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

  1. #36
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    So my parts arrived, and I went through and wired things exactly like my last schematic.
    http://vwtweaked.ca/images/bogen%20a..._tp%20pres.jpg

    After wiring it all up I took the power tubes out, also making sure that the amp was in standby mode now that I have that standby switch in there (so that I wouldn't be supplying power to the OT when I was playing around with the bias circuitry). And then I powered it up, and measured on the red wire supplying the OT, and it read 230V.. So I turned that off right away. I think perhaps the standby switch isn't in there right..
    On the Ampeg the red/yellow wire from the PT goes to ground, and on the bogen it goes back into the filter cap section. Do I need a dpst switch in there to break both lines?

    So since I was anxious to keep going with this I decided to put the power tubes back in, and put a load on the PT to make sure I don't damage it, and a just avoid trying to run this with no power tubes and no load at all.

    After doing that and powering it up, my bias voltage pot didn't seem to actually make any adjustment, I also had a 50ohm power resistor from the cathode of the power tubes to ground, and as I measured over it, I was getting 10v, which is way too high. I should get more like 1V (50 x 0.027ma). So I quickly turned that off.

    Thinking that perhaps I should just stick with the original bias section of the bogen, I wired it up like it was originally (with new components), also, thinking that perhaps the 50ohm resistor to ground from the power tubes wouldn't be the most accurate, I put my multimeter in series from cathodes of the power tubes to ground.
    I powered it up like this, and I seem to be getting like 0.32 on the 10a scale (so 320ma).. I'm sure I have it wired up just like the original schematic. So this is really confusing to me.
    I have the cathode of one power tube connected to the cathode of the other, and then I have a wire from that point connected to my positive lead of the multimeter, and then ground is connected to the negative lead of the multimeter.

    Any thoughts about why:

    1) the current seems to be so high even with the original bias circuitry (new components)?
    (am I measuring incorrectly somehow?)

    2) the bias adjustment pot in the latest schematic I drew up wouldn't change the voltage drop I was getting over the resistor from cathode to ground of the power tubes when I did my initial testing?

    3) the standby switch wiring isn't actually cutting voltage to those filter caps?
    (do I need to cut both red and red/yellow from the PT?)

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    Last edited by thehoj; 09-27-2009 at 04:28 AM.

  2. #37
    Old Timer olddawg's Avatar
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    Since it isn't center taped you might want to use a full wave bridge like this. It makes more sense and it is easier to open the HV B+ line.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  3. #38
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    Quote Originally Posted by olddawg View Post
    Since it isn't center taped you might want to use a full wave bridge like this. It makes more sense and it is easier to open the HV B+ line.
    Would it work the same to just use a dpst switch to open and close the red and red/yellow lines at the same time?

    EDIT: Just tried it and no it doesn't work. I guess because I need that red/yellow connection to come back up for the bias section.

    It's really weird why I'm getting such high current from the cathode of the power tubes to ground.
    Also, if I measure the DC voltage on the output of that bias circuitry I get 0v..
    Is that why I'm getting such high current through the tube? 0v for a bias voltage would mean that I'm not controlling the current running through the power tube at all..
    That's probably what's happening isn't it.. I'm gonna play around with that a bit I think.

    Another strange thing, is that the voltages at each section of the power supply circuitry is higher than the schematic says it should be. If I measure the voltage getting to the plates of each pre-amp tube, I get 200v on v1, v2a & b, and then 298v on v3..
    I checked the datasheets for 12ax7's as well as 6c4's, and the 12ax7 max plate voltage is 300, and the 6c4 is also 300v.
    Regarding this, firstly I'm confused about why I'm getting such high voltages, and secondly is this actually a problem if I'm under the maximum rating?..

    If I did decide to do a full wave, would it be as simple as this?:
    http://vwtweaked.ca/images/bogen%20a...ull%20wave.jpg

    I measured the ac voltage from the red and red/yellow secondaries on the PT, and it measured about 470VAC. Will doing this full wave rec output 470V DC? Would 1n4007's be okay to use?
    And would I still need those 2 40ufd caps in the 460v supply section, or just one of them?

