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

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  • #31
    Interesting.
    Any thoughts on running at 70% plate dissipation vs. more?
    Also if the bias test points make sense?

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    • #32
      Originally posted by thehoj View Post
      Interesting.
      Any thoughts on running at 70% plate dissipation vs. more?
      Also if the bias test points make sense?
      Yes, the 70% is a good idea and your tubes will last longer than the higher dissipation. I just wanted to give you the maximum current that you won't want to exceed. I still think that listening for the results within that range is what you want to do.

      Btw, plate dissipation in watts is not the same as output power. I have no idea how the latter is calculated, especially since it always seems exaggerated!

      And yes, I think the test points are a great idea. I would use 1 ohm, 1 watt, 1% tolerance resistors. Much easier than unsoldering the cathode-to-ground connections everytime you want to check bias! For octal tubes, making an inline bias checker has been a big help to me (great instructions on Hoffman's site) but for these noval pin tubes, it is impractical. This 7868 tube might be the only one we work with that is in this format.

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      • #33
        For octal tubes, making an inline bias checker has been a big help to me (great instructions on Hoffman's site) but for these noval pin tubes, it is impractical. This 7868 tube might be the only one we work with that is in this format.
        Sorry, so you're saying the 7868 is octal? and that it's okay to use the resistor?

        Also, I will definitely use my ear to fine tune the bias, making sure not to exceed maximum current.


        My parts are supposed to be here next wednesday.. It's going to be hard waiting for them!

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        • #34
          No, the 7868 is noval - nine pins - but bigger than the 9 pin miniatures like EL84, 12AX7, etc. As far as I know, it is the only remotely common power tube that comes in this size. So building a socket-and-pin bias checker, which is a good tool to have when servicing octal-tubed amps, is impractical.

          The resistors with test points, however, is perfect for your build.

          Maybe while you are waiting for your parts you can prepare your workbench, get tools organized, and make sure you have supplies on hand like shrink wrap, solder braid, tools, etc - - stuff you can find locally if you need it. If you have bad eyes like mine, a mag light is invaluable.

          Good luck!

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          • #35
            Thanks for clarifying!

            I am in the process of getting things in order.
            I have a good little work table, but I still need to add some additional lighting, and get some extra supplies, so that is definitely the plan for this weekend.

            I'll try to take some pics at various stages, it'll be neat to keep track of the project as I go.

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            • #36
              So my parts arrived, and I went through and wired things exactly like my last schematic.
              http://vwtweaked.ca/images/bogen%20a..._tp%20pres.jpg

              After wiring it all up I took the power tubes out, also making sure that the amp was in standby mode now that I have that standby switch in there (so that I wouldn't be supplying power to the OT when I was playing around with the bias circuitry). And then I powered it up, and measured on the red wire supplying the OT, and it read 230V.. So I turned that off right away. I think perhaps the standby switch isn't in there right..
              On the Ampeg the red/yellow wire from the PT goes to ground, and on the bogen it goes back into the filter cap section. Do I need a dpst switch in there to break both lines?

              So since I was anxious to keep going with this I decided to put the power tubes back in, and put a load on the PT to make sure I don't damage it, and a just avoid trying to run this with no power tubes and no load at all.

              After doing that and powering it up, my bias voltage pot didn't seem to actually make any adjustment, I also had a 50ohm power resistor from the cathode of the power tubes to ground, and as I measured over it, I was getting 10v, which is way too high. I should get more like 1V (50 x 0.027ma). So I quickly turned that off.

              Thinking that perhaps I should just stick with the original bias section of the bogen, I wired it up like it was originally (with new components), also, thinking that perhaps the 50ohm resistor to ground from the power tubes wouldn't be the most accurate, I put my multimeter in series from cathodes of the power tubes to ground.
              I powered it up like this, and I seem to be getting like 0.32 on the 10a scale (so 320ma).. I'm sure I have it wired up just like the original schematic. So this is really confusing to me.
              I have the cathode of one power tube connected to the cathode of the other, and then I have a wire from that point connected to my positive lead of the multimeter, and then ground is connected to the negative lead of the multimeter.

