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Designing for AB2

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  • #46
    Smitty,

    FWIW, I just set up a Marshall recently to feed the choke/screens/preamp from the PT CT of a FWB setup, it worked fine.

    Triode Electronics has a new driver transformer with a 20K CT primary (10mA max DC imbalance) and four separate 20K secondaries (30mA max AC current capability per winding, I just learned this morning), which might work pretty well, for about $20. You could drive it with a P-P 12AT7 and parallel secondary pairs, loading each w/5K to the sec. CT - at least that's how I'd do it, but YMMV.

    The Zener mod works by dumping excess cap-plate electrons to ground during the negative swing - I can't see any corresponding mechanism at work using transformer drive, and with substantial resistive loading it really shouldn't be needed.

    Ray

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    • #47
      With that IT why not "go for the gold" and do an SVT style PI? Gain Stage -> Concertina -> PP gain stage driving two of the windings -> PP CF driving the other two? 12bh7s are purely optional .

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      • #48
        Ray,

        Thanks. I'll check that out.

        Smitty

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        • #49
          Originally posted by Ray Ivers View Post

          FWIW, IMO the simplest drop-in AB2-drive circuit for a Super Reverb output stage is unquestionably a transformer; MV, 12DW7 pre-driver/driver w/12AU7 half driving the tranny primary single-ended (a 12AU7 would also work well if you didn't need the gain of the 12AX7 half of the DW7)...

          Ray
          Ray, excellent suggestion. I'm going to give this a try. With both the DW and the AU.

          Originally posted by Ray Ivers View Post

          ...Zener-regulated negative supply to a low-value bias pot & the tranny secondary center tap, secondary ends (w/loading R's to the center tap) thru low-value stoppers to the output tube grids, badda-bing, badda-boom. In a zero-NFB PA design I wouldn't be surprised if it was the best-sounding option either,...

          Ray
          I've got a few questions on this part. Can you shoot me an email?

          smittyatbluesamps.com

          Originally posted by Ray Ivers View Post

          ...and an excellent transformer (the Allied 6W68HF, about the size of a Fender choke) costs about $14 the last time I checked.

          Ray
          1:1 / 10K:10K at 9W seems perfect. Thanks! Doesn't look shielded. Do I need to worry about placement to avoid hum pickup?

          Thanks,

          Smitty

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          • #50
            Smitty,

            1:1 / 10K:10K at 9W seems perfect. Thanks! Doesn't look shielded. Do I need to worry about placement to avoid hum pickup?
            Maybe just the usual precautions - not too near another transformer, orienting the lams at right angles if feasible, etc. - the placement in-circuit is at just about the lowest-gain point there is, so it shouldn't be too much of a PITA to get the amp quiet & stable. This transformer might even fit inside the chassis if needed, but then you'd want to keep it away from the OT/PT leads and the preamp wiring.

            I'll shoot you an E-mail about the bias circuit.

            Ray

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            • #51
              Re reading this years later, i understand even less than then! Daft me!

              Well, guys, if any of you are about, i hope all is fine in, your lives!

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              • #52
                Hey, glad to see you around
                You had been "lost" for quite a good time
                Juan Manuel Fahey

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                • #53
                  Good to see you Max! Welcome back.

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                  • #54
                    Thanks a lot for the welcome back guys.

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