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headphone tube amp concern

Discussion in 'Electronic Design' started by John Cash, Dec 1, 2006.

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  1. John Cash

    John Cash Guest

    I would like to build a small tube amplifier for my headphone. It is
    working in classic class-A with a penthode 3E5/3S4 working at about 67V and
    a small output transformer 8000 ohms/32 ohms. The cathode of these babies is
    also the heater with a low 1.5V heating voltage. Not top of the line tube
    but, well, they sounds ok for me.

    The prototype I built is working nice but I am concerned about the
    possibility of a tube giving up abruptly while listening to music.

    Is this a possiblity for a tube to cut off abruptly when dying, thus sending
    a loud "pop" in the headphone. Would not be good and probably dangerous for
    listener's ears.

    I don't know if tubes can behave that way or this cannot happen because of
    unknown reason for me.

    Anyone got experience in this matter?

    John
     
  2. Tim Shoppa

    Tim Shoppa Guest

    Tubes are unlikely to fail non-conducting in a really sudden manner. If
    the filament burns out or the emission drops, these are not all that
    sudden.

    Inter-element shorts (say, grid to cathode, or plate to grid) and
    internal arcs are far more likely.

    I do not know of any attempts to mitigate the volume in case of such a
    failure. Hooking the primary of the output transformer straight between
    B+ and ground will undoubtedly make a loud pop but this isn't too much
    different than any output driver in any situation failing shorted. Not
    only have all us had loud pops in our ears, we've also played with
    mercury when we were kids, and you can see that we're all PERFECTLY
    normal here!

    Tim.
     
  3. Did you know that old lead house paint tastes really good?
     
  4. John Cash

    John Cash Guest

    Of course, I know everyone is "normal" here ;)

    So it is more likely a short between grids and/or plate that could produce
    this sudden pop. I was thinking of adding some clipping elements but this
    can be tricky not to have an audible effect on normal sound peak.
     
  5. Eeyore

    Eeyore Guest

    Smells interesting when you burn it off too.

    Graham
     
  6. Eeyore

    Eeyore Guest

    Nothing to worry about honest.

    Graham
     
  7. The problem with putting a varistor or transient suppressor directly
    across the headphones is...?

    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
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    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  8. Eeyore

    Eeyore Guest

    More to the point, why's he worried about a 4 volt impulse ?

    Graham
     
  9. Joerg

    Joerg Guest

    That's a problem. These don't come gold plated ...

    < duck and run >
     
  10. Eeyore

    Eeyore Guest

    That could be fixed !!! Kerching.

    Graham
     
  11. Robert Baer

    Robert Baer Guest

    Tubes do not have that problem; usually the gain slowly decreases due
    to lower filament emission long before the filament would even think of
    opening (and i have never in 50+ years heard of a tube filament opening
    if powered properly).
     
  12. Robert Baer

    Robert Baer Guest

    Do not worry about these extremes.
     
  13. John Cash

    John Cash Guest

    If you never experienced that problem then... I go on with my project!

    Thanks!
     

  14. So, the hundreds of bad tubes with open filaments I replaced in the
    '60s and '70s were imaginary?


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  15. Guest

    Are you prepared to certify that the filaments were "properly" powered?

    Filament widings on power transformers tended to be after-thoughts,
    with a couple of extra turns to deal with peak loadings, that didn't do
    anything good for the life-time of the filament.
     
  16. John Cash

    John Cash Guest

    Robert may be right...

    He probably meant that if filaments are softly powered on or off, not the
    crude switching through a conventional switch.
    Same problem with incandescent lamps, their filaments (usually) break when
    you turn on the switch. Hardly ever they go off in the middle of evening or
    night unless they are stressed by a mechanical shock or severe overvoltage
    peak from the mains.
     
  17. Yes, we had the U series tubes, those had 150mA heaters (mainly radio),
    and the P series tubes with 300mA heaters (TV).
    Those tubes were put in series to work on exactly 220V (Europe).
    Thsere were ceramic decoupling caps from heater to ground, and thse sometimes
    shorted, or a heater touched a cathode or some flasho-over in the TV HV stage,
    and the heater would get a lot more then 300mA as no only a few tubes were on the 220V.

    T1 T2 T3 T4 T5 T6
    0 mains -^---^---^---^---^---^- 220v mains
    | | |
    === === === C3
    | | | etc
    /// /// ///
    ------> + 311V
    | |
    220V mains + |
    bridge rectifier ===
    - ---
    0 mains | |
    /// ///

    You will note high DC / AC voltages between cathode and heater!
    Think wat a defective C3 would do, or a heater-cathode short for tube T5.

    Later European voltages were upped to 230V, and that sure must have cause
    problems with electrolytic caps and heaters and flash over for those who
    did hang on to their tube TVs.
    Before that time I have replaced tubes with defective heaters in TVs too.

    Series tubes saved a transformer.
     
  18. And the early bird gets the worm, which is fine if you like worms for
    breakfast. ;-)

    Cheers!
    Rich
     
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