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    Last edited by thehoj; 09-27-2009 at 05:45 PM.

  4. #39
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    Quote Originally Posted by thehoj View Post
    Would it work the same to just use a dpst switch to open and close the red and red/yellow lines at the same time?

    EDIT: Just tried it and no it doesn't work. I guess because I need that red/yellow connection to come back up for the bias section.

    It's really weird why I'm getting such high current from the cathode of the power tubes to ground.
    Also, if I measure the DC voltage on the output of that bias circuitry I get 0v..
    Is that why I'm getting such high current through the tube? 0v for a bias voltage would mean that I'm not controlling the current running through the power tube at all..
    That's probably what's happening isn't it.. I'm gonna play around with that a bit I think.

    Another strange thing, is that the voltages at each section of the power supply circuitry is higher than the schematic says it should be. If I measure the voltage getting to the plates of each pre-amp tube, I get 200v on v1, v2a & b, and then 298v on v3..
    I checked the datasheets for 12ax7's as well as 6c4's, and the 12ax7 max plate voltage is 300, and the 6c4 is also 300v.
    Regarding this, firstly I'm confused about why I'm getting such high voltages, and secondly is this actually a problem if I'm under the maximum rating?..

    If I did decide to do a full wave, would it be as simple as this?:
    http://vwtweaked.ca/images/bogen%20a...ull%20wave.jpg

    I measured the ac voltage from the red and red/yellow secondaries on the PT, and it measured about 470VAC. Will doing this full wave rec output 470V DC? Would 1n4007's be okay to use?
    And would I still need those 2 40ufd caps in the 460v supply section, or just one of them?
    Diodes are cheaper than switches. Sorry , that schematic wis incorrect. I just took it off osf the 18 watt site. and it was late So your bridge is hooked up incorrectly in your diagram. You are running AC on your B+ line that way. You need to put the switch and the B+ line where the + symbol is marked on the bridge in the diagram. Follow the foreward bias paths of the diodes. Here's a better example.
    Attached Files Attached Files

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    Last edited by olddawg; 09-27-2009 at 06:29 PM.

  5. #40
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    So this would be more accurate then I suppose:
    http://vwtweaked.ca/images/bogen%20a...ave%20rev2.jpg

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  6. #41
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    Quote Originally Posted by thehoj View Post
    So this would be more accurate then I suppose:
    http://vwtweaked.ca/images/bogen%20a...ave%20rev2.jpg
    Yeah that's it. Sorry about the earlier diagram. It was late and you always have to check other peoples stuff for mistakes. Its much more understandable drawn that way. The only thing you might have to do is tweak the dropping resistor values on the half wave bias supply at the at the top but probably not. I believe that most of your problems are resulting from simple mistakes that happen when you re-engineer an existing amp. I didn't read all of the posts, but I always find it easier and usually more economical (especially time wise) to gut a doner amp, wire up a new turrent board from new parts (from a known good and tested design), and replace all of the filter caps. That way you can also start with a new filament dress, etc. It will save a lot of hum headaches when you debug the build.

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  7. #42
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    Yea.. this is becoming a bit difficult to troubleshoot, especially given my fairly limited experience with this type of thing.

    Now I don't know what I had measured earlier, but I've just now measured my ac voltage between the red and red/yellow secondaries on the PT, and I get 188VAC. If I measure the output of the full-wave rectifier to ground I get 260Vdc.. Which makes sense given that AC I suppose..

    So was the circuit before some sort of doubling circuit? I seem to recall hearing that about this PT now that I think about it.
    http://vwtweaked.ca/images/bogen%20a..._tp%20pres.jpg

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  8. #43
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    Quote Originally Posted by thehoj View Post
    Yea.. this is becoming a bit difficult to troubleshoot, especially given my fairly limited experience with this type of thing.

