Bogen CHB 10A Schematic

[Geezer]

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Platefire
I only work on Fender's
Posts: 902
(2/9/05 7:47 pm)
Reply  Bogen CHB 10A Schematic
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 I was given one of these the other day and was wanting a shematic. I done a zillion searches on the net but have turned up nothing. I know some of you have made projects out of this model. Anybody got a schmatic to share??? Plate
 
 
GroundhogKen
Moderator
Posts: 2641
(2/9/05 8:05 pm)
Reply  Re: Bogen CHB 10A Schematic
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 You are in luck my friend.

I used to have one of these when I was a kid--don't know what ever happened to it, so bought one in mint condition on ebay about a month ago.

Last night I sat on my couch with the chassis on my lap and sketched out the schematic.

I will scan it and email it to you. It might take a day or two but you will have it.

The tubes are 7868 and 12AX7. The micropone input is cool for guitar. The phono input doesn't have much gain.

Ken
 
Platefire
I only work on Fender's
Posts: 903
(2/10/05 6:26 am)
Reply  Re: Bogen CHB 10A Schematic
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 Great! Thanks Ken. I fired mine up last night and used an adapter through the phono input. Like you say--not much!
I snipped the input wire on the phono and mic input and hooked the mic input wire to the phono input and wow!
Marshall City!!! but it has hum/buzz issues that I can't figure
out. I think I'm goin to rebuild the power supply to a full wave
voltage doubler and also rewire the input for more guitar frendly set up. I can tell now this will be one screaming little amp. That 7868 is a powerful machine. I've got original Bogen tubes that seem to be working OK. Yes I would appreciate the schematic when you get it done. Thanks, Plate  
 
OldHouseScott
Senior tube assistant
Posts: 155
(2/10/05 8:24 am)
Reply  Re: Bogen CHB 10A Schematic
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 Hey Ken, once you get it scanned, would you mind sharing it with the class?  

OldHouseScott
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GroundhogKen
Moderator
Posts: 2644
(2/10/05 8:42 am)
Reply  Re: Bogen CHB 10A Schematic
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 Yep, I'll share.

The heaters are unbalanced, but snipping one wire and adding a pair of 100 ohm resistors will fix that.

The first 12AX7 stage for the mic input is grid leak biased.

The old style mic jack is a Switchcraft 2501F. I think Mouser has them.

I haven't had too much time to play with mine, yet. As I said I just chicken-scratched the schemematic on notebook paper the night before last while I sat in front of the TV.

The can cap is 20uF 250V, 20uF 400V, 10uF 400V, 5uF 400V.
I replaced it with a 20-20-20-20uF 475V which is a available from (aes) tubesandmore.com or store.yahoo.com/triodeel/twisel.html

It's an exact drop in fit with same height.

Ken
 
GroundhogKen
Moderator
Posts: 2645
(2/10/05 8:53 am)
Reply  Re: Bogen CHB 10A Schematic
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 In mine, the Bogen labeled 7868 appears to be a Sylvania (pointed top). The bogen labelled 12AX7 appears to be a Mullard ( Yee-Hah !). I pick up a spare NOS Sylvania 7868 on ebay for $15. Beware of all the trash out there. Electro-Harmonix has released a new production version of this very rare large 9 pin ( Novar???) pentode. I believe RCA, GE and Sylvania were the only ones who made these, but they were widely used in Fisher stereos.

Data Sheet and Picture:

www.mif.pg.gda.pl/homepag...7/7868.pdf

www.mif.pg.gda.pl/homepag...7/7868.jpg

The power supply is a voltage doubler. That one had me scratiching my head when I first saw it. A full wave bridge might be a good idea.

Ken
 
GroundhogKen
Moderator
Posts: 2648
(2/10/05 9:28 pm)
Reply  Re: Bogen CHB 10A Schematic
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 I will draw these for real over the weekend, but for now here is the chicken scratch version.

home.rochester.rr.com/gro...Supply.gif
The smudgy parts are a 390 ohm 4 Watt resistor and a 20uF 250V cap.

home.rochester.rr.com/gro...+Power.gif
Pin 2 and 6 are the 7868 control grid. The coupling cap is on pin 6 and the grid resistor is pin 2. I didn't measure the pot values.

Feel free to ask questions about parts that are illegible.

