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HT capacitor balancing Rs , necessity ?

Discussion in 'Electronic Repair' started by N_Cook, Feb 2, 2013.

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  1. N_Cook

    N_Cook Guest

    Well I call them HT balancing Rs anyway, in valve amps. Where 2 HV caps are
    used in series to avoid using extra high voltage single caps. I always
    thought they needed 100K to 200K across each to balance out likelihood of
    midpoint voltage drifting up or down and exceeding the rating of one cap of
    the pair.
    Hiwatt AP CP103 ,2005 bouncing back blowing mains fuse , different set of
    output bottles from before.
    Worked for about 6 hours in total and over an hour before problem
    Amp lost sound, owner turned round in time to see the mains neon fail , ie
    after sound failure.
    This amp does not have balancing Rs , unlike its 1970s versions. I will add
    a couple of 220K/1W but anything else to ponder? These amps always seem to
    have had 3A mains fuses and 3A HT fuses for some odd reason - anyone know
    why? so the mains fuse sees more current than the HT fuse in normal use,
    same in abnormal sistuations?
  2. N_Cook

    N_Cook Guest

    I thought about putting a telltale 2A fuseholder in line specifically with
    the caps and then thought better of it. A high voltage in the output Tx whan
    a fuse blows could induce very high voltages across windings but loosing the
    smoothing could mean an even higher peak voltage if the HT or Mains fuse
    should blow.
  3. N_Cook-

    I think equalizing resistors are a good idea. However, you must use
    high voltage resistors.

    A 220K resistor would dissipate 1 Watt at 469 Volts. A higher value
    resistor might be appropriate if DC leakage current through the
    capacitors is low. But you will find that ordinary resistors may fail
    due to dielectric breakdown, even if their power rating has not been

  4. Phil Allison

    Phil Allison Guest

    "N_Cook" <

    ** Not needed in most cases, the electro caps will simply balance themselves
    at a mutually satisfactory leakage current.

    The idea come from series connected, high voltage film caps which DO need
    them or it is snap crackle pop time.

    .... Phil
  5. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"
    ** Had a Marshall 100W head returned to me repeatedly for blowing its AC
    fuse in the first few minutes of the gig - of course, it tested perfect
    while on my bench, no matter what torture tests I tried.

    The cause eventually turned out to be a loose fitting IEC plug that arced
    under heavy vibration (when sitting on a cab) and made the AC power surge
    over and over till the fuse blew. I used my own IEC lead in the workshop so
    initially I never saw the fault.

    In another 100W Marshall, I found that if I turned the amp off and then back
    on again in 3 to 5 seconds, the inrush surge was much longer than usual and
    took out the 2A slo-blo fuse.

    This was due to the tube bias voltage dropping fast and recovering slowly in
    the particular model, so the bias voltage was very low at switch on but the
    4 x EL34 cathodes were still hot and so all drew a heavy DC current for a
    couple of seconds.

    Marshalls do SOOOO many fucking weird things I could write a book on them.

    ..... Phil
  6. Phil Allison

    Phil Allison Guest

    "Gareth Magennis"
    ** The magnetic surge of the power transformer alone is about 50 amps peak
    with a 240V supply - a 2A slo blo fuse can take this once in a while but
    NOT if repeated rapidly.

    A loose fitting plug in the AC outlet or IEC inlet is all it takes and the
    blown fuse that results sends most musos into a panic.

    ..... Phil
  7. N_Cook

    N_Cook Guest

    Switches, wiring and fuses seem ok
    I dummy load test at 8 ohms and this amp is used at 16 ohms. When I set it
    for 8R no sound from the attenuated quality-monitor speaker. Seems ok in the
    16R setting but perhaps when warmed from current over an hour goes open.
    That event at more than half full output would presumably blow a fuse if not
    an o/p transformer. These rotary sw are rivetted in , the 1970 version were
    sockets and double pin jumper selector type.
    2 balancing Rs added . If it bounces back again then I think I will try a
    tell-tale in line with the main caps of a 1.6 amp fuse in parallel with
    vitreous 1R 10W . Difficult to gauge what values so that the fuse will blow
    in aberrant condition but not burn out the R if either mains or HT fuse
    holds and not blow in normal seitch on and use. The minor HTs supply is more
    straightforward , for such a telltale, as tempered by 100R dropper anyway
    and cladding of that vitreous looks as new
  8. N_Cook

    N_Cook Guest

    Before adding the balancing R in the first 10 minutes the difference between
    the 2 caps voltage went from 25V to 35V and still increasing 1V / min or so
    before I decided that was evidence enough for some balancing.

    The 4/8/16R switch is made by SWP with logo of a linearised S in a
    rectangle. I would not want to rely on them for their supposed 250V/8A
    rating. The fixed contacts are just punched holes in the tinplated terminals
    and the moving contact, just a dome, path has 4 crude sprung surface
    contacts along the way
  9. N_Cook

    N_Cook Guest

    I've just realised the mains voltage selector is the same make and design
    just different markings.
    That too is unreliable I now find concentrating on it, so now permanently
    wired for 240V , no bad thing, in case someone mistakes this selector for
    changing oputput for different cabs and in the UK.

    At least 1970s and earlier loop-thru-tag and solder assembly has gone by the
    board by 2005
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