Any technical info on Bogen Challenger CHB10?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Leonc, Oct 26, 2005.

  1. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Looking for schematics in particular. Bought one pretty cheap and have replaced the mic jack, speaker jack and 2-prong AC plug...but would still like it to make improvements on its sound. Any tips would be appreciated. BTW, this amp only has two tubes (12AX7 and oddball power tube) and uses a little bridge rectifier...
     
  2. Jackie Treehorn

    Jackie Treehorn Member

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    I think I've got one of those. I've removed most of the components, though so I couldn't help you schematic wise. I do remember some nonstandard preamp values, and the negative feedback was right at the grid of one of the tubes. It's a 7868 power tube right?

    My plan for that one was to replace the 12ax7 with an EF86, and do a baxandall tonestack. 3 knobs, volume, treble, and bass. I never got around to finishing it, though.

    What do you want it to do? How does it sound stock?
     
  3. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Hey Jackie thanks for the reply.

    Well, it was sounding good at fisrt; I had hooked it to some normal guitar jacks he had on the ends of some alligator clips over at a friend's place. The one thing that had me a little concerned was it really didn't seem that loud. I've heard 12 watt amps that were WAY louder than this supposedly 10 watter. (BTW, we brought it up on a variac over there, it had apparently not been used in quite a while).

    I then took it home and began to do the wiring for the regular 1/4" guitar jacks...and I've done a pretty half-ass job, admitedly...but now it's not got as much volume as it had before. So the main thing I was hoping to do initially was just take some measurments to see where the problem lies and I was hoping that a decent schematic might provide some quick clues...

    But I also noted that the overdrive sound comes on kinda quick and gets really compressed. Not my favorite sound. I'm concernd that it needs a cap job.
     
  4. Jackie Treehorn

    Jackie Treehorn Member

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    Hey Leon,

    Hmmm...16 views and probably 3 are mine...not much interest in the Bogen.

    Well, I dug my chassis out of the closet and it is a CHB10A. I didn't finish clearing the chassis apparently. I would leave the power tube alone for the most part. Three things that immediately spring to mind as changes - the previously mentioned negative feedback - this is the wire that comes off of the speaker connections and either goes to the grid of the power tube or one side of the preamp tube - try it disconnected. You'll definitely get more volume. The plate resistors of the preamp tube are 500k which might explain the distortion coming on fast and compressed. The ubiquitous value for these would be 100k. These are the resistors connected to pins 1 and 6. On my example, the 2 resistors are connected together on one terminal strip with the other sides going to pin 1 and the other resistor connecting to a terminal strip to the right of the tube socket where a short blue wire goes to pin 6. I would stick 100k's there to begin with. While you're at it, the ubiquitous cathode resistor for the 12ax7a would be 1.5k with a 22-25 uf electrolytic bypass cap. You'll need two resistors and bypass caps and each would connect from pin 3 and 8 of the 12ax7a to ground. Another thing to change are the coupling caps which appear to be .01 uf ceramics, which is probably a bit too small. I'd go with a .022 to .047 orange drop or equivalent. There's one from the power tube pin 5 to the plate resistor junction on the terminal strip I previously mentioned. The other one goes from pin 1 of the 12ax7 to the another lug on the previously mentioned terminal strip. I think that went to the volume control but I've removed all that wiring in my chassis. You could just skip the whole tone stack idea and take the middle lug of the volume control and connect it to the grid of the next preamp tube, either pin 2 or 7, whichever the guitar input is not connected to.

    That's 4 resistors to change, 2 22 uf bypass caps to add, and 2 ceramic caps to change to film caps, negative feedback to lift. I bet that would start sounding pretty good. The filter caps may still be good. If it passed signal already, I'd save that for last, unless the voltages are obviously off.

    Anyway, I've rambled! There are certainly other configurations to try, too!
     
  5. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Cool. Some great ideas and info. I will get to this, but not this weekend. Soon hopefully. Thanks Jackie and if anything else springs to mind, don't hessitate. I'll let you know how it's giong.
     
  6. Groundhogken

    Groundhogken Member

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    How did I miss this thead?

    Here's a CHB-10A schematic that I drew up about a year ago.

    http://home.rochester.rr.com/groundhog/CHB-10A Pre and Power.gif

    http://home.rochester.rr.com/groundhog/CHB-10A Power Supply.gif

    I had one of this amps as a kid--don't know what ever happened to it. I bought mint one on ebay last year for $50. They're fun. Electro-Harmonix makes a 7868 tube now, but I haven't tried it. I found a couple NOS 7868 ( Sylvania I think ) for about $10 each on ebay.

    I replaced the electrolytic cap with the new can type that you can buy from Triode or AES. I put the part # on the scheme.

    Hope this helps.

    Ken
     
  7. Leonc

    Leonc Wild Gear Hearder Gold Supporting Member

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    Aw man, how cool it that! Thanks a bunch Ken, that'll help quite a bit!
     
  8. gldtp99

    gldtp99 Member

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    Ken's schematic is terrific-- he posted it on another board when i posted about my CHB10A there--- last weekend some players were over and were laughing and making fun of my "little" Bogen head when they saw it in the corner--- i've re-done the power supply and changed tone caps, etc--- i plugged it in to a 4x12 w/G12H30's and let them play it and they stopped laughing--:eek: :Devil -- it's one screaming little monster when dimed--- i've tightened up the low end response but the amp still has a moderate hum level-- they didn't seem to notice the hum but i did-- i'm considering changing the 7868 output tube (new EH) to a 6L6GC or EL34 if there is enough room height-wise---- maybe the 500K plate res on V1 could be a source of the excess (to me) hum, but maybe i'm too picky because nobody else seems to notice it---- they are too amazed that a 10 watt amp can get such distortion and cool harmonics--- i've also got a CHB14 and several CHB20A Bogens-- the CHB20A has a factory Baxandall tonestack and two ECL86 combination Triode/Pentode PI/Output tubes..........gldtp99
     
  9. Chrome Dinette

    Chrome Dinette Silver Supporting Member

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    I have a Ch8, which may be the predecessor to the amp you are describing. I found the easiest thing to do was gut it and rewire it following a Fender Champ schematic, using a 6l6 in stead of a 6v6 and playing with the cathode resistor value until I got what I wanted.

    If you change the socket, you could use a 6l6 instead of a 7868.
     
  10. Groundhogken

    Groundhogken Member

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    The heaters in this amp run single sided--like a Fender Champ. I haven't tried it yet, but you may get some hum reduction if you let both sides of the heater winding float and then use two 100 ohm resistors as an artificial center tap. The artificial center tap can be grounded or tied to the 7868 cathode to raise the heaters to a higher DC potential.


    I've also considered re-wiring the input stage to be at typical DC-coupled arrangement with a cathode resistor instead of the stock AC-coupled grid leak bias.


    I don't use this amp much, I just keep it around as a reminder of my pre-teen excursions into major single-ended power tube distortion.


    Ken
     

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