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Ampeg Gemini II 7591 bias questions

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  • Ampeg Gemini II 7591 bias questions

    Hello again,

    Hoping to get some guidance on what numbers to use to properly bias this amp, after installing new EH7591s.

    Without adjusting anything except the AC with a Variac, setting voltage to 115 VAC and using an Alessandro biasing unit, I am getting 34 mA per 7591, Volmeter - screen voltage ~ -22, matching the Ampeg drawing.

    When I look up the 7591 spec sheets, for an AB setting, there are 3.

    NJ7P SQL Tube Database Query

    I think this is the correct set of parameters from the above link:

    Push Pull Class AB1 Amplifier
    Plate Voltage ................................. 450 V
    Grid No. 2 Voltage ............................ 400 V
    Grid No. 1 Voltage ............................ -21 V
    Peak Grid No. 1 Voltage........................ 42 V
    Plate Current (Zero Signal) ................... 66 mA
    Plate Current (Maximum Signal) ................ 144 mA
    Grid No. 2 Current (Zero Signal) .............. 9.4 mA
    Grid No. 2 Current (Maximum Signal) ........... 30 mA
    Load Resistance ............................... 6.6K Ω
    Power Output (approx) ......................... 45 W

    Not sure if this is for one PT ot two??

    Confused by the 45 watt power out (thought the Gemini II had about a 30 watt rating)

    Any help with a pin by pin recomendation or other comments would be very appreciated.

    Thanks in advance.

    Neil.....

  • #2
    The 66ma current draw at idle is for two tubes.
    All else is up for grabs as far as I can see.
    How does it sound?

    Comment


    • #3
      Thanks for the quick reply.

      Unless I'm reading it incorrectly, there is only 5 volts diff between the plate and screen(grid 2).

      I'm new to the sound/tone but I'd say it's very close to what I am looking for, matched with my single coil archtop.

      Trying to trace/track down the source of a hum in the reverb - increases as I turn up the reverb pot. Sounds like 60 Hz. Acquiring a scope to help.

      Changed out all the electrolytes but am concerned I did not properly charge them. Also changed both transformers (Mercury Magnetics) and this introduced a low level hum.

      Comments and suggestions are very welcome.

      Thanks!

      Comment

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