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  • dropping resistors not dropping

    Hola mi amigos, I powered up my build for the first time yesterday only to discover that for some reason all of my HT voltages were sitting at B+; the dropping resistors weren't dropping.

    6550 plate: 460
    6550 screen: 460
    12ay7 plate: 460
    12ax7 plate: 455
    12ax7 plate: 455

    There's a 5.6k 10 watt resistor that should be dropping the screen and 12ay7 to about 310, and a 10k 2 watt that should be dropping the 12ax7 voltages. Its as if the tubes aren't drawing current at all. There's a 12au7 for a reverb driver that hasn't been put in yet so I expected them to be sitting a little high but this is clearly a bigger problem. Any hints?

    I wired an ammeter between the power primaries and switch, it says its only drawing .6 amps. The 6550 heaters alone should be pulling more than that. Anyone have a helpful hint on where to start?

  • #2
    Were the tubes plugged in? (I ask this because some builders expected the voltages to drop without tubes.) Were the tubes filaments lighting up? Do you measure about 6.3VAC between the heater pins or about 3.15VAC to ground.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      Yeah tubes are in and I see 6v between the heater pins

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      • #4
        Is ground hooked up? Measure the cathode voltages on the preamp tubes.
        WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
        REMEMBER: Everybody knows that smokin' ain't allowed in school !

        Comment


        • #5
          ok so not sure what's up but i walked back down to my shop to measure the cathodes on the preamp and magically the voltages across the HT have begun dropping. I'm thinking grounding issue, cold joint or something like that. This is what I measured when I walked down-

          B+: 400

          6550 plates: 377
          6550 screens: 180

          12ay7
          PI plate: 162
          PI cathode: 25

          12ax7
          V2A plate: 162
          V2A cathode: 1.7mV

          12ax7
          V1B plate: 148
          V1B cathode: .7V

          12ax7
          V1A plate: 173
          V1A cathode 142mV

          v2b is the recovery stage for the reverb and hasnt been wired up yet. i didnt expect my B+/plate voltage to be this low with load either. hope i dont have to go back to the drawing board

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