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Bathtub curve failure or something more?

Discussion in 'Electronic Repair' started by N_Cook, Jun 10, 2013.

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  1. N_Cook

    N_Cook Guest

    A birthday present of a new set of valves in a 2009 PV classic 30 and after
    a few weeks of use , copious magic smoke and then nothing.
    V7 has a near to short varies [cold] 3 to 6 ohms or so cathode to G3, p2 to
    p9 of EL84 mechanical failure inside, burning out the HT1 to HT2 dropper
    R58, 400R. Anything that could externally precipitate such failure to check,
    before a new valve/s ?
     
  2. Guest

    With the tube/valve removed, is the grid negative with respect to the
    cathode by several volts????
     
  3. N_Cook

    N_Cook Guest

    With the tube/valve removed, is the grid negative with respect to the
    cathode by several volts????

    ++++

    I have to replace the dropper first, ridiculous mechanical design of 3 "wrap
    around" boards with
    something like 2 sets of 27 short link single filament conductors.
    Presumably copper and with age and work hardening , what is the probability
    of one of those links failing in the unwrapping/working on/ rewrapping
    process, then the probabiliy of another such link failing in sorting out the
    first failure. Replacement dropper will be wrapped around the stand off
    pillars, I think.
     
  4. N_Cook

    N_Cook Guest


    So what is the metal of those 50 or so short curved links, that you have to
    bend all of them to unwrap the boards? something specially formulated that
    does not age and work harden? I doubt it , probably deliberately designed
    like that to make repair as awkward as possible. Could easily have been
    soldered eyelets on one edge of each run.
    Perhaps you have never come across ribbon connectors that fail at a solder
    point , only because you've had to flex the ribbon to gain access . Or
    moving the discrete wires to the wire-wrap terminal boards of 60s kit?
    Anyway heatsinked the pcb ends of the pillars and 2x1K 2W plus 2K2 2W in
    there now, could not find a 400R,5W of the required dimensions to otherwise
    go in the available space
     
  5. N_Cook

    N_Cook Guest

    Anyway heatsinked the pcb ends of the pillars

    meaning artery forceps clamped around while soldering
     
  6. N_Cook

    N_Cook Guest

    For a valve/tube amp why does it say minimum load 30 ohm on the back or is
    minimum actually maximum in USA useage?
     
  7. N_Cook

    N_Cook Guest


    So its a two nations divided by a common language sort of thing
     
  8. N_Cook

    N_Cook Guest

    Anyone any experience of reducing the hum in these. Mainly due to a monster
    reverb tank that would normally
    be in a 2x12 cab, lost in the bottom, but in here only a couple of inches
    from the mains Tx
    Testing amp outside the cab then normal sort of , no input, output hum
    level.
    That hum trebled when reassembled. Found some salvaged mu-metal , so bent to
    extract and to wrap partly round this Tx, but hum level now only double, an
    audible improvement, not just by meter. As not a bass amp, how much
    performance reduction in the reverb if a tiddler tank was put in here
    instead. ?
     
  9. N_Cook

    N_Cook Guest

    I wonder where you get new mu-metal from. And, where to learn how to handle
    salveaged mumetal from oscilloscopes/ reform its cloaking character, if
    lost. Would mu-metal only around the reverb tank coils be enough? Tx nearest
    (in tank terms) the transmit end but as the hum is straight , not
    reverberrant, I assume its geting in at the far , receiving, end
     
  10. Guest

    The hum is definitely coming from the output side no matter what. However, just how are you saying that it is not induced on the input side ? It is a continuous wave, an echo would be inaudible.

    However, since some shielding does help it is obviously not induced at the input side.

    I find it hard to believe that the unit was sold like that and you are upgrading a faulty original design. I wonder if the big copper strap around thetransformer (can be internal) has cut loose. They are soldered and if thisthing is new enough to use lead free solder I think that a viable suspect.

    Can you see the salesman in the store saying "Yes it has a reverb but you can't use it because of an annoying hum caused by a design defect" ?

    Anyway, I would have a good look at the reverb unit itself. If this thing isn't junk, it has to have some sort of shielding from OTHER sources of EMI no ? What, you have to keep the thing three meters away from anything with a power transformer ? I tend to doubt it. I think inspection of the reverb unit might reveal a fault there.
     
  11. N_Cook

    N_Cook Guest

    The hum is definitely coming from the output side no matter what. However,
    just how are you saying that it is not induced on the input side ? It is a
    continuous wave, an echo would be inaudible.

    However, since some shielding does help it is obviously not induced at the
    input side.

    I find it hard to believe that the unit was sold like that and you are
    upgrading a faulty original design. I wonder if the big copper strap around
    the transformer (can be internal) has cut loose. They are soldered and if
    this thing is new enough to use lead free solder I think that a viable
    suspect.

    Can you see the salesman in the store saying "Yes it has a reverb but you
    can't use it because of an annoying hum caused by a design defect" ?

    Anyway, I would have a good look at the reverb unit itself. If this thing
    isn't junk, it has to have some sort of shielding from OTHER sources of EMI
    no ? What, you have to keep the thing three meters away from anything with a
    power transformer ? I tend to doubt it. I think inspection of the reverb
    unit might reveal a fault there.

    +++++

    It is a regular complaint by owners of these amps. May be not so obvious
    when they are used in 60Hz countries, 50Hz here. The next time I came across
    one I might explore with small pieces of mu-metal around the coils in the
    tank, and put a scope on the thing. Reading around mu-metal it seems that to
    restore its invisibility cloak effect then you just need a hydrogen furnace
    to re-anneal, I'll knock one up from my pile of scrap
     
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