    Now I don't know what I had measured earlier, but I've just now measured my ac voltage between the red and red/yellow secondaries on the PT, and I get 188VAC. If I measure the output of the full-wave rectifier to ground I get 260Vdc.. Which makes sense given that AC I suppose..

    So was the circuit before some sort of doubling circuit? I seem to recall hearing that about this PT now that I think about it.
    http://vwtweaked.ca/images/bogen%20a..._tp%20pres.jpg
    What do you get with the standby switch on and the filter caps in circuit? If those tubes need 450v you may have to go back to the original power supply configuration or use different output tubes. (EL84s maybe) Its always a good idea to spec out the primary secondary voltages before you start anything. It will tell you what you can use and what you can do. Re-engineering an amp can be a pain. I put a lawnmower engine on my bike when I was a kid. It would go 45mph but that didn't make it a motorcycle and it was dangerous as hell. But it would get you from point A to point B, if you get my drift.

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  9. #44
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    Yea, I definitely need to go with the original circuit design. It's getting me up to the 500v range, but it's higher than I think it should be.. It's like 520v, then 510v, then 410v, then 370v.

    I'm going to put some power resistors before the first stage of that to bring it down to starting at 460v.

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  10. #45
    Old Timer olddawg's Avatar
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    Quote Originally Posted by thehoj View Post
    Yea, I definitely need to go with the original circuit design. It's getting me up to the 500v range, but it's higher than I think it should be.. It's like 520v, then 510v, then 410v, then 370v.

    I'm going to put some power resistors before the first stage of that to bring it down to starting at 460v.
    The voltages will be higher with the tubes out. You will probably have just a little bit higher voltages with the tubes in because line voltages are higher in general than decades ago. You can always put a couple of 10v 5 watt zeners in series with the B+ line if you are still concerned. Its done all of the time and works better than big resistors.

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  11. #46
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    I rewired the bias voltage section, and it seems to be working well now. I'm getting -22v when I measure from the output of it to ground, that seems to be right in the range it should be.

    I also put one 4.7k power resistor in before the first stage of the power supply, that brought things down a little bit, starting at like 510v, and then 500v, 400 and then 360..
    I'm going to go with that for now since it still keeps the plate voltage under the maximum allowed for the preamp tubes.

    I plugged my guitar in, and played through the crappy speaker I had hooked up as a load, and it sounded great. Actually really good full sound. The presence control I wired in even seemed to do it's job.

    Only two problems.
    1) The tone controls don't seem to work very well.. they make very minor adjustments, but also seem to distort the sound depending on where there are.. I'm going to take them right out and rewire them again I think.

    2) When I was adjusting either the volume or master volume control as well as holding on to my guitar I got a sharp pulse of a shock. The pots don't have knobs on them, so I was touching the bare metal of the pot, it's also possible that I just brazed the chassis of the amp.
    I don't know exactly what happened there. I assume one hand on the ground of the guitar, as well as one hand touching something on the outside of the amp that shouldn't be hot.
    I ran through my wiring just now and things look alright.. it's a bit messy, but I really don't see anything touching anything it shouldn't.
    Any thoughts on this?

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  12. #47
    Old Timer olddawg's Avatar
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    Do you have a 3 wire power cord with the chassis earthed? Does your wall socket have an earth ground? If the answer to either of those questions is no you have a potential shock hazard.

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    Yea, I definitely do have a 3-prong cord which is grounded to the chassis, and I know for a fact that the electrical wiring in my house is properly grounded as well.

    I'll re-do the power cabling section as well I think, just to make sure it's not a bad solder joint or something.

    What else could this have been? If b+ was touching the chassis at some point then I'd assume I would have heard a difference in the sound output..

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  14. #49
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    So I checked over my wiring meticulously, and rewired the tone controls. Also powered the unit up and attempted to measure for voltage anywhere it shouldn't be.
    I didn't see any, so as a precautionary measure I put knobs on the pots.

