I want to replace my screen resistor in my xxx can anyone tell me about this?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Igneous, Jul 24, 2005.

  1. Igneous

    Igneous Supporting Member

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    I cant imagine in being too difficult...
     
  2. TheAmpNerd

    TheAmpNerd Member

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    I guess it would depend what xxx amp is?

    For example, if it were something like and older
    Peavey Mace or Classic amp, where in their
    infinite wisdom the manufacturer put the
    resistors under the small circuit board
    where the tubes sockets are, think nicely.

    First you have to drill out all the rivits.
    Then you have to desolder all the
    wires and tube sockets,
    finally you have access
    and can remove the
    resistors and place
    the new ones in.

    Then you have
    the joy of putting
    it all back together
    again. So you better
    have some new hard
    ware and stand offs to
    mount the little circuit boad
    back into the amp and solder
    it all back up again, test it out.

    If you did anything wrong...
    Repeat the process.

    Of course, there are some short cuts,
    you figure those out after you done
    some of it before. You might get
    away with desoldering it in place,
    but then you have to dick with
    it to get the resistors out.

    Sometimes they won't budge,
    and you have to remove the
    board any way. Othertimes
    if you have good skills you
    coax them out and resolder
    on the boards top. Keep
    in mind you need descent
    skills and should bend the
    leads for good contact area
    on the pads before soldering.

    NOW if you were talking about
    an older vintage Fender, Gibson, or Marshall
    amp, well just open her up,
    desolder the old ones and
    solder in the new ones.

    Sometimes on those special amps,
    you get to hack saw the fricking chassis
    bolts off first, because some hack stripped
    them out the last time they had the amp
    apart. Oh the joy of simple repairs.

    Naw, it won't be hard at all, and in fact
    it might not be. : )
     
  3. Igneous

    Igneous Supporting Member

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    Okay, your being fecetious, correct?

    Since the xxx was made in 03, im assuming its new. Just wondering what the process was. is all. Thanks-Ig
     
  4. Igneous

    Igneous Supporting Member

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    Its a Peavey"triple x"
     
  5. TheAmpNerd

    TheAmpNerd Member

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    More then just a grain of truth.
    That is how peavey made those
    amps. Why the stuck the resisters
    on the underside is anyones guess.

    I haven't seen the inside of the
    Peavey XXX, open it up and see.

    If it turns out to be an easy fix
    you are in luck!
     
  6. John Phillips

    John Phillips Member

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    The rivets are a pain, but you don't have to desolder the tube sockets and wiring I don't think... I don't remember having to anyway. When you put it back you can just use sheet-metal screws instead of the rivets. Not too bad.

    Alternatively, you can just figure out where the resistor is and unsolder its lead wires, poke it through the board and let it fall out underneath (remembering to shake it out of the chassis before you power the amp up!) and solder the new one in on top of the board ;). Not that I have ever done that of course (cough :)).

    I haven't worked on a XXX yet - they seem pretty reliable. But modern Peaveys tend not to use rivets, so hopefully it shouldn't be like that anyway. They do use large amounts of hot-melt glue though...
     
  7. Igneous

    Igneous Supporting Member

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    Well, I really dont know which one it would be anyhow. How does one find out?
     
  8. TheAmpNerd

    TheAmpNerd Member

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    Hey John, you must have gotten on of the easy ones.
    One that came in was one of the ones from hell,
    it was a while ago but still.

    If I recall the resistors were stuck to the chassis as
    well. Talk about that 15 min fix that ends up eating
    your lunch.

    Oh come on, say it ain't so.


    Nor I, I think the Peavey designs have gotten better
    for service though. I haven't had any of their
    newer stuff come in.

    The absolute worst amp I've seen for service was that
    Acoustic 100 watter. No access to the board as it
    had wires soldered on all four sides of the circuit board.

    However, you could just barely clip out the faulty
    componants and solder to the existing leads.

    Ah, tales from the bench.
     
  9. John Phillips

    John Phillips Member

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    Sorry :).

    I do always prefer to do the job 'as factory' if possible, but sometimes time or customer's budget (and the experience to spot a potential lunch-eater before it happens ;)) say otherwise...

    What I do know is that in twenty years I have never let an amp leave my bench that wasn't 100% safe, and as reliable as the circumstances allow. If that can't be done I don't work on it.

    That kind of design can be a real pain. I don't like doing that, since it can cold-joint the original solder connections under the board where you can't see them or get at them to fix (as I'm sure you know)... there's no easy answer though.


    Igneous - if you get inside and the resistor is at all hard to get at, you can save yourself some trouble in the longer term if you replace all the screen resistors with higher-powered glazed-ceramic wirewound types (they're usually a grey/green cylinder about 3/4" long x 1/4" diameter - if the originals are 5W, use 7W, or the largest that will fit) - these are the most robust type and will tolerate the short-term overload caused by a tube blowing for long enough for the fuse to go (assuming you have the right value in). That way you'll never need to replace one again.
     
  10. Igneous

    Igneous Supporting Member

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    Hey - Thanks for the 411. I'll take a peak when I get the chance. Whats your fav. place for parts on the web?
     
  11. JerryP

    JerryP Member

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    The tube sockets in the XXX aren't riveted in. You have to remove the screws holding the tube sockets in and a couple other screws on the top side of the chassis and the power amp board will come right out. There's a few connectors that you'll need to unplug too. The XXX uses a double sided circuit board so becareful not to damage the board. They can be a pain if you're not handy with a soldering iron. Change them to 1K 5 watt resistors.
    Jerry
     
  12. TheAmpNerd

    TheAmpNerd Member

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    Who's the mfg?

