Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: labb on August 07, 2019, 10:32:13 am
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Well, we did so good with the wire voltage rating how about we try this one...This is a quote from a well know guru. His initials are R.A.
"One more consideration for resistors: it is sometimes overlooked that resistors have a max voltage rating. The 1/2 watters and some 1 watters usually are only rated for 250-350V. Be sure to get a resistor rated for the appropriate voltage in the amplifier. I use only 1W, 500V min (continuous, 1000V surge) or 2W, 750V rated resistors."
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Geez, never considered this. Research shows: Manufacturers typically list multiple voltage ratings on a datasheet, but most often the primary concern is the maximum working voltage. Maximum working voltage is the maximum amount of voltage the resistor can withstand constantly without arcing. Maximum working voltage is often expressed as Vrms. https://www.powerelectronictips.com/keys-to-understanding-resistor-specs/ (https://www.powerelectronictips.com/keys-to-understanding-resistor-specs/)
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Back when Carbon Composition was the thing, we "knew" that excessive voltage caused micro-arcing between granules, which typically drifted the resistance higher (but sometimes just blow-up). Wire-wounds would arc between turns.
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The working voltage limit is for the voltage across the resistor. Rarely, if ever, is the full B+ across a single resistor. Usually, 250-350V is sufficient.
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That analysis may be overly focused on the steady state idle condition. At power up / with signal, Vpeak will likely push or exceed those limits for all but the lowest HT amp types.
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True, but the 1/2W resistors in common use have a working voltage of 350V and a short time overload rating of 700V.
Interesting article for those wanting to dig deeper: https://www.edn.com/design/components-and-packaging/4321070/Five-questions-about-resistors