              Any thoughts about why:

              1) the current seems to be so high even with the original bias circuitry (new components)?
              (am I measuring incorrectly somehow?)

              2) the bias adjustment pot in the latest schematic I drew up wouldn't change the voltage drop I was getting over the resistor from cathode to ground of the power tubes when I did my initial testing?

              3) the standby switch wiring isn't actually cutting voltage to those filter caps?
              (do I need to cut both red and red/yellow from the PT?)
              Last edited by thehoj; 09-27-2009, 04:28 AM.

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              • #37
                Since it isn't center taped you might want to use a full wave bridge like this. It makes more sense and it is easier to open the HV B+ line.
                Attached Files

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                • #38
                  Originally posted by olddawg View Post
                  Since it isn't center taped you might want to use a full wave bridge like this. It makes more sense and it is easier to open the HV B+ line.
                  Would it work the same to just use a dpst switch to open and close the red and red/yellow lines at the same time?

                  EDIT: Just tried it and no it doesn't work. I guess because I need that red/yellow connection to come back up for the bias section.

                  It's really weird why I'm getting such high current from the cathode of the power tubes to ground.
                  Also, if I measure the DC voltage on the output of that bias circuitry I get 0v..
                  Is that why I'm getting such high current through the tube? 0v for a bias voltage would mean that I'm not controlling the current running through the power tube at all..
                  That's probably what's happening isn't it.. I'm gonna play around with that a bit I think.

                  Another strange thing, is that the voltages at each section of the power supply circuitry is higher than the schematic says it should be. If I measure the voltage getting to the plates of each pre-amp tube, I get 200v on v1, v2a & b, and then 298v on v3..
                  I checked the datasheets for 12ax7's as well as 6c4's, and the 12ax7 max plate voltage is 300, and the 6c4 is also 300v.
                  Regarding this, firstly I'm confused about why I'm getting such high voltages, and secondly is this actually a problem if I'm under the maximum rating?..

                  If I did decide to do a full wave, would it be as simple as this?:
                  http://vwtweaked.ca/images/bogen%20a...ull%20wave.jpg

                  I measured the ac voltage from the red and red/yellow secondaries on the PT, and it measured about 470VAC. Will doing this full wave rec output 470V DC? Would 1n4007's be okay to use?
                  And would I still need those 2 40ufd caps in the 460v supply section, or just one of them?
                  Last edited by thehoj; 09-27-2009, 05:45 PM.

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                  • #39
                    Originally posted by thehoj View Post
                    Would it work the same to just use a dpst switch to open and close the red and red/yellow lines at the same time?

                    EDIT: Just tried it and no it doesn't work. I guess because I need that red/yellow connection to come back up for the bias section.

                    It's really weird why I'm getting such high current from the cathode of the power tubes to ground.
                    Also, if I measure the DC voltage on the output of that bias circuitry I get 0v..
                    Is that why I'm getting such high current through the tube? 0v for a bias voltage would mean that I'm not controlling the current running through the power tube at all..
                    That's probably what's happening isn't it.. I'm gonna play around with that a bit I think.

                    Another strange thing, is that the voltages at each section of the power supply circuitry is higher than the schematic says it should be. If I measure the voltage getting to the plates of each pre-amp tube, I get 200v on v1, v2a & b, and then 298v on v3..
                    I checked the datasheets for 12ax7's as well as 6c4's, and the 12ax7 max plate voltage is 300, and the 6c4 is also 300v.
                    Regarding this, firstly I'm confused about why I'm getting such high voltages, and secondly is this actually a problem if I'm under the maximum rating?..

