# Converted Bogen CHB-35A PA amp to guitar amp, fitted into vox ac15cc cab (pics/clip)



## thehoj (May 12, 2009)

I'm sorta excited about this amp, so I thought I'd post some info about this little project of mine.
EDIT: Just realized this should have probably gone in the Amp Build forum.. If a mod wants to move it that's fine.

So I found this old Bogen PA amp at my moms house in the basement a little while back. The amp had 2 dual triode preamp tubes and a single triode phase invertor, and a pair of 7868 power tubes. The power tubes are pretty uncommon and mostly found in old hifi audio equipment. I was able to get ahold of 2 more sets of NOS 7868's thanks to Ripper on these forums.
And over the past few months, I've worked at redesigning the preamp and tone stack to be more suited to guitar. Rather than multiple inputs I went with cascaded stages.

Once I figured out how I wanted to go for my schematic I basically gutted the whole thing, and replaced all the parts with new ones.. New metal film resistors, new filter caps, new coupling caps, new pots. Using the original OT and PT though, as well as original power tubes.
I converted v1 to 12ax7 wiring (instead of the less common 6eu7 as it was originally).

I based my redesign on the Ampeg portiflex SB-12 7868 (it uses the same power tubes, and is fairly similar) and also quite extensively on the Fender 5e6a (not 5f6a). I've also got a number of my own preferences in there too, like making the negative feedback switchable on and off (not shown in schematic). I had also wired in a MV at one point, but am not running now, I think I prefer the amp without it.
I also made my power tube bias adjustable with a pot.

The two 7868's end up giving me an output power of around 35w from what I understand.

Basically I took it from:
http://www.schematicheaven.com/hifiamps/bogen_chb35a.pdf

to:
http://vwtweaked.ca/images/Bogen BassFlex 08feb10.jpg

I also ended up getting ahold of an empty semi beat up vox ac15cc cab which I was able to retrofit the bogen into, courtesy of another GC forum member (although I can't remember their name now). I've got an old 12" marsland alnico speaker that I pulled out of my Traynor ygm2 in the cab, and I actually like the sound of it.

Anyways, after numerous rebuilds I think I've almost got this thing where I want it.
It's fairly haggard looking, but I'm not really goin for looks on this guy.

Here are some pics of it from before:




















And here it is now fitted into the vox cab:





































EDIT: did a better recording after a few modifications (schematic is accurate for this recording. It is how I'm going to leave the amp). The recording is all amp now.. no effects, the clean part is volume at maybe 1/3 on, the overdriven part has the volume full on. I like the natural overdrive sound of this amp.

http://vwtweaked.ca/music/bassflex feb0810.mp3


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## greco (Jul 15, 2007)

Thanks for the pics...nice work...CONGRATS :food-smiley-004: :bow:

Cheers

Dave


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## Ripper (Jul 1, 2006)

Hey Hoj very cool, you got her all put together. Well done!


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## Wild Bill (May 3, 2006)

Nice job, in the true spirit of "home brew"!

In the words of the immortal band Strange Fruit: "The flame still burns!"


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## brimc76 (Feb 14, 2008)

Nice work hoj, thanks for the details and the pictures.

Brian


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## keto (May 23, 2006)

I've got an old Bogen in the garage, bought it cheap off ebay to scavenge some nice old glass (6L6's + 12AX7's) out of it - more than got my money's worth. Been meaning to see about what it would take to 'convert' it for guitar, just haven't got around to it.


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## thehoj (May 12, 2009)

keto said:


> I've got an old Bogen in the garage, bought it cheap off ebay to scavenge some nice old glass (6L6's + 12AX7's) out of it - more than got my money's worth. Been meaning to see about what it would take to 'convert' it for guitar, just haven't got around to it.


It's definitely worth doing if you want a project that takes a long time. Personally I've really enjoyed the time I've spent learning about tube amps and coming up with a new idea for a schematic, and then trying it all out.
It was very satisfying to get everything together and then come up with a tone stack I liked, and then the preamp sound and gain structure I was after, and just tweaking little bits here and there to get things just how I liked them.
One suggestion I'd have for you though is (depending on what you already have) order a large variety of parts (more than what you think you'll need). Get a whole bunch of resistors, caps, even some extra filter caps (I wrecked a couple of mine at one point), and pots as well.

I ordered all my parts initially from tedweber.com, but I also ordered some from justradios.com (in Canada, which was quicker for the parts to get here).

I'd love to find another old amp to rebuild, but haven't been successful up to now.
At one point I tracked a guy down that had an old hammond organ that he didn't want, it had 6L6s in it and sounded perfect.. But then he stopped replying to emails and stopped answering his phone.. I guess he decided to keep it. 
I'm always on the lookout though.


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## mrd34d (Jan 14, 2010)

Thanks alot for these pics! I've also got a Bogen CHB-35A that I've contemplated converting into a guitar amp. I was looking at your modified schematics, and couldn't help but notice that you still had the Terminal strip included in them. I'm a newb with this stuff, so maybe I'm missing the point?

Would it be at all possible to post a schematic of the amp in it's current state? I'm looking at either modding just my bogen, or somehow integrating it with my "Champagne 22" 60's vintage amp.


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## thehoj (May 12, 2009)

mrd34d said:


> Thanks alot for these pics! I've also got a Bogen CHB-35A that I've contemplated converting into a guitar amp. I was looking at your modified schematics, and couldn't help but notice that you still had the Terminal strip included in them. I'm a newb with this stuff, so maybe I'm missing the point?
> 
> Would it be at all possible to post a schematic of the amp in it's current state? I'm looking at either modding just my bogen, or somehow integrating it with my "Champagne 22" 60's vintage amp.


Terminal strip was still in the schematics because I just never bothered to take it out. I've got a new schematic in the first post now, it's accurate of the current state of the amp.
The small modifications I did was change v1a cathode resistor back to a 1.5K (instead of 820ohm) and I removed all of the cathode bypass caps. I think this improved the sound of the amp quite a bit.
Also, in my latest schematics I don't have a master volume, and I've made the negative feedback loop switchable on/off (although I think I prefer it off). I've also removed the bright switch and just have a 120pf cap permanently attached to the volume pot.
One thing to maybe make note of is that you might not end up with the exact same bias circuitry/components as I have.. It was a little bit of trial and error for me to get the voltages where I liked them.

On a side note, I just tried running this amp in stereo with my Traynor ygm-2, I cranked both of them wide open, threw on a boost pedal in front of both of them, and dear god I have definitely suffered some hearing loss but it was so worth it. The two amps complement each other quite nicely.


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## mrd34d (Jan 14, 2010)

thehoj said:


> On a side note, I just tried running this amp in stereo with my Traynor ygm-2, I cranked both of them wide open, threw on a boost pedal in front of both of them, and dear god I have definitely suffered some hearing loss but it was so worth it. The two amps complement each other quite nicely.



Hahahahaha! That's awesome!! Thanks alot man!


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