    I ended up playing around with the resistors in the filter cap section, just to tweak the voltages I was getting out. The first voltage out 480v is right where I want it, the 450 or so out of the next stage is great too, but the stage after that was sitting close to 400v for some reason, so I bumped the resistor up there to 50K from 22K, that got me right in the zone, I think it was around 270v or so, and then the next one around 250, so perfect. I measured voltages at each tube and it's all right around where I want it.

    I also decided to rewire v1a for a 12ax7, just to make things simpler as far as tubes (I have a few extra 12ax7's).

    Next as for biasing the power tubes, I didn't have a 1ohm resistor, so I took a 100ohm 5w resistor I had (measures 101ohm), and put it between cathode to ground on one of the power tubes, I rigged up my bias test points over that. So if I'm shooting for 27.7ma, I should read 2.77v dropped over the resistor. works fine for me.
    In my initial wiring with no bias control I was getting about 2.2v dropped over the resistor, so a bit low. That being said it did sound pretty good.
    In any case to bring that up some, rather than go back to the ampeg's bias control I just changed out the 39k resistor that was parallel with the 50uf cap, and changed it to a 15k resistor in series with a 25k pot.
    The way I have it hooked up, the higher I have the resistance on the pot set, the colder I have my bias set, the lower I set that resistance the hotter my bias gets.

    So I powered it up and played around with the pot, I can get it anywhere from about 2.2v (22ma) up to 3.5v (35ma) didn't want to go any higher than that, but I suppose I could.

    So all in all I think I'm pretty happy with the layout and the sound is a lot better than I expected at this stage in the game.
    Presence control works great too.

    This is exactly how I have it wired right now:
    http://vwtweaked.ca/images/bogen%20a...res%20rev3.jpg

    The issue I'm tackling now is the tone control.. My Bass is WAY to heavy. I have to have my treble maxed, and my bass right down for it to sound good with the guitar. If I crank the bass pot I can get it into crazy bass territory, just heavy heavy bass.

    What would I need to adjust in the tone control section to bring my bass down and maybe even boost the treble?

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    Last edited by thehoj; 09-29-2009 at 02:10 AM.

  15. #50
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    So I decided to look at entirely different tone options.
    I have limited number of pots to use for this at the moment. I wish I had ordered more of a variety of pots, but I used what I have.
    Playing around with the tone stack calculator, and the classic marshall tone stack, I came up with something that looked good on the graph anyways with the pots I have (2-1meg linear, and 1-25K linear).

    I also decided to use 2 switches I have to add switchable caps over the volume control for added brightness. So I have 1 switch which adds 94pf for a bit of brightness, and then another switch for 220pf for extra brightness. If I put them both on then obviously I have even more brightness. It gives me a lot of different sounds.

    The bright switches in conjunction with the new tone controls give me a huge variety of tonal options.
    What I'm fighting with now is that when I turn my treble pot past about halfway I start to get a really high pitched squeal. If I turn the treble pot back a bit it goes away, if I turn it back forward past half it comes back exactly the same.
    Any thoughts on what might be doing that?


    Here is how the amp is currently wired. I think if I can get rid of the high pitched squeal on that treble pot I'm going to leave it like it is.
    http://vwtweaked.ca/images/bogen%20a...ual-bright.jpg

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  16. #51
    Supporting Member Dave Curtis, dB AudioTech's Avatar
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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"

    Umm, no. Think about it.

    It should be before the diodes and after the bias tap.

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  17. #52
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    Just to keep this thread updated with my progress, I finally have a layout I'm happy with.
    Pretty much every single resistor and cap and diode has been replaced, as well as v1, v2, and v3 tubes. I'm still running the original power tubes, and original transformers.

    In addition to stealing some from the Ampeg portiflex sb-12, I stole much more from the fender bassman 5e6a circuit. I also re-wired v1 for a 12ax7 since they're so much more common than the 6eu7. I'm going to pick up a JAN-Phillips 5751 for v1, since they are supposed to be nice and quiet with a bit less gain, and I'll see how I like that. I have it all wired up now like the schematic I'll post here, and I personally think it sounds amazing.