                    If I did decide to do a full wave, would it be as simple as this?:
                    http://vwtweaked.ca/images/bogen%20a...ull%20wave.jpg

                    I measured the ac voltage from the red and red/yellow secondaries on the PT, and it measured about 470VAC. Will doing this full wave rec output 470V DC? Would 1n4007's be okay to use?
                    And would I still need those 2 40ufd caps in the 460v supply section, or just one of them?
                    Diodes are cheaper than switches. Sorry , that schematic wis incorrect. I just took it off osf the 18 watt site. and it was late So your bridge is hooked up incorrectly in your diagram. You are running AC on your B+ line that way. You need to put the switch and the B+ line where the + symbol is marked on the bridge in the diagram. Follow the foreward bias paths of the diodes. Here's a better example.
                    Attached Files
                    Last edited by olddawg; 09-27-2009, 06:29 PM.

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                    • #40
                      So this would be more accurate then I suppose:
                      http://vwtweaked.ca/images/bogen%20a...ave%20rev2.jpg

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                      • #41
                        Originally posted by thehoj View Post
                        So this would be more accurate then I suppose:
                        http://vwtweaked.ca/images/bogen%20a...ave%20rev2.jpg
                        Yeah that's it. Sorry about the earlier diagram. It was late and you always have to check other peoples stuff for mistakes. Its much more understandable drawn that way. The only thing you might have to do is tweak the dropping resistor values on the half wave bias supply at the at the top but probably not. I believe that most of your problems are resulting from simple mistakes that happen when you re-engineer an existing amp. I didn't read all of the posts, but I always find it easier and usually more economical (especially time wise) to gut a doner amp, wire up a new turrent board from new parts (from a known good and tested design), and replace all of the filter caps. That way you can also start with a new filament dress, etc. It will save a lot of hum headaches when you debug the build.

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                        • #42
                          Yea.. this is becoming a bit difficult to troubleshoot, especially given my fairly limited experience with this type of thing.

                          Now I don't know what I had measured earlier, but I've just now measured my ac voltage between the red and red/yellow secondaries on the PT, and I get 188VAC. If I measure the output of the full-wave rectifier to ground I get 260Vdc.. Which makes sense given that AC I suppose..

                          So was the circuit before some sort of doubling circuit? I seem to recall hearing that about this PT now that I think about it.
                          http://vwtweaked.ca/images/bogen%20a..._tp%20pres.jpg

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                          • #43
                            Originally posted by thehoj View Post
                            Yea.. this is becoming a bit difficult to troubleshoot, especially given my fairly limited experience with this type of thing.

                            Now I don't know what I had measured earlier, but I've just now measured my ac voltage between the red and red/yellow secondaries on the PT, and I get 188VAC. If I measure the output of the full-wave rectifier to ground I get 260Vdc.. Which makes sense given that AC I suppose..

                            So was the circuit before some sort of doubling circuit? I seem to recall hearing that about this PT now that I think about it.
                            http://vwtweaked.ca/images/bogen%20a..._tp%20pres.jpg
                            What do you get with the standby switch on and the filter caps in circuit? If those tubes need 450v you may have to go back to the original power supply configuration or use different output tubes. (EL84s maybe) Its always a good idea to spec out the primary secondary voltages before you start anything. It will tell you what you can use and what you can do. Re-engineering an amp can be a pain. I put a lawnmower engine on my bike when I was a kid. It would go 45mph but that didn't make it a motorcycle and it was dangerous as hell. But it would get you from point A to point B, if you get my drift.

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                            • #44
                              Yea, I definitely need to go with the original circuit design. It's getting me up to the 500v range, but it's higher than I think it should be.. It's like 520v, then 510v, then 410v, then 370v.

                              I'm going to put some power resistors before the first stage of that to bring it down to starting at 460v.

                              Comment


                              • #45
                                Originally posted by thehoj View Post
                                Yea, I definitely need to go with the original circuit design. It's getting me up to the 500v range, but it's higher than I think it should be.. It's like 520v, then 510v, then 410v, then 370v.

                                I'm going to put some power resistors before the first stage of that to bring it down to starting at 460v.
                                The voltages will be higher with the tubes out. You will probably have just a little bit higher voltages with the tubes in because line voltages are higher in general than decades ago. You can always put a couple of 10v 5 watt zeners in series with the B+ line if you are still concerned. Its done all of the time and works better than big resistors.

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