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1-channel BFDR with EF86 Schematic comments please

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  • 1-channel BFDR with EF86 Schematic comments please

    As it is well over a couple of months since I redesigned my last amp, I thought about re-designing this one. Now it has JJ6V6s in the output stage and a 12AT7 LTP (i.e. its looking like a BFDR back end, but with bias vary trem) - everything about that sounds groovy and I may even go to 4 x 6V6s there since there is plenty of juice in the Pt and OT and I've got speakers to burn etc, however...

    The real reason I am posting this today is I am thinking about the pre-amp again, and I have been toying with the idea of an EF86 stage for experimenting with the tonal variation.

    So employing Merlin's excellent book, I knocked up this schematic today, which I've modelled after a BFDR front end (but with the EF86 and CF in place of where the second half of the '12AX7' would otherwise have been).

    I don't want super hi gain out of the EF86 (and the amp is a combo after all), so I was thinking of a lowish screen voltage (around 50-60?) but given the HT is most likely going to sit around 320-330 there (altho' I could easily lower it), I wasn't sure about the screen resistor value, so I guestimated 680k.

    So thoughts about the EF86 anyone? (too noisy? wrong screen resistor value? etc?)

    TIA
    Attached Files
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

  • #2
    Aaah, I was just thinking along similar lines after looking at a Selmer Zodiac schematic with the EF86 as the tone stack recovery stage!

    I think you might be able to lose the ac coupled cathode follower after the pentode stage, since it's driving 1meg in parallel with 3.3meg- doesn't seem like too challenging a load even for a pentode. Dunno if you need an extra triode section for anything though?


    Nathan

    Comment


    • #3
      Originally posted by octal View Post
      I think you might be able to lose the ac coupled cathode follower after the pentode stage, since it's driving 1meg in parallel with 3.3meg- doesn't seem like too challenging a load even for a pentode. Dunno if you need an extra triode section for anything though?


      Nathan
      Thanks for the feedback Nathan

      Funnily enough I'd thought about that variant earlier too, but I was worried that with a ra (which is the same as the effective output impedance in a pentode right?) of 2M5 from the EF86, even a 3M3-odd 'grid load' would result in significant attenuation. (Wouldn't you need quite a bit more impedance bridging? - like 10M or something? - or would you perhaps up the gain factor in the EF86 to compensate? - at the risk of increased microphonics?)

      I'd be curious to hear other's thoughts on which would be better, the one I posted before, or the (variant) attachment on this reply. (FWIW I put the 'extra stage' in parallel with V1 and made it a 12AY7 - which oughta drive the tone stack really well with a low noise floor)

      Cheers
      Attached Files
      Building a better world (one tube amp at a time)

      "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

      Comment


      • #4
        Well, that EF 86 is going to have more gain than the normal 12ax7 stage you'd have there... so it's a question of how much extra gain do you want. I knew the ra of a pentode was high, but I didn't think it'd be THAT high. I guess you could do the math and figure out what your total gain would be in each scenereo....

        Comment


        • #5
          I was looking at the articles on Merlin's site and he says that a pretty good approximation for a pentode is Zout=Ra. So in this case, the output impedance of your pentode stage is 100K. There's definitely no need for a cathode follower IMHO.

          Let us know how this sounds when/if you build it!


          Nathan

          Comment


          • #6
            Hi Nathan - yes you are correct, I was getting ra confused with Ra. 100k it is. That simplifies things quite a bit. Cheers
            Building a better world (one tube amp at a time)

            "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

            Comment

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