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

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

    I've got a CHB-35A bogen PA amplifier, and I'm using it at the moment with my guitar, running into one of the aux inputs.

    Here's a schematic of the amp:

    As you can see from the schematic I'm basically bypassing v1 all together. Additionally I don't feel like I have proper control over the tone for this amp, as I'm sure it's set up for the vocal range.

    It does sound pretty nice, but I feel like I could do so much more with it.

    It was recommended to me recently to replace all the filter caps as that could be affecting tone quite a bit, so I'll be doing that shortly. In addition though I'm tempted to try rewiring and changing some components to utilize the first gain stage, and also have more control over my tone.

    I found some basic schematics for a gain stage and tmb control on weber's site, and just used that in conjunction with the bogen schematics. I figured I would rewire v1 as a 12ax7 since they're so much more common, and if I'm doing some rewiring anyways, it makes sense.

    Anyways, here's what I came up with. Would this make sense? Am I missing anything with this idea?

    I'm just looking at it again, and I probably shouldn't have that line coming from the green wire on the output transformer anymore should I.. I would probalby disconnect any of the other unnecessary connections from that output transformer as well.

    Thanks for any help. I'm somewhat of a newbie to amps, but I have experience in soldering, and some electrical training (from years ago) so I have a basic understanding of the components in this amp.

  • #2
    Hello. Someone with more experience than myself will probably be of more help but you might want to check out some of the Ampeg stuff. They used the 7868's (cool tube) in some of their amps like this:

    This is a bass amp but it's great for guitar. You may be able to get ideas as far as tone shaping etc.


    • #3
      So I took a look at the sb-12 schematic, and I attempted to modify the chb schematic slightly to make the tone controls similarly on v1, also convert v1 to a 12ax7, and then to v2 and still keep v3 feeding into the power tubes.

      Does my schematic make sense? Is it valid? Did I do anything wrong here?


      • #4
        Your V2a is not doing anything for you, and it's grid is grounded. This will cause trouble if you fire it up this way.

        I would remove that triode from the equation, and treat the V2b and the V3 as the same two triodes that precede the power tubes in the SB-12 schematic (V4) . Whether you do this with two bottles (half a 12AX7 and a 6C4) or one (both triodes of a 12AX7) is up to you.

        Btw, before you made any changes, did you try plugging a guitar into the Mic input?


        • #5
          I haven't made any actual changes to the amp yet, I'm just trying to come up with a schematic for the changes that I should make to get more useable tone from the amp, and get more gain out of it.

          At the moment I'm running into the aux input and bypassing v1 altogether. I did try running into one of the mic inputs and it sounded really bad to me, just seemed to be even muddier and just have a very strange tone that I did not like at all.

          Can I not use both triodes of v2 and also use v3?

          If you'd be so kind, What would be the proper way to use v1 v2 and v3 here?


          • #6
            I would be looking for ways to make this simpler rather than more complex.

            You've got a viable circuit there - it could be that changing out the coupling caps will improve the muddiness that you are hearing through the mic channel. You can check the grids for the presence of DC voltage and replace the couplers that are leaking.


            • #7
              Okay, so as a first step if I replace all of the filter caps, and then you're saying the coupling caps as well, I'll see how that does me.
              Are the coupling caps all of the caps between triodes?


              • #8
                Originally posted by thehoj View Post
                Are the coupling caps all of the caps between triodes?
                Yes, they block the DC that is on the plate (output) of a tube, keeping it from the grid (input) of the next stage, while allowing the AC signal to pass.

                Regardless of whether you do the filters or couplers, the bypass or tone caps, take some time to listen to your results. You will get to know what "changing these" sounds like. The filters, for instance, should remove most of the 60hz hum. Working with them will teach you a lot about power supplies, and different values contribute to the stiffness or looseness of that supply.

                The benefit of the schematic that Distorto posted is that, as you go through yours, you might see that Ampeg used a different value here, or this stage is biased slightly differently than what Bogen did. And make some value changes as you go, listening for the results and knowing that the SB-12 is more geared to an instrument than to a voice.

                Speaking of that model Portaflex, here is a picture of mine.


                • #9
                  Okay, so what you're saying is keep it simpler to start with.

