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Valve amplifier loosing 'gain' over 20 minutes

Discussion in 'Electronic Repair' started by Rudge, Apr 15, 2006.

  1. Rudge

    Rudge Guest

    A friend has a 1950's Tannoy PA valve amplifier which works when first
    turned on but the output steadily drops to zero after 20 minutes.

    That's his description of the fault. I have not tested the amp yet.



    Thinking about it, I'm assuming it is a problem with a capacitor.

    I'm not familiar with all the failure modes of old capacitors. I know that
    old electrolytics loose capacitance and become leaky when the dielectric
    degrades.

    Could this fault be caused by a leaky interstage capacitor which is
    affecting the bias of the next valve stage?



    Or could it be a valve problem?
     
  2. I.F.

    I.F. Guest

    Dried out electrolytics usually improve as they warm up, leaky capacitors
    usually worsen with temperature which among other things can cause biasing
    faults - glowing tube plates is a dead giveaway!
     
  3. Arfa Daily

    Arfa Daily Guest

    At this point, that's a bit like saying " My friend says his car engine
    loses power after 20 minutes. Could it be a bad spark plug ? "

    Well yes, it could, but there's also a score of other things it could be as
    well. Same applies with this amp. Until you have looked at it yourself, and
    have some real observations of the symptoms to tell us about, any opinions
    that we might give are pure speculation, and likely to be misleading and way
    off-beam.

    Give us some more info and checks to work with, and we can probably help
    more.

    Arfa
     
  4. The resistors have probably gone high in value, easily checked with a
    meter.

    The "paper" capacitors will probably be leaky, change them anyway. You
    can get polypropylene ones which "look right".

    Be gentle with the electrolytic- if you hear any hissing sounds run!

    I remember replacing the output valves in a Marshall Major back in the
    70's and the output power went from 50W to 330W!

    There are several specialised newsgroups and websites.
    (I remember reading a wonderful account of complete rebuild of Quad amp,
    it wasn't until it was finished that the bloke realised it was a 100V
    line job!.)

    Obligatory safety warning:- I often have to tell the game-boy generation
    at work to keep their fingers out of old equipment with >>50Vinside.
     
  5. Mark

    Mark Guest

    gassy tubes also heat up and draw grid current over a period of time
    like that...

    I had an old Eico scope that did that due to a gassy tube...

    check the plate voltage of all the tubes as it warms up...

    if you have a gassy tube, the plate voltage will DROP as it heats up..

    of course it could be 100 other things as well..

    Mark
     
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