    Anyone know a US distributor?

    Yes, these are beefy and a lot nicer then
    the sand box type.
     
  13. John Phillips

    John Phillips Member

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    I don't know who actually makes them, or where to get them online... I just buy them from a specialist electronics store. They come loose and unmarked or with whatever the distributor's brand is on them. They're all so similar I suspect there is just one actual manufacturer though.
     
  14. JerryP

    JerryP Member

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    You can get them from Mouser Electronics. The part # for the 1K 5W is 280-CR5-1.0K
    Jerry
     
  15. mark norwine

    mark norwine Member

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    Maybe I'm missing something....

    Why do you want to replace the screen resistors in the first place? Is there some design flaw that renders the stock value / size incorrect?

    As for increasing the power rating "just to be safe", I surely would NOT do that.

    When a screen shorts the current skyrockets. The result is usually a blown resistor, and I contend that "that's great!" I think of the resistor as a fuse. If you oversize it, the resistor won't blow......the next "weakest link" might, and depending on the amp design, that could be more disastrous then a couple of screen resistors. I wouldn't count on a slo-blo mains fuse to pop "in time" to save componentry.
     
  16. John Phillips

    John Phillips Member

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    You may have to replace a screen resistor if it's blown (I suspect the one that started this thread was).

    As for using resistors as fuses, I completely disagree.

    IMO an amp should always be well-enough built that nothing ever goes wrong except tubes and fuses. The resistors should be capable of taking the current of a shorted tube until the fuse goes, period - as should all the rest of the components.

    If not, and you have a tube failure at a gig, you can't rely on a new tube and a new fuse getting you out of trouble.

    Not only that, burning resistors can cause other serious damage due to carbon deposits or setting fire to things (like wiring insulation) nearby. Resistors are not fuses and should not be used as such.

    If you build the amp well enough and fuse it correctly, you'll never be in a position where someone has to ask about taking it apart to fix something.

    Yes, it is a good idea to fit separate HT fuses as well as mains primary ones - and many makers do.

    Just my opinion, but I don't think I've ever seen an amp 'saved' by a burning screen resistor when it has the correct fuse value in - even if it only has a primary fuse.
     
  17. mark norwine

    mark norwine Member

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    I don't disagree.....

    With fuses on the B+, with fuses on the filaments {both sides}, with fuses on the mains....you're dead right: resistors ought not be used that way. If all secondary circuits were properly fused & sub fused, all would be right with the world.

    But the overwhelming majority of amps don't have that....they have one mains fuse, usually a SLO-BLO at that.

    And consider the purpose of that fuse: is it to save the transformer? NOPE! It's to kill the mains current if / when the transformer craps itself. A fully melted transformer can / will go up in flames way before the 15A or 20A breaker at the panel trips....thus the purpose of suplemental fusing. The mains fusing...although essential to any right-minded individual....was "designed in" based on the requirements of UL, CSA, LA Fire, etc. The fusing in most amps meets the bare minimum to keep a fire from starting. It's not there to save the amp...it's there to "save the day" after the amp has already gone south. Such is the concept of supplemental fusing in general.

    My contention is this: in a typical {shall we say Fender?} amp...if you put in high-powered screen resistors, they won't blow when a screen shorts to a filament, but the transformer can. I've seen it happen too many times to count {25+ years of amp repair behind me}.

    If you have an amp that's properly fused at *all of the* the critical points, then you needn't "count on resistors to blow". But do you have an amp that's "properly" fused? Or "minimaly" fused?

    I stand by my previous post.

    mn
     
  18. Igneous

    Igneous Supporting Member

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    Finally, an interesting topic. Thanks for the help guys. Much appreciated. I just started dabbling with circuitry with a couple of amps to play around with.

    I asked about the SR because, as is well rumoured, the early PV 3X's need to replace them to work with eurotubes el34L's, which in turn, leads me to suspect that it would be beneficial to replace the SR'c anyway with most el's...?



    -Ig
     
  19. mark norwine

    mark norwine Member

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    Let me see if I understand.....

    I don't know the amp, but as I understand it it comes with 6L6's, right?

    And you're converting to EL34's, right?

    6L6's and EL34's have different operating points. An EL34 typically wants a larger value screen resistor....1K ohms is common.

    6L6 amps are usually found with a lower value.....470 ohms is common. I don't know what the stock value in this amp is, but it might already be 1K.......6L6's will work with the higher value, but EL34's will absolutly not work with the lower value.

    If the amp has 470 ohms...change to 1K ohms {I use 5W}. If it's already a 1K ohm resistor, you can probablly leave it alone.

    Is that the swapout you're talking about doing?

    Another required change is "Pin 1". In a 6L6 there is no "pin 1", so many amp makers use the socket's pin 1 position for some other tie purpose {like a terminal point}. But an EL34 uses pin 1 as the suppressor grid, and it needs to be connected to "something specific".....usually ground {although not always!}. The point is: if you're changing to EL34's, pin one needs to be "dealt with" in one fashion or another. Usually & most commonly, it's "tied to ground" by connecting it to pin 8.

    mn
     
  20. JerryP

    JerryP Member

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    The XXX is setup to use 6L6's or EL34's from the factory. It uses 100 ohm 5W screen grid resistors. Bob at Eurotubes likes to sell guys the JJ E34L tubes even though Peavey says the amp is not designed to use them. Peavey started using 700 ohm resistors in the XXX when the JSX came out to deal with the problem of all the broken amps and to allow owners to use to E34L. I prefer to use a 1K 5W when I do the change. I agree you want the resistor to fail before other items.
    Jerry
     

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