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Bogen CH8 to 3-knob champ with 6L6 conversion filter capacitor questions

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  • Bogen CH8 to 3-knob champ with 6L6 conversion filter capacitor questions

    New member saying hello and hoping to tap into the collective wisdom of this group.
    I am a newbie at this, so please forgive any totally lame statements on my part.

    I have picked up an older Bogen CH8 PA and I would like to mod it like Terry from D-Lab did in this video:
    https://www.youtube.com/watch?v=sMfYSLZj4hc

    I have email Terry about his conversion but he has not replied yet.

    I am getting ready to order the parts, but I have a question about the filter cap can.
    In the schematics that I can find online, the caps are: 5 µF@400V, 5 µF@400V, 5 µF@400V, 5 µF@400V. However, my cap can is: 20Mfd @450V, 20Mfd @450V, 20Mfd @450V, 20Mfd @450V (see pic)
    And it looks like the Fender AA 764 schematic uses: 20µF @450V, 20 µF @450V, 20 µF @450V
    Question 1: Is Mfd the same as µF? (I think it is, but just want confirmation)
    Question 2: Is the schematic for the Bogen incorrect for my amp in terms of the filter capacitors? Or has the cap on my amp been changed.

    Any tips or suggestions?

    Relevant information: I plan on replacing the 6SF5 with a 12AX7 and if I can figure it out, rewiring the 6SJ7 for a 6SL7 preamp tube like in the video.
    Power transformer: Thordarson TS-24R01 Secondary 325V-0V-325V
    Output transformer: Merit A2904

    Thanks.
    Attached Files
    Last edited by tboy; 11-27-2017, 03:40 AM. Reason: fixed link

  • #2
    Welcome to MEF!

    I would consider 5uF to be undersized in a HV tube amp power supply filter. Either your Bogen has been upgraded to 20uF, or the design changed, but the schematic didn't. Either way, it is an improvement. If you want to use the original 4 slot mounting insulator to mount the new can, you could get a 20/20/20/20@450v can, and jumper two of the 20's to make a 40uF for the first node for a little better filtering.

    Mfd is the same as uF, both express microfarads. In dealing with caps, especially in high voltage situations, the tolerances for capacitance or uF is pretty loose, and not highly critical, so the difference between 20 and 40 uF isn't that much. However, the voltage rating is more important. Always use a rating at or higher than designed. For instance, this four cap 20uF can is rated at 475v, where the original is 450v, making the replacement an improvement, especially with today's higher wall voltages. https://www.tubesandmore.com/product...f-electrolytic

    And if you mess up your first amp, I know a guy....
    It's weird, because it WAS working fine.....

    Comment


    • #3
      You really don't need to use a filter cap can. Electrolytic capacitors are much smaller and more accurate than 50 years ago. You could use individual capacitors. It's cheaper than a replacement can cap. You can also "stuff" them in the old can if you want, but I'm sure there's plenty of room in the box either way. Since you are changing and rewiring tube sockets anyway, I would recommend using all new ceramic sockets. They are very inexpensive. I would also consider using a turret or tag board. If it's your first build, point to point construction can be maddening.
      Last edited by olddawg; 11-27-2017, 03:10 AM.

      Comment


      • #4
        Randall, thanks for the info. I am gong to give it a shot and I will keep you posted. If I get in too deep, I may need to take it to that "guy" you know. He has done some great work for me
        Last edited by tboy; 11-27-2017, 04:19 AM. Reason: typo

        Comment


        • #5
          Those are good ideas olddawg. I am looking at using an eyelet board.

          Comment


          • #6
            Since this amp uses a 6L6 single ended... you might want to look at a few Princeton schematics. If the pilot lamp is run off the filament voltage, if you run a neon or LED lamp off of the power switch, you might even free up enough heater current for a Princeton Reverb build. If you use an eyelet or turret board it is easy to mount all of the power supply filter caps on one side. You can also move the OT to the other side if the chassis to give you more room.
            Last edited by olddawg; 11-27-2017, 09:25 AM.

