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Extra Gain Stage - Tremolo?

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  • Extra Gain Stage - Tremolo?

    Okay so I'm working on an amp that uses a 6SL7 for the first two gain stages and then uses a 12AU7 (mu of 20) as a 3rd gain stage. Since I have an extra triode left over in the 12AU7, I was thinking it would be cool to add a trem to the amp but most trems use the 12AX7 (mu of 100) or at least the 12AT7 (mu of 70).

    Here's what I was thinking: I'll use a 12AX7 for both the 3rd gain stage and the trem. The trem will get one half of the 12AX7 so I know that'll work. I can use a 13k plate resistor and a 1.5k/25uF RC on the 12AX7 that is being used as the 3rd gain stage it would have the same gain as the 12AU7. Will this work?

    I used the triode calculator on Adam's Amp page to arrive at the 13k plate load for the 12AX7. http://amps.zugster.net/tools/triode-calculator

    Will this sound okay to use a 13k plate resistor for the 3rd gain stage? I guess if it comes down to it, I can use a 12AX7 with a split plate load and dial in the amount of gain that way.

  • #2
    The gain may be the same, but you won't have the ability to make a very big signal. To make the 12AX7 mimic the 12AU7, you need something like 4.7K in the cathode. Make the circuit look like the Concertina phase inverter in a 5E3 except use a 4.7K on the ground side and 100K on the plate side in place of the 56K resistors. Keep the 1.5K cathode bias resistor. You'll need a cap on the input to isolate the DC if you are coming from a volume control or tone stack or other ground referenced signal.
    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
      Something like this Adjust the 4.7K as necessary to get the gain exactly right.
      Attached Files
      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


      • #4
        Very cool! Thanks!

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