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Vox AC-30/TB "Stolec" needs mystery reverb tank

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  • Vox AC-30/TB "Stolec" needs mystery reverb tank

    I've nursed a circa 1972 AC-30/TB back to life but the reverb tank is missing. This is the infamous circuit-board model for which any technical documentation is nonexistent, so I traced out the schematic and drew it up properly. The tank's missing, though, and I'm stumped for what should be in there. Definitely one with high-impedance transducers (no transformer!) but the tank I tried (4FB3xxx) gave only a ghostly whisper of a reverb. I'm absolutely certain the reverb circuit hadn't been modded in any way, and there were no witness marks on the plywood for anything other than the tank itself. Would a now-unobtanium tank have been used here?


  • #2
    That attachment is not working.
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #3
      You have two cbles to the reverb pan, unplug then from it, turn the amp on and the reverb up. Now touch the tip of each cord one by one. One should hum out the speaker. Does it? That is the return and plugs into the OUTPUT jack on the pan. The other is the drive, and when playing through teh amp ther should be a strong signal there.

      So explore that, to not only find if it was plugged in right, but also to detect which half has the problem.

      Link worked for me.

      You have a reverb volume control on either half, make sure both are up.

      One trick is to drive the reverb with the amp, and connect the OUTPUT jack on the pan to some other amp. get reverb there? And likewise drive the pan from some other amp and plug the pan OUTPUT into this. Does reverb come out now? That all tests the two sides separately.

      Did we verify the tank you used had apapropriate resistance readings at both ends?
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        In haste, I left out some important details:

        Signal at the tank is much attenuated relative to that at V6B's plate. Not at all surprising; C21 creates a HPF that's up in the ultrasonics. Seems that a tank with an input impedance of hundreds of kilohms would be required for this circuit. Did such a thing ever exist? C21 is clearly original.

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        • #5
          Can somebody zip that attachment and repost it?
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

          Comment


          • #6
            You need the infamous homemade VOX piezo based reverb tank, made out of some aluminum U railing, 2 ceramic pickup cartridges (Sonotone/Elac/you-name-it)
            Use office type rubberbands to hold it, guaranteed disintegration in a year or so.
            Very high impedance and thatīs an understatement:

            Mick Bailey picture:


            Read all about it:http://music-electronics-forum.com/t32851/
            Pickup detail:



            This is the standalone version, courtesy of vintagekiki, yours should be very close (I mean drive and pickup section):


            IF similar (it should), you might reform it into a regular one.
            Best would be to get a Fender type reverb tank transformer and a low impedance tank, results would be near classic Fender reverb, about what Mesa gets out of a single 12AX7.
            Juan Manuel Fahey

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            • #7
              Originally posted by J M Fahey View Post
              You need the infamous homemade VOX piezo based reverb tank, made out of some aluminum U railing, 2 ceramic pickup cartridges (Sonotone/Elac/you-name-it)
              Use office type rubberbands to hold it, guaranteed disintegration in a year or so.
              Very high impedance and thatīs an understatement

              My mind is suitably boggled and yeah, I would call that contraption "now-unobtanium" to be sure.

              IF similar (it should), you might reform it into a regular one.
              Best would be to get a Fender type reverb tank transformer and a low impedance tank, results would be near classic Fender reverb, about what Mesa gets out of a single 12AX7.
              That's what I had in mind if the original type of tank was impossible to find. The Cambridge Reverb and the 1978 AC30 schematics will guide me.....

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              • #8
                Vox add on reverb unit - OS/075

                http://www.voxac50.org.uk/images/schematics/closeups/manual_05.jpg

                Who does not know and knows that he does not know - teach him Confucius)
                Who knows and does not know that he knows - wake him Confucius)

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                • #9
                  Reposting the schematics as apparently the download isn't working again.
                  Attached Files

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                  • #10
                    A real big thanks for the re-post. This clears up a lot of questions I had about the amp - especially around the reverb circuit which has been butchered and partly removed in the amp I have.

                    Much appreciated.

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