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Hiwatt AP CP103 ,2005 valve amp

Discussion in 'Electronic Repair' started by N_Cook, Jan 20, 2013.

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  1. N_Cook

    N_Cook Guest

    Blown mains fuse and owner thinks he replaced the HT and mains ones after
    initial failure.
    Amp failed in tune 5 into a gig , playing loud but normal for this amp (does
    "The Who" front label make it louder?) and fuse/s blew. Next day, he has a
    1970 Hiwatt L 100W Mark 4 ? M41.. serial number anyway, and according to the
    owner very rare, this one should be ok when it gets 4 EL34 valves again.
    One of the 2005 EL34, the getter was white so he threw it out ,
    unfortunately . I'm out of contact with him for a couple of days but looks
    as though the other of one of the side pair was partially white and kept
    this one same make and appearance as the others in the amp, now mixed in
    with his box of untested valves. Anyway in the end he robbed 2 EL34 from the
    working vintage one and still blew fuse/s.
    All Rs look and measure ok in the PA section. Powered up with 70 percent
    mains with no output valves, only prea ones , and voltages seem right for
    that mains including bias voltage. Tx resistances seem normal.
    This part white EL34 , 2/3 white , 1/3 mirror Cs getter, blown heater. The
    vertical rod electrically attached to p1 , G3 , but physically on the other
    side of the valve electrically connected by the top "halo", electrode is
    discoloured to blue compared to the other valves. No obvious cracks , I
    suppose deliberately breaking it will confirm was still functional getter.
    But something coming off the rod reacting with the getter while hot and not
    fully converted to a white form ?
    Won't be testing them and his other "spares" until tomorrow.
    One 8 pin black socket (marked Made in Italy , EF ?) is discoloured to dusty
    grey appearance around pins 2,3 and 6, 6? no pin in that one ?.
    Trying a 2.2mm drill shank (not the cutting end) it is close sliding fit to
    the old amp socket pins but slack sliding fit for the 2005 one
    Measured a few EL34 pin diameters, p1 and 8
    2.22, 2.31 mm

    EL24 from the 1970 amp
    2,41,2.41 mm
    2.40, 2.46

    so suspect contacts in the sockets? and loss of bias voltage ?
    Retaining springs for the new version , not considered necessary in the the
    old one
  2. Jamie

    Jamie Guest

    If it's any thing like the EDM machines I used to work on years ago
    that used 4-400Z tubes, the power supply caps get bad and when the amp
    is under heavy load does not get sufficient power. THis also means the
    bias circuit maybe lifting and causes the tubes to start cutting in DC
    current and over heating.

    It was common to see these tubes glowing a mild orange color when
    they were fully working properly however, a bright orange or worst, near
    white is a not so good.

    You need to put a load on the amp, test signal, DMM and scope to
    monitor the B+ and bias voltage under max load.

    A lot of times the large caps may test ok for capacitance but have
    leakage. This leakage, as small as it could be can add extra load on the
    supply and thus the valves as you say (tube) circuit suffers at the
    high end along with having ripple that may not be explainable.

    ALso, high freq oscillation can cause unexplained over heating and can
    be seen on the scope. THis usually is due to a small cap+R circuit that
    has failed in the circuit to neutralize the effect.

    Today we work on machines that employ a single oscillator tube that
    pumps out at the plate around 275k watts. It's water cooled, the heater
    is operated from a 12 volt AC 250 amp transformer and the simplest form
    of thermal protection ever devised is a piece of nylon/plastic fishing
    line tied to a post in the center of the tube assembly back to a wall
    mounted whisker switch actuator arm. If the temps are high enough to
    melt that line it simply breaks and the switch shuts things down.

    That operation uses a hot cathode triode and the heater element is
    made of compressed sheet metal which is also the cathode. THey make nice
    lamps when their service is no longer useful :)

  3. N_Cook

    N_Cook Guest

    The Italian sockets are maybe marked EE, anyway socket pins are now strong
    sliding fit to 2.2mm drill shank.
    Curvey Es like the E of the compliance CE controlle european mark, maker
    name not found Googling
    The 1970 amp is marked Sound City "one hundred" on the gold fascia panel.
    Suspiciously small 63V 100uF bias cap replaced with 100V one in the 2005
    O/p Tx tests good balanced with reverse feed AF sig gen.
    Anyway with valve testing can be pretty sure of the "etiology" .
    Both these amps are stored in the same unheated warehouse, the newer amp
    looks far worse from internal corrosion, than the 1970 one.
    Poor socket pins and corrossion on the valve pins lead to loss of bias. Then
    owner robs a pair of valves from the good amp , but one of those valves has
    a broken polarising nib. Tracing back to my markings on receipt this valve
    was rotated one position out in the base, the one position that connects
    anode to
    cathode via the heater.
    The old valve pins showed no corrossion but the recent JJ ones had bad grey
    dust corrossion , leaving large grey deposits on cosmetic nail sanding
    One valve tested on my Avo as no heater and took a lot of abraiding to the
    pins to get
    the heater current through. Another one tested as no gain until I repeated
    the sanding.
    The JJ valves had no date mark just an E in a cartouche. And marking
    datecode for 2005 ?
    I don't think the pin infil is PbF, but has the bright tin tinning of the
    pin surface turned to tinpest? - those grey patches were certainly
    insulating to 30M , DVM anyway.
    As a puzzle for anyone with the inclination. If the valve with the broken
    nib was placed in the socket with perhaps overheating marks in the bakelite
    of pins 2,3 and 6 what orientation would lead to serious overheating at
    those pins?
    The heater chain on these amps uses "dummy" socket pin 6 as a resistor tag
    from HT2 supply, EL34 are NC and usually NP for pin 6
  4. N_Cook

    N_Cook Guest

    I went to test that part failed one yesterday and over a couple of days it
    had turned fully white, must have been a very tiny leak/self-stopped,
    handling was enough to make it open up a bit
  5. N_Cook

    N_Cook Guest

    Back working again, I'm still trying to find the maker of those Italian
    valve sockets if anyone has any ideas.
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