            Comment


            • #7
              Have a CH8 that I converted

              I am wondering if this amp got finished? I have one I converted - It is a 6AV6 > 6SJ7 > 6L6 with a 5y3 rectifier.
              I did not want to go the D Labs route - as I have built many champs. I wanted to keep the tube compliment and have the 6AV6 as the first gain stage, feeding the 6SJ7. I learned that the early Matchless Clubman had a preamp like that. But I could not find an accurate schematic. It took me a lot of experimenting, but I got there. There is weirdness in the Bogen power supply. The power transformer is too strong, so they bled some power to ground through large resistors. And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids - WTF? I think that CH8 revA schematic has mistakes.
              So - did you get that amp done?

              BMac

              Comment


              • #8
                "And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids "

                The reasoning behind the lower screen voltage is to limit the overall amplifier output.

                After all, it is only a PA amp.

                Comment


                • #9
                  Originally posted by bmccowan View Post
                  I am wondering if this amp got finished? I have one I converted - It is a 6AV6 > 6SJ7 > 6L6 with a 5y3 rectifier.
                  I did not want to go the D Labs route - as I have built many champs. I wanted to keep the tube compliment and have the 6AV6 as the first gain stage, feeding the 6SJ7. I learned that the early Matchless Clubman had a preamp like that. But I could not find an accurate schematic. It took me a lot of experimenting, but I got there. There is weirdness in the Bogen power supply. The power transformer is too strong, so they bled some power to ground through large resistors. And in some Bogen amps like the db15 they ran the 6L6 plates 100 volts higher than the screen grids - WTF? I think that CH8 revA schematic has mistakes.
                  So - did you get that amp done?

                  BMac
                  As was stated before.. look at some Fender Princeton schematics. I doubt the conversion is too complex to copy their power supply architecture for the output stage. I would probably gut it and just build a Princeton if the PT specs are close...

                  Comment


                  • #10
                    No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.

                    Comment


                    • #11
                      Originally posted by Chemtooler View Post
                      No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.
                      What exactly is meant by “too strong”? There is just voltage and current. It shouldn’t matter unless your B+ is too high or it can’t deliver enough current. If the PT is more “robust” it usually just means less sag. I put a PT meant for a 30(ish) watt amp in a 20(ish) watt amp the other day. It may be a little stiffer but it sounds fine and is reliable as hell.

                      Comment


                      • #12
                        Some of these old amps were built around specs for the metal 6L6{nothing}.

                        Design center values were:

                        Plate Voltage 360V
                        Screen Voltage 270V
                        Plate Dissipation 19W

                        The metal shell is connected to pin 1 so designs like this usually have pin 1 on the socket grounded. Some old Fender amps are wired like this. You see it on the layouts.
                        WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                        REMEMBER: Everybody knows that smokin' ain't allowed in school !

                        Comment


                        • #13
                          Originally posted by Chemtooler View Post
                          No, I have not got it up and going yet. I had a bad output transformer and then ran into an issue with the burning out the 6L6 tube. I put it on the shelf, but I am looking to get back into it in a few weeks. I am interested in your comment about the power transformer being too strong. I need to look at that to see if that might have been my issue.
                          Hi - I should have been more specific. Even with a 5Y3, B+ voltage is very high (500+) without dumping some to ground. Your amp is a somewhat different version of the CH-8, so the power transformer spec may be different. Based on real Bogen schematics I've seen for some of the other PAs, I suspect the CH-8 has two 47K large wattage resisters in parallel to ground on the power supply. That would be 23.5K of resistance. This also allows the voltage to bleed off after the amp is turned off. But if that was the only purpose the resistance would be much higher; 220K is common. I used 20K and that brought the plate voltage down to 445.
                          I'd be happy to show you what I did, but you may be looking for something different, as there are a lot of options. Although Old Dawg mentioned a Princeton Reverb, that's push-pull and the CH8 is single ended. I was looking to stay single ended, as I have built many push-pull 6V6/6L6 amps. And I wanted to experiment with a pentode (6SJ7) in the second preamp position. Very few commercial amps did so, the Matchless Clubman being one of them. the early clubman had a 6AT6 > 6SH7 or 6SJ7 in the preamp, later they changed to 12AX7 > EF86.
                          I know that what D Labs did will work fine, as I have built that same configuration in the past - a Champ circuit that can use either a 12AX7 or a 6SL7, and its an easier build as there are accurate schematics out there. Angela instruments has/had a post for that option on the Super Single Ended amp schematic/instructions. I built a Valco circuit 6SL7 > 6V6 champ type amp a few years back, and its a favorite.

