Mystery "clipped" wires in Bassman 135?

Discussion in 'Amps and Cabs [BG]' started by KenB, Jul 2, 2020.

  1. KenB


    Jul 15, 2008
    Yellow Springs, OH
    I have a Bassman 135 amp from 1978.
    I was checking out the condition of the power supply caps under the "doghouse" cover.
    I found a pair of wires that are fed from under the cover through a grommet into the chassis, and simply cut off:


    The wires are soldered into the circuit at this point on the schematic:


    The amp is all original, and has never been repaired or modified.
    I searched the web for images of Bassman 135 amps, and the few pics I found all show the two clipped wires.

    Anyone know what the purpose of the wires could be? Temporary use during manufacturing or initial testing?

  2. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    Off the top of my head, looks like the screen supply node for a non-ultralinear output transformer build. No idea why they'd leave those wires there in that state though.
  3. BassmanPaul

    BassmanPaul Inactive

    I'd remove them for safety reasons.
  4. Take them out. You'll have 400V sitting on the end of those clipped wires just waiting for someone to be poking around inside the cabinet.
    Bill Whitehurst likes this.
  5. Foz


    Jul 26, 2008
    Jax FL USA
    Passinwind likes this.
  6. seamonkey


    Aug 6, 2004
    The amp chassis was probably used for other model heads, and these were for other circuits, reverb, tremolo, but cut off when used in a bass amp.
  7. Any of those circuit using high voltage would have been chassis mounted, not external. Even in those days, they were cautious about leaving high voltage contact points out in the open.
  8. Wasnex


    Dec 25, 2011

    Somebody may have been experimenting with the amp. I believe it's fairly common to disconnect the screen taps from the output transformer, so those wires may have been used to form a 410V screen supply.

    Note the screen connection for the Bassman 100 is essentially at the same point.

    I also found a thread that says the assemblies were shared with other models that used the wires for reverb and tremolo circuits. The thread suggested cutting them out:

    Here's a link to the 135 amp schematic with reverb and vibrato:
    Passinwind likes this.
  9. KenB


    Jul 15, 2008
    Yellow Springs, OH
    I guess all Bassman 135s have those wires.

    I can't imagine what I (or any future tech I would use) would do with them. And they are a hazard. I'm going to remove them if/when I replace the caps.

    The amp works like new, in spite of its age! Even with the original caps and tubes.
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Then leave it alone!
  11. seamonkey


    Aug 6, 2004
  12. Fender4Me

    Fender4Me The Undertaker

    Measure the voltage at them.