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Supro Thunderbolt 6420 question

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  • #1

    Supro Thunderbolt 6420 question

    I am trouble-shooting a Supro Thunderbolt 6420 Bass amp SS-rect.
    http://www.angelfire.com/mech/beansa...hems/S6420.pdf
    The amp smoked while the owner was playing it for awhile at full volume. The amp still powered up and did not blow the 20 amp fuse...
    Interior chassis inspection showed (2) scorched and heated areas. One of them, from the wire( laying on top of the 200 ohm 7W resistor that had flamed and burnt. The other scorched/heated area was around this huge wattage 1000ohm cathode resistor that someone had wired in place of the 7watt original. This res. is 3" long X 3/4" and mounted on a bar. It reads good but has taken a lot of heat.
    I decided to install a new can cap and replace the power and cathode resistors with 10watt while replacing the burnt wires and cleaning up.
    I powered up the amp through a current limiter (no signs of shorts). Powered up under full line voltage and never got any sound....no blown fuse. After inspection I noticed that the new 1000ohm 10W showed signs of heat and got no reading.....apparently fried!
    What could be the cause and what should I test next? Thanks for help and advice!
    S6420 Thunderbolt Link....

    http://www.angelfire.com/mech/beansamps/supro/supro.htm
    Tags: None
  • #2
    The only 1000 ohm power resistor shown is between nodes D and C in the power supply.
    About the only things that could burn it up would be bad power tubes (shorted screens), bad caps, or bad wiring to the caps or power tube sockets. There are 2 caps shown connected to point C, one is part of the can, the other is in the chassis and according to the note may be 20 or 30uf.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      I hope the 20 amp fuse was a joke...


      I agree with g-one. About the only thing that would burn up the 1000 ohm screen node resistor is a failed power tube. and that also follows with the burnt up cathode resistor on those tubes.


      Please fire the amp up with no tubes in it first, next time, in case.

      Also please do not way "no reading" when using a meter. it says SOMETHING on the screen even if it is just zero. Zero is a reading.

      If your new resistor shows open, reverse your meter leads and check again. ANy residual voltage stored in a cap can confuse your meter.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        I just got back to working on the Supro after a couple of weeks set aside. After going over my cap job work, I realized that I made a wiring goof on the 20mfd cap that resides inside the chassis. I made that right and replaced the 1K R and all is well now. WHEW!
        It is a nice sounding amp and has popularity with the early Led Zeppelin 'folklore'. I still sound like 'me' when playing through it however.. ;-)
        Thanks Enzo and g-one....

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