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Vox AD30VT small combo, 2007, PbF

Discussion in 'Electronic Repair' started by N_Cook, Oct 22, 2010.

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  1. N_Cook

    N_Cook Guest

    Was intermittant no output other than low level hiss/hash, even on first
    powering up with me. Now permanent no output except background, but phones
    output ok and its diverter sw ok. Output hybrid ok with feed to its input or
    either pin2 or pin 6, IC2, JRC 2082DD further back. No springing back to
    life with prodding to either board
    I suspect a control signal is not getting to DG211 an sw from the SM laden
    top board. Anyone more insight ? to avoid a load of signal and control line
    tracing. Probably end up bridging phones across to o/p permanent and no
    bypass of output
     
  2. N_Cook

    N_Cook Guest

    Although only -15V on the valve it is necessary to have something in the
    socket of some pinning like ECC82 or 83, to give the faux valve signature
    delay-on via the heating of heaters and then produce a gating signal or
    something. Otherwise any valve effects are produced in the digital stuff of
    course, the valve just being sham, or anyone know any different? The phones
    output is direct it would seem. Bad contact between some pins and socket ,
    compounded by low V and low I presumably, would seem to have been the
    problem. I was testing without the valve in place as previously meeting
    these I thought the valve was for show only, but a marginally bit more than
    that.
     
  3. N_Cook

    N_Cook Guest

    In the end it was duff PbF solder meeting 50 yearold technology, the
    pcb-type pins of the valveholder. After redoing solder could simulate the
    reported previous intermittant situation of half power and no power by
    hovering a variously angled hot valve over the socket pins. The 2 cathodes
    are tied to -15V , beyond that quite complex, traces to njm2082 dual FET
    opamp and the DG211 an. sw. etc, certainly no proper anode Vs
     
  4. Meat Plow

    Meat Plow Guest

    Last one I worked on had some bad solder joints. Can't remember much more
    detail.
     
  5. N_Cook

    N_Cook Guest


    The previous 2 I looked at needed the o/p hybrid replacing.
    The an. sw. IC is functional at switch on presumably for muting but not with
    absent valve, tripled up control line goes to 3.3V for a second then 0V
    after that
     
  6. N_Cook

    N_Cook Guest

    I've never seen the like, the -15V cathodes , next to nothing anodes and the
    numerous weird-looking electrolytic caps are the same as this AD30VT . The
    11 way selector for the amp types is on the digital board but some control
    line could go down to the valve area. The real test would be twin beam
    scoping of phones o/p and speaker o/p for differences. But I imagine musos
    would smell a rat if the "sound" through the phones was different to the
    speaker. Phones o/p is there whether valve is present or not, immediate o/p
    as no heaters must make them a bit suspicious surely.
    Next time I power up my valve tester I will try the lowest anode setting
    possible
     
  7. N_Cook

    N_Cook Guest

    With this vox in running order whether good ECC83 in there or bad ECC82 with
    (250V measured ) CH/R of 3 megs.
    There is 6V ac across the heaters
    cathodes are -15V wrt to ground
    grids are -14.6V
    anodes are -.04V and -.1V
    I'd not noticed before than my valve tester goes down to 20V anode setting,
    only ever use 250 to 350V.
    I wonder what gain you get with a good ECC83 running at 20 anode volts and
    zero bias , i doubt negative negative bias is settable on it. I will have to
    try progressively lowering from 250V and -2V to 20V, 0V
     
  8. N_Cook

    N_Cook Guest

    I did not expect that
    one triode of a good ECC83 on an Avo 160
    250V an, -2V g, gain 2.7
    200,-1.5, 2.7
    150, -1, 2.5
    60, -.3, 2.3
    40, -.1, 2.1
    40,0, 1.7
    Without putting an micro-ametter in there I cannot measure the anode current
    with any confidence at those low levels
     
  9. Meat Plow

    Meat Plow Guest

    Don't they call that a starved cathode configuration?
     
  10. Meat Plow

    Meat Plow Guest

    Hmmm makes sense but there is another mode they run these effects units
    in that utilize a 12AX7 called starved cathode. Unless I'm completely
    mistaken.
     
  11. Meat Plow

    Meat Plow Guest

    LOL I've only heard it referred to as starved cathode :) My Peavey Tube
    FEX uses two 12AX7's for gain and tube effects. I'm sure they run in a
    starved mode. Worked on a Mesa Boogie tube pedal with a 12AX7 in it that
    ran off a 12 volt wart. If you want some tube sound and gain the Mesa is
    the way to go although you'll pay a pretty penny for a vintage model.
     
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