    I recently picked up a Traynor YGM-2 (which I love) and I was sure the Traynor would end up being the better sounding of the two.. But I honestly don't know now, this Bogen just sounds so nice to me. I'm going to record a few clips to upload.

    Since I stole from the Portiflex and the Bassman I'm calling it the BassFlex... Corny I know.

    Anyways, here's the schematic I went with and I think this is how I'm going to leave it:
    http://vwtweaked.ca/images/Bogen%20BassFlex.jpg

    Next up will be building a cabinet for this amp.

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    Last edited by thehoj; 10-19-2009 at 03:48 PM.

  18. #53
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    This amp has been torn down and built up a few times now, for one reason or another. I'm pretty sure I'm leaving it as is now though, the biggest problem I had with the amp was noise, buzzing and I thought humming.
    Turns out I really wasn't identifying humming very well. The only real problem I had was a lot of buzzing that got louder with the volume control.

    So what I did is spend some more time being meticulous with the signal path, short leads on all components in the signal path, shielded wire with one end of the shield grounded on every piece of wire in the signal path right up to the power tubes basically and the thing is silent now. Like eerily silent, other than a nice expected very low hiss there is no noise/buzz even when cranked wide open. The thing is noticeably quieter than my Traynor YGM2 which I always think of as being pretty quiet.

    In fact since I prefer this amp with no negative feedback loop (but have left it on in the past since it reduced noise) I disconnected that and I still have no buzzing.

    This is where it stands now.

    http://vwtweaked.ca/images/Bogen%20B...%2020jan10.jpg

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  19. #54
    Member RWood's Avatar
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    This is really cool. Hoj. Nice work. Do you have a gut shot of the final version that you could post? It's great that you got it so quiet.

    Since we discussed this amp last fall I have acquired at CB-35a to work on - looking forward to doing some of the same things you did.

    RWood

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  20. #55
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    I'll definitely get some pics up, it ain't pretty, and it's still in a "in progress" state, but I'll take a pic of the guts.
    One thing I did a bit differently this time is put all of the filter caps on top of the chassis. I'll build a little protective covering over them later. But It just helped to get some of the clutter out of the underside.

    I have it running through a cab I have that is actually pretty good, except the speaker in it right now is the old 12" marsland out of my traynor. So the speaker's not the greatest, but still gives a pretty good idea of how it'll sound in the end.

    I'll try to get a recording of it also.

    I think I'm gonna pick up an eminence private jack speaker to go with this amp eventually.

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  21. #56
    Supporting Member Dave Curtis, dB AudioTech's Avatar
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    If it were me, I'd put in some blocking caps to keep DC off the master volume.

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    Quote Originally Posted by Dave Curtis, dB AudioTech View Post
    If it were me, I'd put in some blocking caps to keep DC off the master volume.
    Interesting, probably would have been a good idea, I've since decided to remove the MV altogether though.. Simpler that way and I use pedals for OD so I don't mind not being able to OD the preamp at low volumes.

    And yes, I'm still gonna get a pic up and some sound clips.. Just trying to find my camera.

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  23. #58
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    So I got the amp fitted into this used (beat up) vox ac15cc cab I got for free, using the old marsland alnico speaker from my Traynor YGM-2. And it JUST fits in there (about 1" clearance between the back of the speaker and the OT).
    I had to rig up a few different types of angle brackets and some 2x8" boards, but it is in there rock solid now.

    It's pretty haggard, but I don't really care, it's in there sturdy. I actually think the marsland speaker sounds quite good with this amp too, I know a lot of people don't really like them.





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  24. #59
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    I like that.

    Good use of an old Vox box.

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    EDIT: Did a different quick recording, the first bit sounds a bit distorted, I don't think that's the amp, just my recording method.
    Please forgive my playing, I wasn't trying to demonstrate my playing abilities (starting lessons next week ).

