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Bogen CHB-35A conversion (B-12/B-15??)

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  • Bogen CHB-35A conversion (B-12/B-15??)

    Basically, I'm wondering if this could be a good idea and any problems that might get in my way....

    I have a Bogen CHB-35A I picked up at a thrift store and have messed around changing tone control, adding a grounded 3-wire plug, 1/4 jacks, etc. I looked at schematics for the old Ampeg B-12 and B-15 bass amplifiers and I see that the circuit seems similar. I love the sound of the old 60's B-15's and it seems like it would be possible to wire up V1A/V1B of the Bogen like V1 of the Ampeg, and V2A/V2B like V3. I guess I would then just ditch the 6C4 tube? Would there be some other application for this? Is there anything wrong with my logic here?

    I'm slightly confused in the CHB-35A design. It seems like V3 in the Ampeg schematic is doing what is happening between V2B and V3 of the Bogen schematic. Would anyone mind helping break this down for me?

    Thanks a lot, I've been enjoying reading this discussion forum recently and learning a lot from it.

  • #2
    You will not need to copy the B15 exactly. Keep the 6C4 since you will need a phase inverter in any case. Put a gain stage in front of it and the B15 tone controls in front of that.


    • #3
      Dear J Martin

      thanks for replying to my two posts...
      I'm still a bit confused by the V2B (1/2 12AX7) and 6C4 sections on the Bogen schematic. What does the 1/2 12AX7 directly before the 6C4 do?

      I'm now thinking that the SB-12 would be a better circuit to follow because it has 2x12AX7 and 2x7868 while the Bogen has 1x12AX7, 1x6EU7, 2x7868.


      • #4
        The 6C4 has no gain at all so it needs some type of gain stage to drive it. You could incorporate the functions of both tube stages in a 12DW7 tube or for more drive use a pentode/triode like a 7199, 6AN8, or a 6U8, etc. The 12DW7 is still in current production. For guitar amp purposes, consider the 6EU7 the same as a 12AX7 with a different pinout. I have redone countless old PA amps over the years for friends and customers. My philosophy is not to build a clone of anything. Its more like a variation on a theme.


        • #5
          Editable Bogen schematic

          You can find an editable version of that CH-35 here:

          If you need to download a program to make edits, you can find that here:

          Maybe you can post what your final schematic version is after tweaks on this forum?

          With respect, Tubenit


          • #6
            Thanks a lot. I was wondering if a program like that existed for OS X


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