Versatile Amp

Discussion in 'Shock Brother's DIY Amps' started by shortcircuit, Aug 20, 2019.

  1. shortcircuit

    shortcircuit Tele-Meister

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    This is the final (yea, right) evolution of a build that I started about 1 year ago (https://www.tdpri.com/threads/single-ended-6bq5.869737/). Since only the power supply section remained the same, I think this iteration deserves its own thread. On to the schematic:

    Omnivox.png

    The amp can go from a super clean tone (cold clipper off, 5879 cathode on full NFB) to face melting overdrive (cold clipper on, 5879 cathode on full bypass), and everything in between.

    Guts:

    Guts.jpg

    Outside:

    Outside.jpg
     
  2. FRANKENJAG

    FRANKENJAG TDPRI Member

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    We know it's final until you have another project you need to procrastinate on. So what's on your bench now?
     
  3. shortcircuit

    shortcircuit Tele-Meister

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    All right, you got me:D. This amp was a proof of concept. There's an all octal 6V6 push-pull version of this in the works with the pentode in the first position instead. I got my hands on a couple of those nifty red, metal canned 6SJ7, the 5693.
     
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  4. tubelectron

    tubelectron Tele-Meister

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    Fine job ! Congratulations... :cool:

    -tbln
     
  5. Mexitele Blues

    Mexitele Blues Tele-Meister

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    I feel like I learn a lot trying to read your circuits but I've got to ask a couple questions to wrap my head around this one. Switch B is a 4 pole that switches 12AX7-2 in while warming the bias on the 5879? Where would center bias be on that tube? And what does the heater secondary attached to the power tube cathode do?
     
  6. shortcircuit

    shortcircuit Tele-Meister

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    The center bias of the 5879 is provided by the 3K3 resistor. This stage is a direct ripoff of the normal input of a Gibson GA-40. So when the cold clipper is switched in, I want to run the 5879 warm, basically like the second and third stages of a JCM800.

    The 6.3V rail is tied to the cathode of the power tube via a virtual center tap (the 2 220 ohm resistors) to elevate it above ground. It reduces hum from the heaters.
     
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  7. shortcircuit

    shortcircuit Tele-Meister

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    More procrastination...I liked the amp so much that I converted my all octal 6V6 single ended amp to the circuit.

    Octaltron SE-HG.png

    I thought I had a horrible ground loop problem after I finished it up. There was a crazy 60 Hz buzz that mostly went away when I touched the chassis, but when I touched the power transformer, the buzz became horrible. The amp also squealed and oscillated whenever my hand came close to the 6SJ7 tube, weird. I spent the entire afternoon and evening trying to chase it down. After I took a break and had a beer, I looked at the amp again and noticed that I had hooked up the input backwards. 5 minutes later, the amp was working great. This episode further reinforced the notion that beer is my friend.

    Guts.jpg
    Outside.jpg

    There's the 5693 tube. Does it sound any different from a regular metal 6SJ7 or a glass 6SJ7GT? Not that I can tell, but it looks like a little fire hydrant, and it's skookum as frig.
     
    Last edited: Aug 25, 2019
    D'tar, FRANKENJAG, sds1 and 1 other person like this.
  8. shortcircuit

    shortcircuit Tele-Meister

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    Applied the basic premise to my pair of all 7-pin amps.

    Pipsqueak EVO.png

    PE-Innards.jpg

    Double Trouble.png

    DT-Innards.jpg

    I wired them so that the bypass switch dumps the signal from the input pentode to the phase inverter. I designed the input stage of the smaller amp around the 5654/6AK5 tube, and the bigger one around the 6AU6 tube. With pins 2 and 7 of the pentode socket wired together, I can pop in a whole bunch of different different pentodes for giggles.
     
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