Ken
 
Platefire
I only work on Fender's
Posts: 904
(2/11/05 1:19 am)
Reply  Re: Bogen CHB 10A Schematic
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 Mine works but buzzes like crazy. I think I will gut it all the way down to the transformers and terminal strips and completely rewire everything. Ken, have you determined what value those the pots are?? I think I will remove the mic pot and install a 1/4" jack in that slot and make the phono volume the regular volume and leave the tone same location, just rewire for a Princeton type tone stack. The can cap from AES is kind of high but I guess that would save you having to find space and installing terminal strips for the filter caps??!! I think I will also give my cover a new paint job. I would be nice to install some kind of handle on the case. As far a extras, think I will install a pilot light and standby switch. I don't particularly like the slide on/off switch but with that rectangular slot, would be hard to install another type switch unless I reamed out the slot and installed a pilot light in it's place. I want to keep it pretty simple.  Plate

Edited by: Platefire at: 2/11/05 1:19 am
 
Tiny Daddy
I only work on Fender's
Posts: 1090
(2/11/05 8:48 am)
Reply  Re: Bogen CHB 10A Schematic
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 Ken I don't think you can replace the doubler with a bridge without dropping the B+ by half. However the doubler diodes could be replaced with a bridge if you use only 2 of the diodes.
Plate, regarding rectangular power switch hole, suggest you just file out larger and install a snap-in rocker switch for 88 cents.
 
mrr3000gt
I only work on Fender's
Posts: 555
(2/11/05 9:01 am)
Reply  Another challenge for Ken
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 OK Ken, I have another schemo challenge for you:

As I mentioned in another thread, I had a customer recently give a complimentry Hammond H-100. What I dont have is a readable schematic for the AO-29 and AO-44 reverb components in it. The one I downloaded from Captain Foldback is such bad resolution it is not good for reading. All other Hammond Links I have found for the H-100 refer to this website for schemo.

Do you happen to have a H-100's AO-29 amplifier schematic handy???

 
 
Platefire
I only work on Fender's
Posts: 905
(2/13/05 12:09 am)
Reply  Bogen Blaster
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 Well I used up a whole roll of de-soldering Brade tonight removing all the preamp components. Man them Bogen folks
poured the solder to every joint. The mechanical joints were
so tight I believe it would have worked without solder. Har!
I've got plenty of room for mounting filter caps on terminal
strips so that's what I'm going to do, just remove the can cap and put a cover over the hole. I will also remove all the speaker lugs, the phono jack and mic connection and just put a metal plate over those hole to cover them up and mount the speaker jack in that plate. I think I will build the full wave doubler power supply like the
"little Bear" amp I finished a while back.

Hey Ken, don't know what I'm doing wrong but I couldn't get the whole page of your schematic sketch to print out on one page like I normally do. Maybe I just need to copy the file
and print in photo editor. I'll give that a try and see if it works.
Got to make a material list so I can make an order and get the rest of the parts I need. Guess I try that rocker power switch Old House recommended. Ye Haw---this is good fun!
Plate  

Edit: Hey Ken!!!I copied the schematic files to my computer and they printed out just fine but I'm seeing some difference in resistor values than I've found on mine but most everything else is the same.

Edited by: Platefire at: 2/13/05 12:43 am
 
mrr3000gt
I only work on Fender's
Posts: 560
(2/13/05 1:29 am)
Reply  Re: Bogen Blaster
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 I have a Bogen AP-30 integrated preamp/stereo (four 7591-powered) I have not been able to find ANY information on, let alone a schemo....

Any suggestions??

Its a great sounding stereo. (all tube)
 
Tiny Daddy
I only work on Fender's
Posts: 1095
(2/13/05 2:15 am)
Reply  Re: Another challenge for Ken
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 I just looked at the AO29 schematic. Those old drawings used M for 1000, so for example a resistor labeled 56M would be 56,000 ohms. It seems someone may have put MEG in the place of M on some of the resistors.
 
GroundhogKen
Moderator
Posts: 2651
(2/13/05 10:32 am)
Reply  Re: Another challenge for Ken
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 OK, here are new schemes for the CHB-10A

I did these in Orcad. Since the drawing has component designators (R1 C5 etc.) on each part now, if anyone has different circuit values just let me know and I will add them as alternate parts notes.

home.rochester.rr.com/gro...Supply.gif

home.rochester.rr.com/gro...0Power.gif

I still

[Geezer]

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 OK, here are new schemes for the CHB-10A

I did these in Orcad. Since the drawing has component designators (R1 C5 etc.) on each part now, if anyone has different circuit values just let me know and I will add them as alternate parts notes.

home.rochester.rr.com/gro...Supply.gif

home.rochester.rr.com/gro...0Power.gif

I still haven't measured the pots yet, sorry.

Ken

 
Platefire
I only work on Fender's
Posts: 907
(2/13/05 4:55 pm)
Reply  Great Job Ken!!
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 Wow!!! Great Job Ken! Thanks! This will something of great use to anybody in the future making a project out of this amp----namely me!!! Har! Believe me if there was any
schematic for this on the net, I would have found it because I have searched diligently off and on for a week now---and nothing! So special thanks to you for your effort!

