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Bogen CHB-20A Conversion

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  • #1

    Bogen CHB-20A Conversion

    Hello All, newbie here. Great forum!

    I just "won" a Bogen CHB-20A on eBay and was wondering if anyone here has any other info on this amp other than a schematic (I found one of those)?

    My life as an EE started in the early 70's, back when they taught tubes. It's been a while since I've worked with them, but I think I know enough to keep from getting electrocuted/burned. The question I have is this: the amp is untested, sold as is. What is the safest sequence to power it up? Should I do it with no tubes to start and measure voltages or what?

    The goal I have for this amp is to build a "quick and dirty" head that will give me a little more power than the Ruby 386 based SS amp I currently use.

    Thanks,
    Brian
    Some people are like slinkies, not good for much, but they bring a smile to your face when you push them down a flight of stairs.
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  • #2
    What is the safest sequence to power it up? Should I do it with no tubes to start and measure voltages or what?
    There's a lot of things you can do for safety, and that's a good one.

    I usually keep things simple.

    You can test the electrolytics with an ohmmeter. They usually need replacing in old amps, but sometimes they last.

    After doing it a bunch of times without, I built a simple light bulb current limiter using a lamp and an inexpensive three prong cord. Pros have a varistor with an ammeter, but they're somewhat expensive.
    See the birth of a 2-watt tube guitar amp - the "Dyno Tweed"
    http://www.naturdoctor.com/Chapters/Amps/DynoTweed.html

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    • #3
      Welcome.

      I use a dim bulb tester when I run an old amp for the first time. If there's a short you won't harm anything. here's a link

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      • #4
        Thanks John_H, good link!!
        Some people are like slinkies, not good for much, but they bring a smile to your face when you push them down a flight of stairs.

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        • #5
          Bogen conversion

          I've done 5 Bogen conversions including several CHA-20 amps. I've drawn up schematics and layouts using paralleled terminal strips for the Bogen conversions.

          What are the tubes in your amp?

          What kind of tone are you looking for?
          I don't know what a "quick and dirty head" is?

          With respect, Tubenit

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          • #6
            Originally posted by Tubenit View Post
            I've done 5 Bogen conversions including several CHA-20 amps. I've drawn up schematics and layouts using paralleled terminal strips for the Bogen conversions.

            What are the tubes in your amp?

            What kind of tone are you looking for?
            I don't know what a "quick and dirty head" is?

            With respect, Tubenit
            Hello Tubenit, thanks for the response. By "quick and dirty", I need something other than my little "Ruby" Solid State amp and thought a quick change of the input jack and few other tweaks on the Bogen would get something reasonable going (with the addition of a speaker of course).

            The amp has ECL86 (2) finals and a 6EU6 pre.

            For now, a clean tone will do and after I play around with it for a while I can decide what I really want.

            Pointers to any of your schematics or other modification notes would be much appreciated.

            Thanks again,
            Brian
            Some people are like slinkies, not good for much, but they bring a smile to your face when you push them down a flight of stairs.

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            • #7
              Bogen conversion

              I have gutted all 5 of the Bogens and rebuilt them into what I wanted rather than trying to adapt the Bogen topology and component values.

              A "quick and dirty" based on your description would possibly to simply replace the old electrolytics and install a input jack in place of the mic jack and try it. If you don't like the tone, then post the schematic and we can offer suggestions for modifying the Bogen with minimal changes.

              I am presuming you know how to safely discharge filter caps (which can contain lethal voltage) and safely work on the amp? Correct?

              With respect, Tubenit

              Comment

              • #8
                Originally posted by Tubenit View Post
                I have gutted all 5 of the Bogens and rebuilt them into what I wanted rather than trying to adapt the Bogen topology and component values.

                A "quick and dirty" based on your description would possibly to simply replace the old electrolytics and install a input jack in place of the mic jack and try it. If you don't like the tone, then post the schematic and we can offer suggestions for modifying the Bogen with minimal changes.

                I am presuming you know how to safely discharge filter caps (which can contain lethal voltage) and safely work on the amp? Correct?

                With respect, Tubenit

                I have a very deep respect for charged caps and the potential contained therein. (mostly from college pranks, but they weren't necessarily deadly).
                Some people are like slinkies, not good for much, but they bring a smile to your face when you push them down a flight of stairs.

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                • #9
                  I have a Bogen CHB50 that sounds great

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