                          Comment


                          • #14
                            Originally posted by bmccowan View Post
                            Hi - I should have been more specific. Even with a 5Y3, B+ voltage is very high (500+) without dumping some to ground. Your amp is a somewhat different version of the CH-8, so the power transformer spec may be different. Based on real Bogen schematics I've seen for some of the other PAs, I suspect the CH-8 has two 47K large wattage resisters in parallel to ground on the power supply. That would be 23.5K of resistance. This also allows the voltage to bleed off after the amp is turned off. But if that was the only purpose the resistance would be much higher; 220K is common. I used 20K and that brought the plate voltage down to 445.
                            I'd be happy to show you what I did, but you may be looking for something different, as there are a lot of options. Although Old Dawg mentioned a Princeton Reverb, that's push-pull and the CH8 is single ended. I was looking to stay single ended, as I have built many push-pull 6V6/6L6 amps. And I wanted to experiment with a pentode (6SJ7) in the second preamp position. Very few commercial amps did so, the Matchless Clubman being one of them. the early clubman had a 6AT6 > 6SH7 or 6SJ7 in the preamp, later they changed to 12AX7 > EF86.
                            I know that what D Labs did will work fine, as I have built that same configuration in the past - a Champ circuit that can use either a 12AX7 or a 6SL7, and its an easier build as there are accurate schematics out there. Angela instruments has/had a post for that option on the Super Single Ended amp schematic/instructions. I built a Valco circuit 6SL7 > 6V6 champ type amp a few years back, and its a favorite.
                            There are are versions of the Princeton that use a single ended architecture and have a higher output than a Champ. You may be correct that all of the Princeton Reverbs were push-pull. You might want to peruse the schematics online. There was a long thread on this forum a while back about 6L6 single ended Princetons also. People seem to like them.

                            Comment


                            • #15
                              Originally posted by olddawg View Post
                              There are are versions of the Princeton that use a single ended architecture and have a higher output than a Champ. You may be correct that all of the Princeton Reverbs were push-pull. You might want to peruse the schematics online. There was a long thread on this forum a while back about 6L6 single ended Princetons also. People seem to like them.
                              Yes - there are single ended Princetons; but there were never single ended 6L6 Princetons as all single ended Princetons were 5 watt 6V6 amps.
                              Tweed Princetons - single ended - same as champ circuit, but with an added tone control. Same wattage (5 watts) as Champ, 1-6V6, but depending on vintage, a larger speaker.
                              After the tweed era - all Princetons were push pull 2-6V6.
                              Brownface Princetons - push pull, 2-6V6.
                              Blackface & Silverface Princetons - push pull, 2-6V6.
                              All Princeton Reverbs - push pull, 2-6V6.
                              Scratch built, kits, or converted PA amps that people sometimes call Princetons - Any of the above and more, including single ended 6L6 amps. Some people even change the power tube cathode resistor and use a 6L6 in SE Champs and Princetons, but the power transformer is working harder than it was designed. Dave Hunter's two-stroke started out as a Champ type circuit with parallel 6V6 and then moved to a single 6L6. Weber's Maggie is the same. Yes people like them, I have built them and like them.
                              The single ended Champ type circuit was used by just about everybody back in the day: Fender, Gibson, Valco, Webster, etc. It was not an original Fender circuit, they are all derived from a Western Electric circuit.
                              Princetons are Fenders, and they are all good amps as far as I am concerned. I have rebuilt several and scratch built a few.
                              Chemtooler referred to what DLabs had done with their Bogen CH8- gutted it and rewired it with a Champ type circuit (same as a Princeton type circuit.) Terry hooked up the preamp sockets for the use of either a 9 pin 12A_7, or an octal 6SL7.
                              What I did with my CH8 was to use a Matchless Clubman type preamp 6AT6 (7 pin triad) into a 6SJ7 (Octal Pentode) with the 6L6 power tube. Different - not better, not worse, just different.

                              Comment

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