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Converting Bogen chb-35a to more guitar friendly amp (newbie inside)

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  • #61
    New CHB35a conversion completed.

    I've documented my conversion of a Bogen Challenger CHB335a for Bass Guitar, Guitar, Microphone and Music player use. I used information found here and on other web sites. My conversion notes can be found at:

    CHB35a Amp conversion notes & pics

    Hope my notes help someone!
    Last edited by sjcarter; 02-17-2010, 08:47 PM. Reason: Added web link.
    S. Carter :D

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    • #62
      Originally posted by sjcarter View Post
      I've documented my conversion of a Bogen Challenger CHB335a for Bass Guitar, Guitar, Microphone and Music player use. I used information found here and on other web sites. My conversion notes can be found at:

      CHB35a Amp conversion notes & pics

      Hope my notes help someone!
      Neat, interesting to see a different take on the amp.

      I've changed things a slight bit again. I'll post my final schematic revision here, and a new recording of the amp first at about 1/3 volume, and then at full volume. This recording is done with absolutely no pedals in front of the amp.
      I really like the sound of the OD I'm getting out of the amp now.

      Here's the final schematic: http://vwtweaked.ca/images/Bogen%20B...%2008feb10.jpg

      And here's the final recording: http://vwtweaked.ca/music/bassflex%20feb0810.mp3

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      • #63
        Originally posted by thehoj
        I really like the sound of the OD I'm getting out of the amp now.
        Sounds really nice, Hoj. You've done a great job sticking with this amp and also documenting it for everyone.

        In this last change, do you bring back a switchable NFB?

        RWood

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        • #64
          Originally posted by sjcarter View Post
          I've documented my conversion of a Bogen Challenger CHB335a for Bass Guitar, Guitar, Microphone and Music player use. I used information found here and on other web sites. My conversion notes can be found at:

          CHB35a Amp conversion notes & pics

          Hope my notes help someone!
          If you haven't done so already, you should install a 3-wire AC cordset. I didn't see it mentioned in your notes. It's the first thing I do with old 2-wire amps.

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          • #65
            Originally posted by RWood View Post
            Sounds really nice, Hoj. You've done a great job sticking with this amp and also documenting it for everyone.

            In this last change, do you bring back a switchable NFB?

            RWood
            Yea I did bring that back, I don't use a presence control anymore though. The other thing I did which improved the sound a lot for me was change the cathode resistor on v1a to a 1.5k (instead of 820ohm) and I removed all of the preamp cathode bypass caps.

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            • #66
              I just bought a Bogen CHB 100 and saw this post. When I try thehoj's links to his schematics, they now lead nowhere (not surprising since I'm 10 years late to this party). Anyone have a copy they could share?

              Thanks,
              MG

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              • #67
                I have one of these and a CHB-35A in the shop right now that I am working on (slowly.) I can post up the preamp changes I made sometime if it's any help to you. It will likely take a little while before I get it finalised.

                Andy

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                • #68
                  That would be very much appreciated; thanks, Andy!

                  MG

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                  • #69
                    I did a whole bunch of modding and work to a CHB 100. I have a second one here to duplicate what I did on the first one. They're pretty cool amps, though the chassis layout is poor, especially where the transformers are concerned.

                    Greg

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                    • #70
                      hi Greg, what are a couple things you’d recommend as a mod to these? I’m using it as a test bed (first time tinkerer) so I’ll try whatever will get it to sound decent with guitar. Thanks, MG

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                      • #71
                        I gutted mine and started over with my own circuit. The transformers are too close together with the same orientation on the stock chassis, so if you turn the gain up then it will hum more than it should. I found that out the hard way but I am stuck with it for this amp. I did a parallel stage on the input into a pentode, then a LTP phase inverter into the power tubes. No negative feedback. Yes I added one preamp tube. I optimized my layout and grounding. I'm getting about 55-60 Watts RMS and it sounds pretty killer. I'm sure you can come up with something to make it sound great! The 7868 tubes are nice too...I would keep them rather than change to something else.

                        Greg

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                        • #72
                          I hadn't noticed the transformer orientation. I'm going to take a similar approach and start with a new preamp circuit, but use existing tubes. I'll leave the rest as is from the cathodyne phase inverted forward and see how it sounds.

                          MG

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                          • #73
                            Originally posted by moonsgin View Post
                            I hadn't noticed the transformer orientation. I'm going to take a similar approach and start with a new preamp circuit, but use existing tubes. I'll leave the rest as is from the cathodyne phase inverted forward and see how it sounds.

                            MG
                            Yeah I started with that too, but then gutted it to start over. Ideally I would have moved the transformers away from each other a bit. They are both "Z" mounted, and since they are so close I suspect their magnetic fields interact with each other too much with higher gain. My amp isn't a super hummy beast, and it is unnoticeable in a band context on stage when the music is being played, but the perfectionist in me says that it would hum less if I had made a new chassis with relocated iron.

                            I suggest to remove the NFB and see how it sounds. I liked mine a lot more when I did that. I didn't change the power section much at all but I used a LTP phase inverter as it has a distinctive sound that I liked.

                            Greg

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