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Thread: 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:
    http://schematicheaven.com/hifiamps/bogen_chb35a.pdf

    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?
    http://vwtweaked.ca/images/bogen%20modified%20idea.jpg

    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.

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    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:

    http://www.schematicheaven.com/ampeg..._portaflex.pdf

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

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    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.

    http://vwtweaked.ca/images/bogen%20a...onv%20idea.jpg

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

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    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?

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    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?

    http://vwtweaked.ca/images/bogen%20a...onv%20ques.jpg

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    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.

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    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?

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    Quote 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.

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    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.

    http://vwtweaked.ca/images/bogen%20s...mod%20rev2.jpg

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    Last edited by thehoj; 08-22-2009 at 02:10 PM.

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    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.

    http://vwtweaked.ca/images/bogen%20s...mod%20rev3.jpg

    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)?

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

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

    Quote 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.

    Quote 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

    Quote 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.

    Quote 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.

    Quote 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.

    RWood

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    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?

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    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:
    http://vwtweaked.ca/images/bogen%20s...0stage%201.jpg

    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 http://www.netads.com/~meo/Guitar/Amps/PA2Guitar/
    I've drawn up the schematic to match his suggestions: http://vwtweaked.ca/images/bogen%20s...e%201%20v2.jpg
    Is this a better way to do it then how I am in the first schematic?

    EDIT:
    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:
    http://vwtweaked.ca/images/bogen%20s...mod%20rev5.jpg

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    Last edited by thehoj; 08-24-2009 at 07:46 PM.

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    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.

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

    http://vwtweaked.ca/images/bogen%20s...od%20rev2a.jpg
    (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.

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    Last edited by thehoj; 08-25-2009 at 10:15 PM.

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    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.

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    Last edited by thehoj; 08-25-2009 at 11:01 PM.

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    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..?)

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    Last edited by thehoj; 08-26-2009 at 08:07 AM.

  19. #19
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    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!

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

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    SB-12 pics

    Quote 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
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    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?

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    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

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    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?

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    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?

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    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?

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    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!

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    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?

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    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.

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    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.

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

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    Quote 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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    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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    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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    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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