                  I've come up with a new schematic that works with the mic 1 input I've already got, and uses the 6eu7 that's there. But I change caps, and rewire the tone controls between v2a and v2b to match the ampeg.

                  In the schematic I'm linking, I've outlined the caps that I will change the value of and replace with new caps in red (do these new values appear to be more targeted towards guitar tone?), I've outlined other caps in green which I will simply replace but keep the same value, and then I've outlined some sections in purple that I'm slightly confused about (should the volume pots be 1M?), I don't quite understand what they do, and whether I should replace those or not, and what kind of bearing they have on tone, or overall operation of the amp.
                  Should I have the 150 cap before the mic1 volume? What does that do?

                  Also, is it safe to assume that if they don't list a unit amount for a cap, but simply a number, that they're referring to pF?

                  Am I cascading v1a into v1b properly? And is it okay to leave the connections to REM #1 and #2 disconnected? I don't understand what these were for..

                  Finally, how much of a bearing do resistor values have on tone?

                  Any suggestions/input on this schematic are appreciated, as in, are the adjustments I'm suggesting more targeted toward guitar tone? I don't have any electrical components right now, so I need to make an order for parts, and I'd rather not get a whole ton of extra parts that I won't need in the end.

                  Last edited by thehoj; 08-22-2009, 12:10 PM.


                  • #10
                    So as I'm mentioning above I'd like to figure out if technically I've cascaded all the gain stages together correctly, and just make sure that I haven't done anything I shouldn't have, but I'm also struggling with a different issue now..

                    I'm trying to determine whether I've got the correct values for my tone controls.
                    I'm coming around to the idea that I might like TMB controls, and so some fender tone controls are looking interesting to me.


                    I've been playing around with a tone stack calculator, and it bases component values on your source impedance to that circuit. How much of the prior circuitry play into that source impedance value?

                    I understand that I need to include capacitor reactance and resistor resistance in this impedance value, but does all of the prior circuitry up to that point play into that value (tubes included)?


                    • #11
                      Nice job on the schematic composite - that really looks cool.

                      Originally posted by thehoj View Post
                      do these new values appear to be more targeted towards guitar tone?
                      Originally posted by thehoj View Post
                      should the volume pots be 1M?
                      Yes, 1M or 500k will work fine

                      Originally posted by thehoj View Post
                      Should I have the 150 cap before the mic1 volume? What does that do?
                      It's probably in there as an RF snubber. Most guitar amps use a grid blocker resistor there, anything from 10k to 68k, but unless you are picking up radio signals then you can simply wire the input signal straight to the grid socket pin.

                      Originally posted by thehoj View Post
                      Also, is it safe to assume that if they don't list a unit amount for a cap, but simply a number, that they're referring to pF?
                      Yes, as long as the value makes sense there. I agree that the 150 is pF

                      Originally posted by thehoj View Post
                      Am I cascading v1a into v1b properly?
                      I think you are asking for trouble doing this - I would go straight to the volume control and leave that extra triode for another input, or leave it out. But hey, there's nothing wrong with experimenting.

                      Originally posted by thehoj View Post
                      And is it okay to leave the connections to REM #1 and #2 disconnected? I don't understand what these were for..
                      Yes, I would get rid of those. That's the B+ circuit for the input tube, which is the 240V from the final filter cap. It looks as though it was made to be accessible to something else, which is just a shock waiting to happen.

                      Originally posted by thehoj View Post
                      Finally, how much of a bearing do resistor values have on tone?
                      Lots. Is there a specific one that you are wondering about?

                      Aside from the cascaded input channel this looks good to me - - other eyes please respond.



                      • #12
                        Wow, thank you so much for your detailed reply!

                        You've probably also seen my other reply about an alternate idea for tone controls, it's just another idea I'm tossing around, I'm trying not to flip flop around here too much with ideas, but I just keep coming across new information.
                        And I am interested in input impedance calculation.

                        As to your other suggestion of leaving out the v1b gain stage, why would you say that that is asking for trouble?
                        If I do leave out v1a though, does it seem redundant to have mic1 volume into master volume? Would it make sense to simply remove one of those volume controls?


                        • #13
                          So as a preliminary test before I redo the tone, I decided to try and get rid of
                          what I don't need, and wire up the mic 1 input properly.

