Problem with the bias on a Bassman 100

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mastervolume, Jan 3, 2006.

  1. Mastervolume

    Mastervolume Member

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    I have a bassman 100 on my bench right now and when I run a bias probe in one of the jacks I am reading 2ma.

    I have 460ish on the plates. 320 on the out of the PI. -58 on the balance.

    Screen resistors look ok on a meter.

    I'm stumped.

    Any ideas?
     
  2. John Phillips

    John Phillips Member

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    If the proper B+ is present at both the plate and screen-grid pins, it must be either a bad tube (although I assume you've checked that already) or a poor contact between one of the socket contacts and the tube - or a bad ground connection from the socket to the chassis. I would check that first, since it's made via the same screw that holds the socket in. If it's corroded or loose it might not pass much current.

    I would also check again that the screen resistor is definitely OK - if they're the original carbon-comp ones these can crack and work intermittently... like when you're touching the lead wires with your meter probes :). Best to replace them all with wirewounds for reliability anyway, if it hasn't already been done.
     
  3. Swarty

    Swarty Member

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    The ground connection would be soldered to the chassis. Most likely a bum tube but could also be a bum solder joint on the socket or a crapped out socket. Per John's suggestion double check the screen resistor.
     
  4. John Phillips

    John Phillips Member

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    Yes... you're right, they solder the tags down as well. I was thinking too much of Marshalls ;).
     
  5. Wakarusa

    Wakarusa Member

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    Where on the balance? (input? output?)
    If you've got -58 on the actual output grids they'll be very close to cutoff.
     

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