    Gives a basic idea of the sounds, was runnning through my pedal board, but only had a reverb pedal enabled, and then in the distorted parts I was running through a boss ds-1 with monte allums tri-gain mod. This amp is LOUD, I had the knob turned about 40% of the way on..
    I want to crank it wide open and try another recording, but I have to mind my neighbors at the moment.

    Anyways, I'm really happy with the sound.

    I'm playing an '08 US telecaster with a seymour duncan stacked neck pickup, and a SD lil '59 bridge humbucker.
    For the clean parts I'm playing with both pickups on, the distorted sections are just bridge pickup.

    http://vwtweaked.ca/music/bassflex%202.mp3

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    Last edited by thehoj; 01-26-2010 at 06:05 PM.

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    New CHB35a conversion completed.

    I've documented my conversion of a Bogen Challenger CHB335a for Bass Guitar, Guitar, Microphone and Music player use. I used information found here and on other web sites. My conversion notes can be found at:

    CHB35a Amp conversion notes & pics

    Hope my notes help someone!

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    Last edited by sjcarter; 02-17-2010 at 09:47 PM. Reason: Added web link.

  27. #62
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    Quote Originally Posted by sjcarter View Post
    I've documented my conversion of a Bogen Challenger CHB335a for Bass Guitar, Guitar, Microphone and Music player use. I used information found here and on other web sites. My conversion notes can be found at:

    CHB35a Amp conversion notes & pics

    Hope my notes help someone!
    Neat, interesting to see a different take on the amp.

    I've changed things a slight bit again. I'll post my final schematic revision here, and a new recording of the amp first at about 1/3 volume, and then at full volume. This recording is done with absolutely no pedals in front of the amp.
    I really like the sound of the OD I'm getting out of the amp now.

    Here's the final schematic: http://vwtweaked.ca/images/Bogen%20B...%2008feb10.jpg

    And here's the final recording: http://vwtweaked.ca/music/bassflex%20feb0810.mp3

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  28. #63
    Member RWood's Avatar
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    Quote Originally Posted by thehoj
    I really like the sound of the OD I'm getting out of the amp now.
    Sounds really nice, Hoj. You've done a great job sticking with this amp and also documenting it for everyone.

    In this last change, do you bring back a switchable NFB?

    RWood

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  29. #64
    Supporting Member Dave Curtis, dB AudioTech's Avatar
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    Quote Originally Posted by sjcarter View Post
    I've documented my conversion of a Bogen Challenger CHB335a for Bass Guitar, Guitar, Microphone and Music player use. I used information found here and on other web sites. My conversion notes can be found at:

    CHB35a Amp conversion notes & pics

    Hope my notes help someone!
    If you haven't done so already, you should install a 3-wire AC cordset. I didn't see it mentioned in your notes. It's the first thing I do with old 2-wire amps.

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    Quote Originally Posted by RWood View Post
    Sounds really nice, Hoj. You've done a great job sticking with this amp and also documenting it for everyone.

    In this last change, do you bring back a switchable NFB?

    RWood
    Yea I did bring that back, I don't use a presence control anymore though. The other thing I did which improved the sound a lot for me was change the cathode resistor on v1a to a 1.5k (instead of 820ohm) and I removed all of the preamp cathode bypass caps.

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    I just bought a Bogen CHB 100 and saw this post. When I try thehoj's links to his schematics, they now lead nowhere (not surprising since I'm 10 years late to this party). Anyone have a copy they could share?

    Thanks,
    MG

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    I have one of these and a CHB-35A in the shop right now that I am working on (slowly.) I can post up the preamp changes I made sometime if it's any help to you. It will likely take a little while before I get it finalised.

    Andy

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    That would be very much appreciated; thanks, Andy!

    MG

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    I did a whole bunch of modding and work to a CHB 100. I have a second one here to duplicate what I did on the first one. They're pretty cool amps, though the chassis layout is poor, especially where the transformers are concerned.

    Greg

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    hi Greg, what are a couple things youd recommend as a mod to these? Im using it as a test bed (first time tinkerer) so Ill try whatever will get it to sound decent with guitar. Thanks, MG

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