    Remember that power is a 'power law' thing. Required power dissipation quickly builds as cathode voltage increases. If the tubes are pulling more that 25v worth of current across the cathode resistor, then dissipation is above 2.5W and there isn't a 2x safety factor any more. Plus a 10W resistor has a lot more surface area than a 5W, so doesn't have the same surface temp for the same heat dissipation.

    edit: I don't think this requires a 10W resistor, but I see why these are justifiable arguments.

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    Member E biddy's Avatar
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    But if it is dropping more voltage, that would be because the value of the resistance has increased, and that wouldn't that still keep the power down?

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    Quote Originally Posted by E biddy View Post
    But if it is dropping more voltage, that would be because the value of the resistance has increased, and that wouldn't that still keep the power down?
    No, apart from long time effects in carbon compound resistors, resistance won't change when within rating. Rather, higher voltage drop means increased cathode current at high power. Just measure.

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    Bent Member Chuck H's Avatar
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    Remember that idle current is not the same as what the amp makes and even wastes when conducting signal into a heavily clipped waveform. A 5e3 could probably get the current through the cathode resistor to 4 watts on peaks for tiny intervals. Resistors typically accept this without the need to rate them for tiny peaks. Even averaged out I'm sure the 5W cathode resistor is fine. The bigger issue is heat. Some would argue that the due to the laws of physics it all balances out. Wattage dissipated as heat will be the same in a 5W resistor or a 10W resistor. That is, the 5W resistor will be hotter but have a smaller surface area while a 10W resistor will will be less hot over a larger surface area. The result is that the net heat inside the chassis is the same. But...

    Sometimes there is proximity to consider. Will there be heat sensitive components like electrolytic capacitors near the resistor? This is almost always the case with cathode resistors. Though you CAN locate the bypass cap elsewhere it's not done very often. So in that case a cooler surface temperature for the cathode resistor can help preserve the bypass cap.

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    Whether the part itself will survive is just one consideration. As was mentioned, a larger wattage resistor spreads the heat over a larger area. Same amount of heat, but not all in one spot. And that can make the difference between baking a nearby cap or not.

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    Supporting Member Randall's Avatar
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    I had a boutique hand made Tone King in recently. It had 4 6V6s with two sharing a 10 watt cathode resistor, and two sharing another. This amp was meticulous, every lead and wire bent perfectly in the direction the builder wanted it to go. Point being, both 22uF/25v cathode caps were installed right alongside it's 10 watt resistor, when they could have been separated some. Seems the designer wasn't worried about enough heat from those big 10 watters baking those little caps.

    I did bend them away a little, though.

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    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    I had a boutique hand made Tone King in recently. It had 4 6V6s with two sharing a 10 watt cathode resistor, and two sharing another. This amp was meticulous, every lead and wire bent perfectly in the direction the builder wanted it to go. Point being, both 22uF/25v cathode caps were installed right alongside it's 10 watt resistor, when they could have been separated some. Seems the designer wasn't worried about enough heat from those big 10 watters baking those little caps.

    I did bend them away a little, though.
    Knowing how to make things look good and knowing what works best or longest are mutually exclusive.

    Witness the reverence for HiWatt wiring. HiWatt amps are known to have some degree of buzz and hum due to daisy chained grounds that were done to preserve a neat appearance. It's the same with the cathode cap next to the cathode resistor for power tubes. It's done all the time in the original amps, sure, but those caps DO fail. And often sooner than other electrolytics in the same amp. It surely could be due to the heat from the cathode resistor, which DOES get quite warm or even hot depending. The decision to place the cathode cap elsewhere is along the same lines of not putting photographs in a sunny window. Perhaps they would look good there, but it's only a matter of time before the poorly chose location causes damage.

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    I'm not a factory so I don't worry abkut saving pennies on a thousand units, but for the price of resistors, I use 10W for every power tube cathode resistor. Sometimes 20 or 30 if it has 6L6s or EL34s. I call it cheap insurance.

    Justin

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    Quote Originally Posted by E biddy View Post
    ...If you have approximately 20V dropped across a 250ohm resistor...
    That voltage may increase massively when a class AB amp gets heavily overdriven, eg 35V. The point being that things are a lot nicer if part specs accommodate worst case conditions, the engineer's job is to work out what conditions cause that worst case. Idle is just part of the picture, other aspects include power up, power down, max square wave, 1/2 power square wave, inductive load etc.

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