                          I also decided to disconnect the output wires that I need from the OT from the screw plate on the back of the amp and wire those to a 1/4" speaker output.

                          As far as the input side of things,
                          I attempted to use both v1a and v1b and I seemed to get nothing on the output, basically just some squeaks and squaks.. So I tried wiring it up even simpler like you suggested, just going from v1a to v2a, and when I powered it up it was basically letting out a really loud feedback squeal constantly as soon as the tubes powered up.
                          So I tried putting a .1uf/100v cap between the output of the volume control to the input of v2a

                          Like this:

                          And I still got the squeel.

                          Then I noticed that I wasn't actually grounding the black (ground) wire from the OT to the chassis anymore, I just had the black wire going to the ring of the output jack, and the yellow (8ohm) wire going to the tip. So I grounded this jack to the chassis like it was before when it was attached the screw strip on the back of the amp.

                          Now I'm basically getting no output from the amp.

                          As it's wired up in the schematic I posted here should that be okay? Is there more to coupling triodes?
                          And should the ground of the OT be the same ground that is grounding all the other circuits?

                          Following some directions on stage seperation from
                          I've drawn up the schematic to match his suggestions:
                          Is this a better way to do it then how I am in the first schematic?

                          Holy crap I'm an idiot, I don't think I have B+ hooked up to the plate anymore..

                          Maybe this will work better for me:
                          Last edited by thehoj; 08-24-2009, 05:46 PM.


                          • #14
                            So how did that work out?

                            What kind of voltages are you getting on the plates? On the cathodes? It would be helpful for you to start a voltage chart.

                            With regard to the squeals and squaks, did you by any chance change or swap the brown and blue wires that are between the plates of the power tubes and the OT? If you did, swap them back - they are interfering with the negative feedback circuit (the green wire going from the OT secondary back to the cathode of V2B).

                            Speaking of the output tubes' plates, do you see the connection on the Bogen drawing from the center tap (red) to the V4 plate? I would remove that, along with the resistor and the capacitor. Look on the SB-12 how the red OT wire comes straight off the first filter cap, and copy that.

                            The output jack is drawn by Bogen as being grounded. It's certainly easy enough to try both ways, though.


                            • #15
                              Okay, so I have it wired up like I initially wanted to (I've included V measurements on each of the plates):

                              (The tone is definitely not how I want it to be, But this is how the tone controls are for now (still stock), and as is obvious I'm focusing more on pre-amp wiring and volume control of that right now.)

                              I ended up using the same 240v source through 220K resistors into v1a and v1b, just like v2a. Although I'm curious why v2b has 260v source. Should v1b have that source?
                              Also, on v2a and v2b, why is there b+ -> 220K resistor -> cathode? Should I leave that or take it off? I don't have that on v1 at all?

                              Anyways, it works.. basically.
                              My first volume control seems to work pretty good, and then the second one is INSANELY sensitive.. like on half way to 1 it's just crazy loud, and if I go much past that it starts to squeak really weird (not like a guitar feedback squeal). Is there some reason these pots might be ultra sensitive? Which components would control this sensitivity?

                              I also tried doing away with the v1b and just using one volume control, and I definitely had much more control over my volume, but also had a bit more of a hum noise than I would like (Didn't hear it as much with the two volume controls strangely). In general is a hum likely from poor grounding?

                              I would run with just v1a, but the only problem with that is that I seemed to have almost no clean headroom.. basically just crunch right off the bat. Is there a better place I could locate the volume control(s)?
                              Ideally I'd like to have a pre and then master sort of control, but as long as I could get some decent clean headroom with one volume control that'd work fine. Would it make sense to put the first volume control before v1a, and then the second one after v1b?

                              Also, I didn't swap the brown and blue wires between the plates of the v4 and v5 and the OT.
                              And, when you say remove "from the center tap (red) to the V4 plate", what is the reason for this out of curiosity?.. Strangely enough I don't think my amp matches this portion of the schematic.. It's hard for me to see what's going on, and I think I need to take a photo of it, I'll do that shortly.

                              It's coming along good though, I actually have sound now.
                              Last edited by thehoj; 08-25-2009, 08:15 PM.


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