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5f2a mod as power amp for external pre?

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  • 5f2a mod as power amp for external pre?

    hi guys.
    my 5f2a sounds great.
    i was wondering if i could use it as a power amp section for an external hi gain tube preamp.
    my idea was to create an insert point right after the 0.022uF cap (the one before the 6v6). i would lose the NFB,i'm not raving about that but i read some people says it's not so important after all -i guess i'll see-

    do you guys think it's a good idea? do you think it's gonna damage the amp?
    my problem is that hi gain pedals does not sound good in my amp (too much saturation and EQ character from the amp).
    i thought bypassing the preamp with an external pre could be the answer.
    what do you think?
    (i'm not an expert)

    thanks guys

  • #2
    What kind of external preamp do you mean? Selfmade, 19'' rack version?
    It could work, but I'd feed the external preamp signal *before* that 0.022uF cap (the signal through it).
    You could try to feed the NFB into the last stages cathode of the external pre - just like in the 5F2A schem - don't know if it works like I imagine, but worth a try. It would keep the signal a little on the clean side. Don't know if you aiming for this.
    An external pre won't damage the amp as long as everything is wired correctly.


    • #3
      yeah, something like a rack preamp, i would just use it for recording. something like the Anvil from Musikding, or similar.
      why *before* the cap? is that cap important to the sound? there's 190v before the cap!
      today i tried connecting a pedal directly after the cap, and it worked. very low volume. maybe a preamp could give higher output to the power tube.
      then i tried to connect the pedal directly after the volume pot. much more volume, some saturation from the power tube.
      i'm not sure about wiring the NFB to the external preamp, too many chances of wrong wiring.


      • #4
        Originally posted by pietro_moog View Post
        there's 190v before the cap!
        You gotta disconnect the "old" preamp, when you plug in the external one (use a plug with switch). I'd use the cap before the PI *because* there might be some 190v or whatever on the output of the other preamp (depends on how it's built and where you cut the signal line) and you don't want it to hit the grid of the PI.


        • #5
          cool, thanks.
          so you're saying i have to disconnect the internal preamp?
          can't i leave it there? is that a problem? (other than the NBF)


          • #6
            Insert the signal at the volume control. If the gain is too high, use a 12AT7 or 12AY7. You may have to reduce the value of the feedback resistor to compensate for the lower gain tube or make it variable.
            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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