Announcement

Collapse
No announcement yet.

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

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #17
    So I've decided I'm going to basically just try to do a straight conversion to the ampeg, since it's obviously set up pretty good as far as values for pots caps and resistors go.

    I'd like to add a master vol though, so I drew one into my preliminary schematics which sits just before my power tubes. Apparently I can use a dual pot to control the signal going to both tubes.

    Here's my schematic:
    http://vwtweaked.ca/images/bogen%20a...0mastervol.jpg

    Any suggestions or critiques are welcome.

    Also, does it make sense for me to add a choke to this system to help remove any hum I might end up getting?
    I was reading a site regarding chokes, and it mentioned that I would likely use a "choke for a capacitor input supply". For filtering the DC supply to the screen grids of the output tubes and the preamp section. It says "Just add up the current requirements of the screens and preamp tubes, and add a bit more for margin". So if the max screen-grid current of each 7868 is 15ma, and then 2ma per 12ax7 (or equivalent.. from what the site says).. That means about 40ma or so (on the high side). Should I theortically be able to use this universal choke in the amp? (model W022699 from weber) https://taweber.powweb.com/store/022699sch.jpg https://taweber.powweb.com/store/magnetic.htm
    It lists it's rating at 120ma @ 9H. This should more than do it right? Am I figuring that out right? Where would that go in my schematic above? Is this worth doing?

    I do still have a number of questions that I posted a couple posts above, and would greatly appreciate if anyone wants to give me their thoughts on them, as well as my new schematic.
    Last edited by thehoj; 08-25-2009, 09:01 PM.

    Comment


    • #18
      Okay.. so I think I posted too much information and questions..
      I'm going to summarize my questions at this point in this post:

      I'm going to work towards this layout, which is basically the ampeg plus a master volume control (also including my estimation of a standby switch and choke):
      http://vwtweaked.ca/images/bogen%20a...%20standby.jpg


      1a) Why is there 240v running to the cathodes on v1a and v2b in the original diagram?
      1b) Would it be a bad idea to remove this to match the ampeg schematic?

      2) Have I layed out the master control in a good way?

      3) The power supply section of the schematic doesn't seem to match my actual amp. Am I missing something? (I've posted actual pics of the amp in a post above).

      4a) Is it worth adding a choke to this design?
      4b) will it help to remove unwanted noise?
      4c) Is the universal weber choke (W022699) rated at 120ma @ 9H suited to this application?

      links to choke:
      https://taweber.powweb.com/store/022699sch.jpg https://taweber.powweb.com/store/magnetic.htm

      4d) Have I placed the choke in a good location for this schematic??

      5) Have I placed the standby switch in a good location for this schematic? To me it makes sense.. The heaters all get voltage once power switch is turned on, and then once standby switch is turned on power is applied to everything else. (Or should it go right off the red PT terminal, or somewhere else..?)
      Last edited by thehoj; 08-26-2009, 06:07 AM.

      Comment


      • #19
        1a) Why is there 240v running to the cathodes on v1a and v2b in the original diagram?

        I didn't notice that initially, but that doesn't look right to me. The B+ is already entering from the plates of that tube. More enlightened minds than mine feel free to comment, but that looks like it would make for one very unhappy tube. Was your Bogen originally wired like that?

        1b) Would it be a bad idea to remove this to match the ampeg schematic?

        No, I think it is a very good idea

        2) Have I layed out the master control in a good way?

        It looks correct to me - other readers please respond.

        3) The power supply section of the schematic doesn't seem to match my actual amp. Am I missing something? (I've posted actual pics of the amp in a post above).

        Remember that schematics and layouts don't necessarily line up visually, as far as what terminals are connected to which wires, etc. But they should be similar, electrically. If you are referring to that odd wire on the schematic to the plate of one of the 7868s, that should be fairly easy to find, if it is there. You'd start at pin 9 of first one, then the other 7868 and follow it to see if it at any point strays off to an R/C network that ends up at the center tap (red) wire. If you find it, remove it. If you don't, don't worry about it - maybe the owner before you removed it.

        4a) Is it worth adding a choke to this design?
        4b) will it help to remove unwanted noise?
        4c) Is the universal weber choke (W022699) rated at 120ma @ 9H suited to this application?

        links to choke:
        https://taweber.powweb.com/store/022699sch.jpg https://taweber.powweb.com/store/magnetic.htm

        4d) Have I placed the choke in a good location for this schematic??


