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Thread: 470 ohm screen-grid resistor power rating

  1. #1
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    470 ohm screen-grid resistor power rating

    Hey guys,
    just a quick question. On the Fender amps that I've had the pleasure of looking at, a few of them have had the 470 ohm screen-grid resistors replaced with large 5W sand-cast resistors. Is there a reason for this?

    I've worked on a few amps where the stock resistors were blown when the corresponding tube failed, and I was always under the impression that the resistor failing was a good thing and that upping the power rating of these resistors would put other components in jeopardy if the tube was to fail.

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    Old Timer Leo_Gnardo's Avatar
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    There have been several discussions about this on MEF. Which is the right thing to do? It's up to the tech and/or amp owner. Some techs say use fuses for fuses, others favor low power resistors which fuse under fault conditions.

    Some amps have current sensing cathode resistors, either stock or added-on, usually 1 ohm. Should we use quarter watt resistors that will theoretically handle the current but disappear in a puff of smoke should the tube short? Or something sturdier like a 3 or 5 watt wirewound? Same answer.

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    Supporting Member mozz's Avatar
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    Sometimes you need a larger resistor because smaller ones can't handle the voltage. I've used 1 watt many times with no failures, yet.

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    Quote Originally Posted by Leo_Gnardo View Post
    ...Some techs say use fuses for fuses...
    The problem with implementing that approach is that even the simplest repair may end up with a lot of re-work (fitting upated resistors and fitting fuses) and an amp full of fuses.

    An advantage of using resistor wattage as a ghetto fuse is that is it very circuit specific, eg a shorted tube may cause a cathode or screen grid resistor to fuse, taking just that tube out of action, possibly allowing the amp to limp along until a repair can be effected.
    Whereas it becomes unfeasible to put a fuse in series with every such resistor and so a few fuses end up covering more circuits each, potentially making the amp less resilient to component failure, eg a shorted tube blows a generic fuse and shuts the amp down.

    If considering using a resistor as a fuse, avoid carbon types, metal types are better, and with a flame retardant coating. Keep the resistor body well away from other stuff. Actual fusible resistors are available, so investigate those first.

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    g1
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    Marshall uses 5W screen resistors, even in their amps that use 5881/6L6.
    But they also usually have HT fuses, which Fender does not.
    Two different approaches, both are valid.
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    Quote Originally Posted by g1 View Post
    Marshall uses 5W screen resistors, even in their amps that use 5881/6L6.
    But they also usually have HT fuses, which Fender does not.
    Two different approaches, both are valid.
    I understand that either of those approaches are valid, but I'm questioning the trend of replacing 1W resistors with 5W resistors in an amp where no HT fuses exist (and aren't added). In that resulting arrangement, what happens when a power tube shorts?
    Last edited by waspclothes; 11-08-2017 at 08:12 PM.

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    Senior Member Enzo's Avatar
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    When a power tube shorts, often as not it then takes out the mains fuse. If the short is to screen or causes way excess screen current, the resistor takes a hit, It may burn up as it usually does in the stock Fender 1w case, or it may survive happy as a clam with a 5 watt. Though I do have to replace teh 5w on Marshalls now and then.

    In my experience, when Fenders burn up their 470 ohm screen resistor, I rarely see damage go any further. Fuses may help, but not always. I don't see 5w as increasing damage potential. I often use rectangular resistors there. If I no longer have 1w in my drawer, I grab 3w or 5w and install them. I don't think any further about it. I have other things to obsess over.

    Bottom line is this: 5w resistors are more likely to survive a tube failure, and in my personal view confers no particular risk to the amplifier. Others are free to disagree. On the other hand, I don't purposely increase teh wattage of the resistors. If I have 1 watts in stock, I use them. The whole deal is just not a thing with me.
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    Maybe off topic but certain amps that have a very crappy thermal design with no ventilation, tubes hanging under chassis, very high plate and screen voltages, power tubes all bunched up close to each other, etc. sometimes benefit from 5W screen grid resistors just for the fact that they can dissipate heat better than 2W ones.

    In certain amps (ampeg V4 and orange OR120 come to mind) I have seen screen grid resistors fall out or become disconnected because they get so hot the solder melted. Resolder it and it's fine. The amp and the tubes did not have a problem, the resistor just fell out.

    Also these days these two amps are perfect candidates for turn it up almost all the way to get it to distort and then use an overdrive pedal to get it to distort more, so you know that screen current is flowing heavily. I don't really see people wanting to do that with Fenders for the most part.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by nsubulysses View Post
    ......In certain amps (ampeg V4 and orange OR120 come to mind) I have seen screen grid resistors fall out or become disconnected because they get so hot the solder melted. Resolder it and it's fine. The amp and the tubes did not have a problem, the resistor just fell out.......
    I see that a lot in certain Carvin amps. I'll put 5W resistors in there just because they are better at dissipating the heat. I've tried resoldering the existing resistors and it doesn't take long before the amp is back in the shop.
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