weird tube experience

Discussion in 'Amps and Cabs' started by Coldacre, Jan 17, 2015.

  1. Coldacre

    Coldacre Supporting Member

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    so I bought some NOS tubes for my fathers 60th Anniversary Fender Woody Pro Junior. some JAN Philips 5751's from the 80's and some Tesla EL84's, dating back to 1964. matched pair. we put the tubes in, turn on the amp and after one magnificent full sounding chord, the volume drops off by 60%..... I smell burning..... and I immediately look at the tubes to see one of the Tesla's red plating.

    oh well. they were a cheap eBay buy and thems the breaks. I switch out the Tesla's for the stock JJ's and the amp sounds great again.

    so I took the Tesla's home and thought to myself that I'd put them in my AC15 and take a photo of the offending tube to show it to the seller. wasn't interested in filing a claim, just more of a heads up.

    I put the tubes in, turn on the amp..... and they're fine. :huh I start playing and there's nothing wrong! I played for half an hour. no issues. I can't for the life of me work out what's going on??? both Tesla's tubes are looking the same, the volume is consistent and they appear to be fine. how is this possible?
     
  2. davebc

    davebc Member

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    That's really weird, but I've had amps/tubes do similar weird stuff that I was never not smart enough to figure out.:huh
     
  3. gkoelling

    gkoelling Member

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    Read this page from kcanostubes.com The Pro Junior is listed as being OK but perhaps things have changed since Mike posted the article? Maybe there's something different in the Woody?
     
  4. DaveKS

    DaveKS Member

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    Does woody need biasing? Sounds like theyz not quite as matched as seller would have you believe, at least not at voltage the woody running at.
     
  5. Coldacre

    Coldacre Supporting Member

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    fixed bias so nothing needs adjusting there. I'll just have to assume that the Pro Junior Woody pushes the power tubes harder than the Vox - which I really didn't think would be the case, but there you go.

    I just didn't expect to see a red plating tube suddenly function as per normal..... :eeks
     
  6. boyce89976

    boyce89976 Supporting Member

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    Fixed bias means that the bias needs to be adjusted, cathode bias typically self-adjusts.
     
  7. Jeff Scott

    Jeff Scott Member

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    Yep, fixed bias means you need to replace/add resistors to perform a bias adjust on a fixed bias amp, or turn a pot if there is one in the bias circuit. The term fixed bias just means it is just that, fixed at a specifically set voltage.
     
  8. Coldacre

    Coldacre Supporting Member

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    ok, that go's a long way into explaining why it red plated! thanks guys.

    odd that it still works though.
     
  9. bgh

    bgh Supporting Member

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    The other option is to stick to tubes that are rated for that voltage. Some of the better tube suppliers can tell you which tubes are rated for a particular amp. This is the way Mesa sets their amps up - they are all fixed bias.
     
  10. The Eristic

    The Eristic Member

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    The factory fixed bias on the Pro Jr is notoriously hot, there's a relatively simple adjustable bias mod you can do (or have done) that makes it much more workable with a wider variety of tubes.
     
  11. jcs

    jcs Member

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    This is why anytime I sell NOS & ANOS power tubes out of my horde I INSIST the current draw be checked IMMEDIATELY.

    Cathode bias is only "self adjusting" to a point.......thankfully, Coldacre caught the tube before it melted down because once a tube is "red plating" it is already running WAY too hot at that point.

    ALWAYS at least observe the power tubes in a darkened room from the second you turn on the amp if you CANNOT or will not check the current draw initially with a bias rite or similar device.
     
  12. Coldacre

    Coldacre Supporting Member

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    I'm interested in what that mod is... but I'm not sure I want to sacrifice the amazing tone that the amp currently has. its a beast and it sounds huge! is it relatively straight forward?
     
  13. The Eristic

    The Eristic Member

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    It's just installing a bias trimpot on the board, not too tough. Should find it pretty easily with a quick google. As to changing the tone, you can still run the tubes hot if that's the sound you want.
     
  14. Blue Strat

    Blue Strat Member

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    Oxidation on one or more of the tube pins or loose sockets. All it takes is for ONE pin on either EL84 to not make contact, and only one tube will operate and hog all the current in a cathode biased amp. This results in severe overheating, distortion, and odors. Sometimes it will kill the remaining operating tube, sometimes not.

    Get some contact cleaner and clean the tube pins and sockets.
     

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