        My opinion is that this is adding unneeded complexity to your amp. Might it help? Sure. Only one way to find out. Is it a good use of your time and resources? That's another consideration.
        In looking at amp schematics, I generally only see chokes in the higher-power amps with 6L6, EL34 or larger tubes. There are some exceptions. Typical Fender black/silverface amps use a 90mH choke, however, so your 120, if you use it, is on the large side. And keep in mind that using the choke means that you can lower the value of the first filter cap to 20uF

        5) Have I placed the standby switch in a good location for this schematic? To me it makes sense.. The heaters all get voltage once power switch is turned on, and then once standby switch is turned on power is applied to everything else. (Or should it go right off the red PT terminal, or somewhere else..?)

        It looks good to me. You want to have the bias voltage on when the amp is turned on, as well as the filaments of course, so you have done both.

        Good luck!

        Comment


        • #20
          Thanks again for all your help, I'm gonna order my parts soon and get to work on the rest of this.

          Comment


          • #21
            SB-12 pics

            Originally posted by thehoj View Post
            So I've decided I'm going to basically just try to do a straight conversion to the ampeg
            That being the case, I thought you might benefit from seeing a few gut shots of an SB-12. Here are some from mine, along with a picture of the schematic that was glued inside the amp. It's got voltages listed, too, which Piazza's does not.

            Keep us posted on your project,

            RWood
            Attached Files

            Comment


            • #22
              That's really nice to see, I was wondering what kind of voltages were coming out of the power supply.

              I still haven't ordered anything, so I may slightly adjust some of the components I order.

              Now I guess since I'm attempting to recreate this amp, I should probably shoot for similar voltages.

              I see that they're a bit different all around.
              If you look at the original bogen setup on the original schematic, you can see that the voltage right off of the red tap just after the diode is 450v.
              Right after the red tap on the ampeg, the voltage is 420. Would adding a 1.8K 10w resistor (the same resistor in the bogen schematic that drops the voltage to 420 for the second v output) before that first voltage output be a good way to drop it to 420, and then use the same cap and resistor sizes as the ampeg to achieve the remaining voltage outputs?

              Like this:
              http://vwtweaked.ca/images/bogen%20a...dby%20rev2.jpg

              I also adjusted the section for the bias voltage to be the same as the ampeg, including the bias adjusting pot. Is this all a good way to do this? I'm wondering if it's not since the voltage rectification is using diodes instead of the tube like in your case.
              Should the diodes be different values?

              Also, would it be worth having that .05uf 1kv cap right on the input side of the PT?

              And one other thing, in the ampeg the schematic shows the blue and brown on the input side of the OT reversed to the bogen one. I know you mentioned that I shouldn't reverse these, but why would I not match the ampeg in this case? Is the OT different?

              Comment


              • #23
                On closer inspection, the original Bogen schematic uses a voltage-doubler circuit to achieve its 450VDC. Look how the diodes are oriented, and how they connect to the first two filter caps. The top cap is not grounded, it is connected in series to the bottom cap's + end and the Red/Yel wire from the PT is connected to that point. This is a design that is used to increase the high voltage capability on an otherwise lower-voltage PT. Take a look at Silvertone 1484, Guild Thunderbass, among others that employed this design. This technique, combined with solid state rectification, is what enabled the amp to hit the 450V threshold.

                The Ampeg uses a more traditional tube-rectified circuit without the voltage doubler. See how the B+ comes off the rectifier's cathode, then right on down the line to the various high voltage nodes? It also has a centertap which is simply grounded, where the Bogen creates a centertap with a reverse-biased diode.

                With this in mind, I think you would be wise to use the Bogen power supply scheme, without the choke, and keep it wired exactly as it is now (but with new filter caps). You may, however, take the fixed-bias circuit directly from the Ampeg drawing. I might use a 25k pot instead of 15 to give you a wider range of adjustability. The diodes do not have to be different values, although if you're placing an order get a supply of 1N5408 and 1N4007 - they are very inexpensive.

                Don't worry about the .05uF cap off the PT

                Leave the brown/blue OT primary wires connected as-is, and keep the green neg fdbk line coming off the same secondary terminal. You may change the R value to what Ampeg uses, although I see they do not use a capacitor there. This is an area that you might condsider tweaking later, either with a cut switch or a variable pot which becomes a presence control.

                RWood

                Comment


                • #24
                  Interesting.
                  So I've modified the schematic yet again:

                  http://vwtweaked.ca/images/bogen%20a...and%20pres.jpg

                  I've still got the standby switch in there too, Is that in there at the right location given the fact that there is a voltage doubling happening in the power supply section?

                  I'm quite interested in the presence control, I didn't realize that's where that would go in the schematic. Would it make sense then to say use a 250K linear pot to have some adjustment below and above the 100K resistor? Or would a 100K pot make more sense.

                  And, how would the power supply circuit section be modified for bias control? I know I added bias control in the schematic where I changed most of the power section, but I also changed much of the other components and so perhaps wasn't doing it correctly for this type of circuit.

