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2005 Vox AC30 , AC30CC2X

Discussion in 'Electronic Repair' started by N Cook, Apr 4, 2007.

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  1. N Cook

    N Cook Guest

    snap almost
    Cured the main troubles in valve preamp and amp, suspect EL84 and 2 badly
    soldered pcb wire links (lead free solder again, note only 2 or 3 years
    olkd) . But there is a low level throbbing rumble noise
    obviously associated with the tremolo oscillator on the effects board, as it
    cycles up and down with varying tremolo pitch and depth.
    The owner lives with this but is there a simple cure, assuming its a
    grounding/screening/decoupling/smoothing problem.
     
  2. N Cook

    N Cook Guest

    Can anyone say for what mucic reason there is a valve/tube rectifier in this
    amp rather than 1 or 4 silicon diodes ? Loads of op-amps so obviously not
    overall faithfull to 60s technology
     
  3. J M Goodey

    J M Goodey Guest

    The message <ev4rjj$qac$>


    A valve rectifier will start providing H.T at the same time as the O/P
    valves draw current.
    If a diode bridge is used the H.T may rise above the working voltage of
    the smoothing caps until the O/P valves start to draw current.
     
  4. N Cook

    N Cook Guest

    I'd not considered that, but a thermistor plus Si diode/s would avoid that
    potential problem. o/p valves for the soft limiting in overdrive but a
    thermionic rectifier seems unjustified/unnecessary instead of diodes plus
    thermistor.
     
  5. Ron(UK)

    Ron(UK) Guest

    Maybe it`s the monetary and snob value of having the words vacuum tube
    rectifier in the blurb. Some amps have both ss and valve rectification,
    switching from one to the other does produce a variation in the sound.

    Ron(UK)
     
  6. If the PS is producing a variation in the sound it's poorly designed.
     
  7. Ron(UK)

    Ron(UK) Guest

    No, it`s intended that way, an amp with a valve rectifier has a certain
    amount of sag in the ht voltage. On ss amps, there is often a resistor
    in the feed to reproduce the same sag.

    Ron(UK)
     
  8. Ron(UK)

    Ron(UK) Guest

    We are talking guitar amplifiers here btw, not hi fi.

    Ron(UK)
     
  9. N Cook

    N Cook Guest

    This throb noise is due to loading of the HT which goes up and down with
    tremolo speed and depth. With EL84s in and all 12AX7 pulled the throb is
    still there, no signal throughput of course, even makes the standyby LED dim
    in sympathy when at the deepest depth. Only stops if I disconnect the ribbon
    that goes to the tremolo pots to kill the tremolo oscillator
    Leaving, likely ,just a JRC 2147D high voltage dual op-amp (+/- 28V not
    300V) and a 2SC2910 associated with the tremolo area and something marked
    SiLN D25D or SiLN D250 a 3 pin TO92 device, the S being the Nazi SS
    lightning flash S.
    Anyone recognise the Logo ?
    Whatever this is thyristor ? is connected via 220K to the HT line.
    I thought tremolo just bent the frequencies in a cyclic fashion, not
    modulating the HT.
     
  10. N Cook

    N Cook Guest

    Angled/Jagged Si logo is probably Supertex, so from their site its a LND250
    500V, 3mA ,1K RDS on, n ch MOSFET , even as a SOT23 package
     

  11. More importantly, Why did he leave all those monkeys here in
    Florida? Those movies were filmed at Silver Springs, a few miles from
    Ocala. A few years after they stopped making the Tarzan movies there
    was a monkey epidemic in Ocala. The nasty little boogers were all over
    the place stealing food, and flinging crap at people. In fact, it kind
    of reminds me of some of the critters I encounter on USENET. :(


    --
    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
     
  12. N Cook

    N Cook Guest

    The LND250 had failed, didn't pass the DVM diode, lead swap, test.
    Erroneously replaced witha non-depletion type n channel Mosfet but at least
    the rumbling had gone. Owner always ran it with minimum tremoplo speed and
    depth which gave minimum throbbing.
    Still no tremolo function. Now to find a 400V to 500V depletion mode mosfet
    from somewhere.
    BSS126 and BSP135 are a couple near equivalent device types found
     
  13. N Cook

    N Cook Guest

    What I like to do. If something is as rare as hen's teeth then bodge up a
    work-around with standard parts.
    This high voltage , but depletion type Mosfet, is an active part of the
    tremolo circuit , there is no separate oscillator. So re-biased the mosfet
    up a few volts.
    Using a TO220 size standard IRF740 enhancement type, fudged pinning, bled
    off the 28V line with a 2.7V zener, couple of resistors and a preset pot, to
    the gate, now have a fully working work around this tremolo problem. Full
    range of .2 to 20 Hz or so and 0 to excessive depth. Its a bit too critical
    on preset setting but the owner had not used the tremolo for ages.
     
  14. N Cook

    N Cook Guest

    So as research is not wasted
    For the archives , some high voltage, depletion type , n channel mosfet
    type numbers
    LND250
    LND150
    C633 , Teledyne
    BSP135
    BSS126
    DN3135
    DN3145
    DN3545
    DN2540
    BSR58 , BSR56 ?
     
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