1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Reviving a old amp

Discussion in 'Shock Brother's DIY Amps' started by bogzz, Aug 8, 2009.

  1. bogzz

    bogzz TDPRI Member

    Posts:
    7
    Joined:
    Feb 7, 2004
    Location:
    Rochester,New Hampshire,USA
    Hello all
    I'm new to the forums. I have an amp that my brother built in the 60s based on a fender design. He was a keyboardist .
    I would like to get it working again. I tried changing the 2 lead plug to a 3 lead plug. .All the tubes glow but when I plug in the guitar it just hums real loud.

    So I set it aside until I figured thru this forum I might get some advice on getting it back in working order. I so have some electrical/electronical experience and understand the dangers of working on this amp.

    thanks
     
  2. jefrs

    jefrs Doctor of Teleocity

    Posts:
    13,321
    Joined:
    Nov 20, 2007
    Location:
    Newbury, England
    Lights on but no-one at home? - try changing the valves.

    Bad hum and no sound, usually a bottle. Start with the rectifier then o/p valves. Check HT fuses.

    One amp I have has an R and a cap between the two anodes (to kill rf). It shorted and red plated the bottles: bad rasping hum and no sound.

    Have you changed the psu capacitors? - if it has been left idle the electrolytics die. Can you test them? - if not then replace. The preferred series and shape of caps have changed since 1960 but values are rather arbitrary, anything in the region 30 to 50 µF will probably do.

    Double check your mains 'polarity' and that you have earthed correctly and the power secondary is also earthed at its 0V. Use a star earth point. Hum can be lessened if the transformers are raised off the chassis by a couple of millimetres, not electrically isolated but it breaks electromagnetic induction to a steel chassis (that's why an aluminium chassis is better), a washer will do, ideally a sheet of Paxolin or GRP (blank pc board).
     
  3. shadowfan

    shadowfan Tele-Meister

    Posts:
    203
    Joined:
    Dec 13, 2008
    Location:
    Atlanta, GA US
    Check the grounding of the input jack? your cable's ? Just a thought but missing grounds would give similar symptoms.
     
  4. Papa Joe

    Papa Joe Friend of Leo's Silver Supporter

    Age:
    91
    Posts:
    4,234
    Joined:
    Jun 30, 2007
    Location:
    Swanton Ohio
    Power supply caps will do that..
     
  5. shadowfan

    shadowfan Tele-Meister

    Posts:
    203
    Joined:
    Dec 13, 2008
    Location:
    Atlanta, GA US
    +1 on that too.
    If a 1/4" plug (no cable) is inserted in the jack that would be a good test for the power supply caps, since any resulting hum would not come from an essentially ungrounded cable
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.