                  Also, is it generally recommended to use linear pots for the volume controls (and presence), and then audio pots for the tone controls? What kind of pot for the bias control?

                  Another thought crossed my mind about caps as well.. I'm having trouble finding some of the exact values of caps that I need.. Is a .047uf going to be basically interchangeable with .05uf? And if I can't get the exact cap value I need can I put them in series to add up to a value I'm after? Is there any disadvantage to that?
                  Last edited by thehoj; 09-01-2009, 01:59 AM.

                  Comment


                  • #25
                    I've still got the standby switch in there too, Is that in there at the right location given the fact that there is a voltage doubling happening in the power supply section?

                    yes

                    I'm quite interested in the presence control, I didn't realize that's where that would go in the schematic. Would it make sense then to say use a 250K linear pot to have some adjustment below and above the 100K resistor? Or would a 100K pot make more sense.

                    250k is too wide of a swing, 100k might be as well. Take a look at this thread for some ideas:
                    http://music-electronics-forum.com/t14320/

                    And, how would the power supply circuit section be modified for bias control? I know I added bias control in the schematic where I changed most of the power section, but I also changed much of the other components and so perhaps wasn't doing it correctly for this type of circuit.

                    I thought you had it nailed pretty well - my only suggestion was to add a 25k pot. The voltage-doubler business all comes after the bias supply, so you may copy the SB-12's adjustable bias circuit.

                    Also, is it generally recommended to use linear pots for the volume controls (and presence), and then audio pots for the tone controls? What kind of pot for the bias control?

                    Volume controls and many tone controls are usually not linear but logarythmic or "audio" tapers. They correspond with the relative increases in volume that our ears like to hear. Use linear for bias, tremolo, presence, certain tone controls. Borrow liberally from known circuits.

                    Another thought crossed my mind about caps as well.. I'm having trouble finding some of the exact values of caps that I need.. Is a .047uf going to be basically interchangeable with .05uf? Close enough for our purposes, yes And if I can't get the exact cap value I need can I put them in series to add up to a value I'm after? Is there any disadvantage to that?
                    Capacitors in series decrease the overall capacitance while caps in parallel increase it. This is opposite of the effect of resistors in series and in parallel. So yes, you can combine them, but most values that we use are commonly available. What are you having trouble finding?

                    Comment


                    • #26
                      New schematic as per usual:
                      http://vwtweaked.ca/images/bogen%20a...ias%20pres.jpg

                      Thanks for the link to thread on the presence control, also clarification on pots, that makes sense.

                      As for the caps, I guess it was just the .05uf I couldn't find. I'll use .047.
                      If I were to spend a bit extra on nice caps, which would be the ones to spend extra on?.. the coupling caps between stages?

                      As for the bias control section, if I wanted to match where I was to start with, I should measure the bias voltage right when I turn it on, and adjust it so that I get -23v to start with right?
                      Is there much danger in really low or high bias voltage for a short while?

                      I should be placing my order in the next day or two, so I'm excited to start really fiddling around with this amp.
                      I'll order some extra resistors and a few caps of different similar values so that I can try different things as well.

                      Any suggestions on my best chances of reducing noise and hum for this amp? I plan on following suggestions from this article on grounding http://www.geofex.com/Article_Folder...nd/stargnd.htm

                      Any more thoughts on this new schematic?
                      Last edited by thehoj; 09-02-2009, 03:39 AM.

                      Comment


                      • #27
                        If I were to spend a bit extra on nice caps, which would be the ones to spend extra on?.. the coupling caps between stages?

                        You can get real broke in a hurry buying expensive coupling caps. Resist that temptation and get a wide variety of values in a good workingman's cap like the Mallory 150. Get the 630V versions in all the common values: .01, .022, .047, .1, .22uF. You can see them on pg 838 of the Mouser catalog

                        As for the bias control section, if I wanted to match where I was to start with, I should measure the bias voltage right when I turn it on, and adjust it so that I get -23v to start with right?

                        That might get you close but I like to start with the highest negative voltage available and monitor the tube's current draw, then gradually raise the current by decreasing the bias voltage. I do this by interrupting the cathode-to-ground connection and putting an ammeter in series there

                        Any suggestions on my best chances of reducing noise and hum for this amp? I plan on following suggestions from this article on grounding http://www.geofex.com/Article_Folder...nd/stargnd.htm

                        No, that looks like a good resource.

                        Any more thoughts on this new schematic?

                        Looks good to me - it's time to stop drawing and start soldering!

                        Comment


                        • #28
                          I did indeed end up ordering a whole schwack of parts this afternoon.

