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'56 Bassman bias problem

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  • '56 Bassman bias problem

    Hi Guys,

    I haven't been around for a while but a project fell in my lap and has me stumped. I'm working on a friend's 5E6-A 4X10. It is bone stock except for fairly new filter caps. The RCA 6L6s are pulling 66 ma each. I subbed in a new bias cap and subbed in a UF4007 in place of the selenium rectifier (which still works fine). I switched tube positions and subbed in a pair of TADs to be sure. They only pulled 61 ma but you get the idea. I'm using the shunt method. Screens are 410 and B+ is 420.

    On very low gain I ran a 1k sine wave into the jack and it looked fine all the way through the amp -- so I'm guessing there aren't any hidden parasitics lurking. The PI caps test good (old Astrons). The crux of the issue is that the bias tap puts out 46 volts -- right on the schematic. At the grids it is -41 volts which is a pretty efficient exchange. I'm not losing bias volts anywhere. I did put another 20k resistance on the bleed resistor to see if it would make a difference and it did raise the bias voltage a volt and the rail by 5 -- so it is open to persuasion.

    So, two questions: any ideas and does the shunt method work when the B+ goes to both the center tap and the screens? Thanks for your help, Skip

  • #2
    Sorry, the B+ is 428 at the first cap and entrance to the choke, screens are 423 and the plates are 417. This uses a capacitor input filter. sh

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    • #3
      got it fixed. thanks for looking

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      • #4
        Well???

        What did you find it to be?
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Leaky PI coupling caps methinks....
          The farmer takes a wife, the barber takes a pole....

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          • #6
            yes, one of the coupling caps was a little leaky and I also added some resistance to the bleed resistor. Cheers, sh

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