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  • Boggie Mod Questions

    I've got a couple questions to ask of some more experienced solder fume breathing amp constructors..

    First off I've had this Bogen chb35a sitting around for about 15yrs and now have got enough nerve to start working on it...

    I have replaced all the filter caps only to find out that the Output Transformer is burt.. So I ordered a hammond 1620 which appears to be a suitable drop in judging from another thread I read here or perhaps another forum.

    When I took a good look at the tube sockets they were very dirty, as if someone had spilled some kind of food into them and mashed it around.. yuck!..... So needless to say I replaced the sockets with new ceramic ones as the would be home to the new tubes I purchased as well.

    My whole idea with this amp is to convert it to guitar use... I have already modified the schematic to a Fender tone stack, but I would also like to make V1A (6eu7) switchable as a "dirty" channel.. I've been looking at the possibility of cascading V1B into V1A but I don't know if I'm even going about it the right way.

    I modified it on the schematic (attached) to show V1B outputing into the grid of V1A via switch.

    Does this look correct? Or is this even possible? If so what has to be changed in order to do it this way?
    Attached Files

  • #2
    The original schematic has a HiWatt look to it at the 12AX7 stage, if that appeals to you at all. There's nothing wrong with your tone stack though.
    It's not too far from 'functional' if it weren't for one thing (that is can see). Notice V1B is directly connected to the input and output of V1A. You'll need a large (220K - 470K) resistor between the Volume center wiper and the right side of your Channel switch to isolate the input/output of V1A.
    Also, remove the 220K resistor from V2A pin 3. a grid stopper might also be nice to add.
    Other than that, I think it should work, aside from fine tuning. You might want to adjust the 2.5M resistor above the Volume pot depending on how much drive you want. There are a lot of dropping resistors on the B+ lines for V1 also. If it does not respond when you fire it up, check the voltage on plates of V1 and adjust you dropping resistors accordingly.

    The .1uF at the input of V1A is not absolutely necessary. If you remove it, the 1M resistor may do a better job of reducing a 'pop' when you hit the switch.
    You might also experiment with a resistor under the V1A Volume pot to ground. When engaged, your Drive signal should be at unity gain with that volume all the way down.
    Good luck.
    Black sheep, black sheep, you got some wool?
    Ya, I do man. My back is full.

    Comment


    • #3
      Originally posted by GibsonLover View Post
      The original schematic has a HiWatt look to it at the 12AX7 stage, if that appeals to you at all. There's nothing wrong with your tone stack though.
      It's not too far from 'functional' if it weren't for one thing (that is can see). Notice V1B is directly connected to the input and output of V1A. You'll need a large (220K - 470K) resistor between the Volume center wiper and the right side of your Channel switch to isolate the input/output of V1A.
      Also, remove the 220K resistor from V2A pin 3. a grid stopper might also be nice to add.
      Other than that, I think it should work, aside from fine tuning. You might want to adjust the 2.5M resistor above the Volume pot depending on how much drive you want. There are a lot of dropping resistors on the B+ lines for V1 also. If it does not respond when you fire it up, check the voltage on plates of V1 and adjust you dropping resistors accordingly.

      The .1uF at the input of V1A is not absolutely necessary. If you remove it, the 1M resistor may do a better job of reducing a 'pop' when you hit the switch.
      You might also experiment with a resistor under the V1A Volume pot to ground. When engaged, your Drive signal should be at unity gain with that volume all the way down.
      Good luck.
      Wow thanks for the help! I've posted a couple times on other forums and this is the best info I have gotten so far! (its not easy being green)

      I would like to ask a couple questions about your reply if you don't mind..

      The 220k-470k resistor you suggested adding on the wiper of the volume pot of V1A, should it be located in a way that does not block the path of V1B's output?

      And removing the 220k resistor from pin 3 of V2A would let the full 240v go directly to the cathode of V2A correct? Will this Damage anything?

