Where to buy wirewound resistors for screen?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by FuzzOff, Jan 2, 2008.


  1. FuzzOff

    FuzzOff Member

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  2. rooster

    rooster Member

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    Dale is good stuff, or at least, always was. I generally use 5W, but that may be a bit of overkill.

    rooster.
     
  3. Trout

    Trout Member

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  4. Wakarusa

    Wakarusa Member

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    For what it's worth, Mouser part # 660-MOS3CT631R471J works well. It's a 3W metal oxide flameproof instead of a wire wound. The advantages are first that they're $0.20 each and second that they're length is very close to the distance between pins 4 and 6 on an octal socket (so they don't get in the way of other stuff).
     
  5. John Phillips

    John Phillips Member

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    I don't agree at all.

    Resistors are NOT fuses and should not be used as such. When they destroy themselves, they can burn (literally) which is a very serious risk to surrounding parts and wiring - including the power cord in some amps. If they burn the filament wiring loom - which is common - you can end up with a far more serious short on the PT anyway. Even if they don't burn anything else, the heat and soot produced can carbonise the tube socket to the point it becomes conductive (this also can short the PT badly) and will have to be replaced.

    A failed screen resistor will never blow a tube - it's the other way round. But if the bad tube does blow the resistor, you then can't get the amp running again without a trip to the bench, even if you put a new tube in, because it won't work without the resistor (this is a common problem).

    If it is necessary to allow a resistor to burn in order to protect the PT, there is one of three things wrong with the amp design: either the PT isn't robust enough, the primary fuse value is too high, or the amp needs a secondary (HT) fuse.

    Fuses are designed to fail safely, and can be mounted in holders so they can be changed without taking the amp apart.

    If you don't think the amp is robust enough to survive a tube failure for long enough for the fuse to blow without burning out the screen resistors, the amp is badly designed. If that's the case, fit a HT fuse, do not try to use a resistor to do the job instead, which it was not designed to do and which can cause other problems.

    Any really well-designed and built amp should never blow anything other than tubes and fuses. All the other components should be spec'ed heavily enough to survive until the appropriate fuse blows.

    Sorry, I don't normally disagree with you, but this advice of using screen resistors as fuse is one of my pet peeves. I've seen so much damage caused by doing this and never one single case where it definitely saved anything. I always up-rate screen resistors to 3W (470 ohm) or 5W (1K ohm) glazed-ceramic wirewound (the type with the highest short-term overload rating), and I've never seen one case where a transformer has failed as a result either.


    Here's a good example of what happens when you use a resistor as a fuse:

    [​IMG]

    A tube shorted, the resistor caught fire, and actually the fuse then blew. The owner then (rightly) pulled the obviously blown tube - apparently it was still glowing red when he quickly looked round the back - and its pair on the opposite side, replaced the fuse and tried to get through the gig. If the resistor had not burned out, he would have... but unfortunately the charred remains shorted to pin 5 via the cap leg which it is directly underneath, feeding the B+ voltage into the bias supply, blowing the bias cap and frying the remaining two tubes. The only survivor was the one good one pulled initially. So that's three expensive power tubes destroyed and a fair amount of repair work needed, where if a wirewound resistor had been used, only the first tube and a fuse would have blown.
     
  6. Blue Strat

    Blue Strat Member

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    Haha! I knew John would object to OTM's statement. :crazyguy

    It's all true but it's only theoretical unless the user installs a B+ fuse (which virtually none will). So the point is moot.

    The resistor doesn't kill the tube (and possibly the OT at the same time)...the POWER RATING OF THE RESISTOR allows the (over conducting)tube to fry which often takes out the OT.

    The only good answer is to install high wattage screen resistors AND a B+ fuse. Simply installing high wattage screen resistors is IMHO a bad idea.
     
  7. John Phillips

    John Phillips Member

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    I have never seen one case where it caused damage provided the primary fuse rating was also not incorrectly increased (in which case all bets are off anyway, really).

    But I do agree that fitting a HT fuse as well is the best solution, by far, and it puzzles me why so many amps don't have them (even now).

    If you have an old Fender, the easiest and best place to put the HT fuse holder is where the ground switch was...
     
  8. Blue Strat

    Blue Strat Member

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    I have, serveral times. One time that I remember well was a Bassman head. It distorted at very low volume. First thing I do is check the bias on the power tubes (which is like a doctor using a stethoscope as it gives indications of MANY things all at once). The power tubes were running at about 17.5 mA which seemed odd. That's not low enough to notice anything sonically.

    Then I noticed that the whole back of the amp was very hot, especially in the power tube area. DOH! The power tubes (NOS JAN Philips 6L6GCs) were REALLY running at 175mA each and didn't really seem to mind being abused like this. The correct line fuse was installed and didn't mind either (never did the math on this). Eventually this would have killed the power tubes, OT, or both.

    Turned out the bias supply cap was leaky (not visibly/physically leaky, electrically leaky) and running the power tubes wide open.

    I've also seen several other amps with fried OTs and correct line fuses.
     
  9. John Phillips

    John Phillips Member

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    So have I, but I doubt that using the resistors as fuses would have changed the outcome in any case.

    If the tubes were running happily at 175mA, would even stock screen resistors have blown before the OT or PT overheated? Probably not.

    If a tube shorts via the plate, it will also not blow the screen resistor because the excess current isn't going through it... but it's plate shorts that kill OTs, not screen shorts, because the current in a screen short doesn't go through the OT anyway (although admittedly could potentially blow the choke). Yes, it's true that blowing the screen resistor will usually shut the tube down - but not always if the short is directly from the plate, in fact I think the amp in my pic didn't, since otherwise the fuse wouldn't have blown after the screen resistor (which was open circuit, to the tube).

    So I can't see any real logical justification for using the screen resistors as substitute fuses - only one 'maybe' reason - and many very good reasons for not doing. It's the worst of all possible worlds IMO - worse than over-spec resistors and no HT fuse.
     

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