5W or 10W cathode resistor

Discussion in 'Amp Tech Center' started by mherrcat, May 30, 2019.

  1. mherrcat

    mherrcat Tele-Holic

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    I'm getting ready to do some bias tuning on my 5E3 clone and have various resistor values in both 5W and 10W ratings. Is one or the other a better choice?
     
  2. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    a 5W / 250R seems to be the go to choice for starters. They do get a bit hot, but most folks have used them for oh about 60 years or so... your call :)
     
  3. mherrcat

    mherrcat Tele-Holic

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    I have several, 250, 270, 300, 330, in both 5W and 10W ratings. I had seen some mentions of using 10W resistors in that position and was just wondering. Some of the 5W are considerably smaller physically; like half the length and diameter of the 10W.
     
  4. Andy B

    Andy B Friend of Leo's Gold Supporter

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    10W if it fits.
     
  5. JD0x0

    JD0x0 Poster Extraordinaire

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    10 watt with aluminum casing that you can secure to the chassis. Wont be going anywhere, and it will be able to dissipate that heat quickly. You shouldn't ever have to worry about that resistor going bad.
     
  6. Nickfl

    Nickfl Tele-Afflicted

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    A 250r resistor in a typical 5e3 is probably dissipating about 1.5 watts at idle, even with the rule of thumb of using a resistor rated for 3x what it will see at idle, a 5w should be fine with room to spare. A 10w would be fine too and run a bit cooler but it's not necessary.
     
  7. Phrygian77

    Phrygian77 Tele-Afflicted

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    Resistors power dissipation will derate at different temperatures depending on their construction. Wire wounds usually derate at lower temperatures than metal film, for example. That's something that I think people tend to forget. In this case, a 5 watt wire wound away from the tube sockets should be fine. Just because I'm somewhat eccentric, I'd use 2x 3W 500 ohm metal films in parallel. :D
     
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  8. mherrcat

    mherrcat Tele-Holic

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    This occurred to me yesterday as well.

    So while we are on the subject of different types of resistors, I have some 3W 470 Ohm and 1K Ohm resistors in both metal film and metal oxide that I was planning to use for screen resistors and a couple of 5.6K metal oxide I planned to use for grid stoppers.

    Are metal film or metal oxide preferred for those positions?

    All the cathode resistors I have at the moment are wire wound.
     
  9. dan40

    dan40 Tele-Afflicted Silver Supporter

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    You mentioned that you had 250, 270, 300 and 330r resistor to experiment with. I have found in my amps that such small changes in value won't make much of an appreciable difference in current draw. You could probably start with the 330r and see if the bias cools a bit but it may take a slightly higher value to see much of a change.
     
  10. Asmith

    Asmith Friend of Leo's

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    I'd use a 5 watts if it's common enough. The way I see it a larger wattage resistor will generate the same heat. The resistor will still drop the same voltage across it and will conduct the same current and produce heat as a result. If 5 watts is enough its enough.
     
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  11. mherrcat

    mherrcat Tele-Holic

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    Using the bias calculator on Rob’s website and with the 270r that’s in the amp (measures 268), and a 5Y3gt, the 6V6s are running at 107%.

    I had posted all the other voltages I measured in another thread regarding the 335-0-335 PT and B+ voltages so I’m not going to bring all that up here again.

    I am still curious about metal film vs metal oxide resistors though...
     
  12. JD0x0

    JD0x0 Poster Extraordinaire

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    Bigger wattage has a bigger body, it can and is meant to dissipate more heat. Especially if you get one of the aluminum cased ones and secure it to the chassis.. More heat dissipation means extended component life.
    [​IMG]
     
  13. Asmith

    Asmith Friend of Leo's

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    As long as the resistor has enough space around it then it shouldn't matter, a heat sink is nice if heat is a concern but this isn't going to be on a PCB or in a confined space. Your right in thinking generally a larger resistor won't get as hot but as far as the resistor is concerned (as long as it's used as intended) the dissaption in watts is what matters not the temperature. If it's only idling at 1.5 watts the 5 should be plenty.
     
  14. mherrcat

    mherrcat Tele-Holic

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    Correction on my last post: with the 5Y3 the tubes are at 91.7%; with a 5V4 they are at 107.5%.
     
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