Saturday, 15 November 2014

May I? Vox in a Box

Thought i'd post this here too in case someone wants to try it out. There's an old development thread on FSB for this AC30 emulation circuit. There are number of schematic versions with slight differences, but the basics are the same for all of them. It is a nice high gainer with some noise content, so i'd suggest using 220µ for the power filter and maybe adding a 100p cap between germanium transistor's base and collector. I built one to see if the layout works and it's good to go.


19 comments:

  1. Excellent Miro! By the way guys, which one is your favorite Vox in a box (the closest to the real thing) Galileo, Rog Brittania, CB30, The Englis Channel or May I?...
    Cheers!

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    1. I didn't try the ones you listed though the CB30 is on my todo list. I have built a copy of Wampler's old CrankedAC with some slight mods and I like that quite much. I don't have an AC30 to compare to, but it is a nice crunch box.

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  2. Dammned it! Just when I thought I was wrapping up my last VOX-like build ;)

    I'm wrapping up the Galileo but speaking for both the English Channel and its next gen, Britannia - the latter is my favorite. Of course that very well may change ofter the Gali.

    I can't speak to the CB30 (perhaps that is yet another build)
    BTW - thanks mirosol for the great work (not to mention everyone too).

    Cheers
    Chris

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    Replies
    1. Ah - forgot to add. Can the voltage for the drains on Q3 and Q4 be posted please?

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    2. Please let us know your thoughts when you complete the Galileo, Chris.. :)

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    3. My board sounds the best with both drains around 6,3-6,4 volts. With supply at 9,6V. This can be used with 18V with no issues. Just use +25V rated caps for electrolytics.
      +m

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  3. What Hfe should we use for Q1? Or does this make much difference?

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    Replies
    1. I used MP38A at 81 hFE on mine. But since the first stage is pretty much ROG treble booster, i think anything should work fine.
      +m

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    2. Or anything with decent levels of leakage.
      +n

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  4. How can I best measer the voltages of the drains?
    Were do I connect the + and - on the vero of the DC meter?
    Cheers Leon

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    Replies
    1. + probe to drain (leg of transistor or vero next to it)

      - probe to ground

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  5. Im having a weird issue with mine. The gain control will cut out the audio when adjusted, but when left in place it will sound fine. Ita almost like the change lags. Its also extremely loud, way over unity gain at even the lowest volume. Ill check for shorts or missing cuts tonight.

    I used an AC176 for my NPN, had one lying around.

    ReplyDelete
    Replies
    1. The gain control behavior is to be expected. It's a control that changes the bias of that Ge transistor on the fly. Consider that a feature.

      And it should be loud, but you should still check your wiring for the volume pot. Like travis said, on minimum setting there should be no signal at all.
      +m

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  6. If the minimum volume setting is loud then something is wrong with your build.

    Lug 1 of the volume pot is ground, so when you turn it all the way down it should be silent.

    Check the link between the volume 1 wire and the ground rail

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  7. Definitely an issue with the pot. Mine is really tight inside so I noticed a short when I removed the board.

    I was wondering about that gain control, too. I kind of like the idea as well, its almost Super Hard On-ish.

    I guess Im all sorted, thanks for the help!

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  8. For wiring, is this correct?

    Cut1/Volume3 =
    vero -> Cut1 -> Volume3

    Cut 2&3
    vero -> Cut2
    vero -> Cut3

    trying to figure out the difference between "/" and "&" in diagram.

    Tks.

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    Replies
    1. that's right, but the easiest way for the second is this. you can take one wire from the board cut2&3, and take it to the pots lug2 or lug 3, then with a jumper connect lugs 2 and 3 together on that pot.

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    2. & if the connections are at the same pot, / if there are two separate pots for the connections.

      To be completely honest, the cut 3 doesn't need to be connected at all. The pot will work exactly the same regardless of the lug 3 is connected to lug 2 or not.

      For the volume 1 and cut 3, i'd personally take two wires from the board.
      +m

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