      Also I'm still kinda bad with some of the terminology lol, what might a "grid stopper" be? I've been through so much info in the past couple months I may have heard it before and forgot

      Again thanks alot! It's alot easier to learn when you can ask

      Comment


      • #4
        Replace the wire from the V1A volume wiper to the right side of the channel switch with a 220K resistor.
        You might find some other schematics that use a 2-section switch so that the input of V1A is grounded when it is not used. That might help keep noise and the chance of oscillation down.
        A grid stopper is just a series resistor with the grid to prevent overload. You'll see anything from 1.5k on output tubes to 68K on preamp tubes.
        You may have also noticed that both sections of V1 are grid leak bias. Be aware that they tend to overload with pedals, and your second stage may get overwhelmed by the gain of the first stage. Look carefully at the difference between yours and any Fender preamp schematic stage and you'll see the difference. The cathode will go straight to ground on grid leak, and Fender will have a ~1.5K resistor. Both work, just a different way to bias the tube. It gives you more options if you decide to take a different path.
        There should be no 240V at all going to the cathode of your first 12AX7 stage. Remove the connection and resistor entirely, just the resistor to ground is good. Again, look at (almost) any Fender preamp schematic for the differences and you'll pick it out.
        Good luck.
        Black sheep, black sheep, you got some wool?
        Ya, I do man. My back is full.

        Comment


        • #5
          Originally posted by GibsonLover View Post
          Replace the wire from the V1A volume wiper to the right side of the channel switch with a 220K resistor.
          You might find some other schematics that use a 2-section switch so that the input of V1A is grounded when it is not used. That might help keep noise and the chance of oscillation down.
          A grid stopper is just a series resistor with the grid to prevent overload. You'll see anything from 1.5k on output tubes to 68K on preamp tubes.
          You may have also noticed that both sections of V1 are grid leak bias. Be aware that they tend to overload with pedals, and your second stage may get overwhelmed by the gain of the first stage. Look carefully at the difference between yours and any Fender preamp schematic stage and you'll see the difference. The cathode will go straight to ground on grid leak, and Fender will have a ~1.5K resistor. Both work, just a different way to bias the tube. It gives you more options if you decide to take a different path.
          There should be no 240V at all going to the cathode of your first 12AX7 stage. Remove the connection and resistor entirely, just the resistor to ground is good. Again, look at (almost) any Fender preamp schematic for the differences and you'll pick it out.
          Good luck.
          Thank you for the help GL!

          I'm getting a better understanding of how this all needs be. My output transformer is being delivered tomorrow so I'm going to get that put in and get the amp fired back up to make sure my cap job is solid and its modification from there..

          I'm going to try to keep this post updated. I know there seems to be alot of people converting these Bogens, so maybe this might serve to help someone else too!

          I got a new layout for the front and back of the chassis drawn up too and have plans to place the amp inside a old echophone radio box I have It may look pretty neat when it's finished as well as sounding good (hopefully)

          Comment


          • #6
            well my output transformer is installed, but I seem to be having the same problem as I did before I replaced it..

            I turn the amp on and I get a loud buzz followed by a harsh squeal and no output from the guitar..

            So I dove for the power switch.. and decided to do some voltage reading without the tubes..

            My output tube plates are at 520v
            plates of V1 are 360v
            plates of V2 are 395v

            power transformer before rectifier 260v
            B+ after rec/doubler 520v

            I've heard the voltages at the tube plates will be higher if the tubes are not present, but these voltages seem rather high with that taken into account..

            Any Ideas?
            Attached Files

            Comment


            • #7
              I'm Guessing I made a typical noobie mistake freaking out on the the voltage reading without the tubes in...Lol

              I put the tubes in and ran the amp on my iso transformer set to a 117v main line voltage as printed on the chassis and every thing seems to be at the correct levels..

              The screens of the output tubes call for 420v on the schem and measure
              440v so that is the only high reading I had but it is within the 20% tolerance...

              So with that out of the way I need to find out what is making all the buzzing and screeching sounds.

              In the original schematic there were two remote lines connected to terminals on the back of the amp leading back to the plates of V1 through two 470k resistors and a 330k resistor.... I removed the wires to the terminals but left the resistors on the plates.. Now I have read (correct me if im wrong) that adding a higher resistance to the plates of a preamp tube will increase the gain... Would anyone be able to tell me if having these three resistors on the plates of V1 be overloading the later stages with too much gain and thats the cause of all this noise and no guitar output?

              Would removing these two 470k resistors and just leaving the 330k help? Only one resistor is necessary right?
              Attached Files

              Comment


              • #8
                Well I seem to be answering my own questions quite often here... yet another mistake by a noob.... I didnt have the OT grounded lol looks like I have an extra one for another project.. Obviously before I start another project I need to make sure these kinds of things dont happen anymore....

                On a lighter note It's on to the modification from here. Time to get some parts ordered!

                If anyone needs to wire up a hammond 1620 on a 3 way impedance selector I made a diagram after a hour of headache, but it works!
                Last edited by skid20; 05-02-2010, 03:37 AM.

                Comment

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