A few of my components vary from yours. My amp still
has a Bogen service tag where apparently it was sent back to the factory for service. The tag has " Bogen Division Lear Siegler Inc". The tag hasn't got any specific info on the repairs made but has several numbers as follows: 52-9071-01,
02038 and a stamped number 1122 with a initial by it. I was trying to dig a date out of that??? They may have moded the original at that time. On mine R1 is 10 Ohm/5W, R2 is 60 Ohm 1/2 W and R3 is 120 Ohm 1/2 W. Also after C7 I have a 410 Ohm by color code(y/bn/bn) but reads 468 Ohms on my meter. This resistor is not a carbon comp resistor like the rest so I suspect it's a mod. Everything else pretty much looks identical to yours.

On the pots VR1 is 1M, VR2 is 1M and VR3 is 2M. My pots are in very good shape so I will probably re-use them along with the knobs.

Thanks a Bunch!! Plate  

Edited by: Platefire at: 2/13/05 4:57 pm
 
Platefire
I only work on Fender's
Posts: 915
(2/22/05 9:26 pm)
Reply  Bogen CHB-10A Rebuild
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 Got a question on this one. I remember someone a while back
using a pot in place of the grid resistor to ground(normally 1 meg) on the section of grid between the last preamp tube and power tube. I think it was Jeff but can't remember for sure. It seems like a good idea since the resistor has such an effect on tone and gain. There is got to be a good reason this isn't a standard feature on amps these days. Before I drill another hold for a pot in my chassis and try this can somebody give me a good explanation why this may be a bad idea???? Plate  
 
phsyconoodler
Hey get your own solder
Posts: 271
(2/22/05 10:23 pm)
Reply  Re: Bogen CHB-10A Rebuild
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 Plate,
Don't do the grid resistor pot! It cannot take the current and can cause a surge of electricity to go into the chassis or you,if it fails.Bad idea!A rheostat designed for that purpose and about 20watts,would be a good choice.Available at mouser electronics.
Check your inbox,I left you a message.
Phsyco  
 
Platefire
I only work on Fender's
Posts: 916
(2/22/05 11:38 pm)
Reply  Re: Bogen CHB-10A Rebuild
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 Hay Phsyco

I was thinking it would be safe on the oppisite side of a coupeling cap from the DC but if the cap ever failed could be bad. I think I will just do like I did on my last one and experment with different fixed resistors until I find the sweet spot. In expermenting with the different resistor on the little Bear amp I noticed a lot of difference in gain and highs/lows. I was thinking it would be cool to get all these variations with a varible control as long as it didn't make the amp un-stable.
I will read your e-mail before I make my fineal decision.
Thanks, Plate

 
j allen shaw
I only work on tube amps
Posts: 492
(2/23/05 2:41 pm)
Reply | Edit
  Re: Bogen CHB-10A Rebuild
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 Plate,
If you're referring to R5/1meg (grid to ground) on Ken's schem linked above, you can put a 1meg linear (or audio if that's all you have) pot in there to find the sweet spot, then replace it with the correct value resistor.
I think you are right, no danger on that side of the cap...measure the voltage, the cap should be blocking it. If the pot goes bad, all you do short all the signal to ground (no output).

I think phsyconoodler was thinking you were talking about placing a pot on the plate supply....I posted a while back, saying that the variable plate voltage control indeed was not a good idea, because of the risk of failure/shock, and have deleted the feature from all my schematics.

Jeff

Edited by: j allen shaw at: 2/23/05 3:03 pm
 
Platefire
I only work on Fender's
Posts: 917
(2/23/05 10:45 pm)
Reply  Re: Bogen CHB-10A Rebuild
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 Hello Jeff

I knew I remember some varible control you was expermenting with. So it was plate voltage on the preamp tube. Once I get the amp fired up and going I may just shoe in a temporary 1 meg as a test to see how it goes before I drill an extra hold.
Getting ready to drop a new power supply and filter caps. So once I get it up and running I can do a little expermenting.

Thanks, Plate
 
j allen shaw
I only work on Fender's
Posts: 686
(5/21/05 7:15 am)
Reply | Edit
  Re: Bogen CHB-10 Rebuild
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 Well,well.....glad I found this thread before it fell off the board!

I picked up a Bogen CHB-10 (no "A") FREE this week. The only difference I see so far from the "A" model is that mine has an EZ81 tube recto instead of the SS diodes.

Woo-Hoo! Maybe I'll try some "Schedule 40" mods on it!
 

Jeff