                          "That might get you close but I like to start with the highest negative voltage available and monitor the tube's current draw, then gradually raise the current by decreasing the bias voltage. I do this by interrupting the cathode-to-ground connection and putting an ammeter in series there"

                          To get the highest negative voltage would that mean I have the resistance on the pot as low or high as possible?

                          When you say to put my ammeter in series of the cathode to ground connection, and monitor current, It shouldn't matter which power tube I insert my ammeter between cathode and ground right? what am I after for current?
                          Why do I not want to get voltage reading instead?

                          If I am after current, does it make sense to throw a 1/2w 10ohm resistor from cathode to ground on both power tubes, then I could measure voltage drop over that resistor and calculate current.

                          Also, I picked up bias test points to install.. Those would be to measure voltage right, so where would I stick those in the circuit.
                          Are they of any value if I'm actually after determining current?
                          Last edited by thehoj; 09-03-2009, 03:36 PM.

                          Comment


                          • #29
                            I did some further reading on biasing.

                            Rebuilding a Deluxe Reverb Reissue Tube Amplifier with Mikey

                            This makes sense to me.

                            The bias is set to make sure the proper plate dissipation rating (in watts) of the output tubes is reached. The maximum plate dissipation rating for a single 6V6 tube is about 14 watts. Since it is best to operate at about 70% of maximum plate dissipation, the bias is adjusted to bring the plate dissipation down to about 10 watts (.7 * 14 = 9.8). The calculation of the proper bias setting is done like this:



                            The formula to calculate plate dissipation says that P = E * I, which means that Power (watts) = Volts * Current Draw (amperes). Let’s say the plate voltage is 400V. The current draw (the unknown variable we need for setting the bias) can be calculated like this:



                            P = E * I

                            10 = 400 * I

                            10/400 = I

                            .025 = I (amperes)

                            check_your_bias.gif
                            In this example, since .025 amperes = 25ma (milliamps), and because of the 1-ohm 1% resistor, the bias setting for a power tube will be right when the voltmeter reads 25mv. The next time I put in a set of matched power tubes, I would just set the bias to 25mv.

                            When I pick up my amp, Mikey will measure the actual plate voltage (it probably isn’t exactly 400) and we’ll play with the bias to find my favorite bias setting that puts each tube close to 10 watts. Since the amp uses two 6V6 tubes in class AB (push/pull), the output of the amp will be about 20 watts, which in my opinion is just about the right amount of power for most gigs in smaller rooms.
                            I looked up the 7868 datasheet http://www.pmillett.com/tubedata/HB-...art_2/7868.PDF
                            and it points out that the maximum plate dissipation for the tube is 19w.
                            So if I follow the instructions from this link, my optimum plate dissipation (70% of max) is 13.3w.
                            I should then measure plate voltage again, which if is the same as before should measure 480V.
                            So my current will be 13.3/480, which equals 27.7ma.
                            And if I set up 1 ohm resistors (is 1/2w okay?) from cathode to ground, I would read 27.7mv
                            That's what I want to ideally read from cathode to ground then right?

                            new schem:
                            http://vwtweaked.ca/images/bogen%20a..._tp%20pres.jpg

                            The site also indicates that that plate power dissipation is where we get the amp's output power rating from.. If that's the case, when I have this running at about 70% of max plate dissipation, I'm running the amp at roughly 26watts or so.
                            So the factory rating of this amp at 35w would be if we were running at nearly the absolute max that the plates can dissipate.

                            Is 70% a good percentage to set my bias voltage for?

                            btw..
                            I know I'm asking a lot of questions (dumb questions in a lot of cases), but it's all starting to come together for me.
                            Last edited by thehoj; 09-03-2009, 04:25 PM.

                            Comment


                            • #30
                              To get the highest negative voltage would that mean I have the resistance on the pot as low or high as possible?

                              I get so easily confused by all that stuff, so rather than work it out in my head I simply put a voltmeter on it and see what happens when I turn the pot. You can do this before you put the power tubes in, after you build the adjustable bias control.

                              When you say to put my ammeter in series of the cathode to ground connection, and monitor current, It shouldn't matter which power tube I insert my ammeter between cathode and ground right? what am I after for current?

                              The tube data sheet calls for 19W max plate dissipation per tube. So at 450 volts, that comes out to 42mA of plate current through each tube. Since you'll be measuring cathode current, keep in mind that it includes both plate and screen current, so you can go up to around 47mA for each tube (or 94mA if you are taking both cathodes to your ammeter, before ground). These should be your upper limits. Ampeg had the SB-12s set to dissipate 16.8W per tube. Once you get in-range, take a listen and decide for yourself which sounds best.

                              A schwack, huh? How does that compare to a shload?

                              I see that you have a more recent reply - I will read that now.

                              Comment

                              Working...
                              X