# Bogen CHB-35A - rebuilt for the millionth time (it's staying this way)



## thehoj (May 12, 2009)

So over the past year or so I've tried quite a few things with this Bogen CHB-35A that I had kicking around. It's a bit of an oddity because it runs on 7868's in the power section. I think they sound really good, and so I designed a new amp around that original bogen power section.

I have pics of the original gut shots in this other thread, as well as my first attempt at doing a rebuild on it, and then retrofitting it into a vox ac15 combo cab.
http://www.guitarscanada.com/amp-tech-section/31000-converted-bogen-chb-35a-pa-amp-guitar.html

I've since gutted it again and built it with a completely different preamp (which I happen to really like). The preamp is pretty much a dead on marshall 2204 layout right up until the PI. I only have a single triode for the PI, so I have to use a different type of PI than the 2204, and so from that point on I'm using fairly close to the original bogen layout.
I also built the whole thing on a turret board to keep things tidier. I think it's turned out really well. It's still a beast of an amp, not pretty at all, but I really like it and it's much more solid than it was before. I wouldn't hesitate to take it out gigging now.

Here's the schematic I'm running with currently (red lines are shielded cable):
http://home.westman.wave.ca/~vwtweaked/bogenrebuild/bogen2204 08july10 jp.jpg

The Presence control is a bit of an oddity also.. It's wired into the tone stack, and it almost seems to be a combination presence/depth control. I like what it does, so I'm leaving it.
I also wired in a switch to have low or high gain on v1b. It gives a nice contrast.

Here are some pics of the newest build:























































I've learned so much from this amp, It's really been an awesome tool to have around, and I'll be leaving it as it is now.. Might change out the speaker but that's it.


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## phoenixjazz (Jan 8, 2011)

*Questions on your circuit*

Hi,
I've got a CHB 35a and I've been planning a mod to it for guitar. I've been looking over your project and I'm thinking I'll duplicate it.

I was wondering if you would answer a question or two.
On R10, R11, R5 and the capacitor just under the connection to E what is the symbol in front of the 7o? I can't make it out.

Also for pin 3 on the output tubes verify the resistor value. Is it One ohm? what wattage are you using?

Thanks for any help you can give.

Regards,
chris

phoenixjazz at gmail dot com


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## thehoj (May 12, 2009)

Wow, sorry about the late response, I hadn't noticed that someone posted.
that's a 4. So they're all 470. That is 470K for the resistors, and 470pF for the capacitor.

And the resistor on pin 3 of the output tubes is a 1ohm 1% resistor, it's so that you can measure the voltage drop across it and then easily tell how much current you have going to ground through the power tube. It helps you bias your power section much easier.


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## the-patient (May 19, 2009)

This is a very cool post!

I have a couple old Bogens that I found in my grandfather's basement that I intend to overhaul, and this provides a great starting point.

I was wondering if I could ask where you found the majority of your information to learn about tube amps and stuff?

Thanks for the hard work!


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## thehoj (May 12, 2009)

The majority of what I learned is from the forums at Music Electronics Forum

In all honesty Looking back on this now, all I've really done is mashed the preamp from a marshall 2204 and the PI / power section from the bogen together. Done some tweaking here and there, and made it easier to work with on the turret board.

I learned a lot, but definitely didn't design anything new here.



the-patient said:


> This is a very cool post!
> 
> I have a couple old Bogens that I found in my grandfather's basement that I intend to overhaul, and this provides a great starting point.
> 
> ...


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## the-patient (May 19, 2009)

Thanks a lot for the reply! I like the sounds of your idea, I'm thinking of adapting it into my CHB100, but maybe changing the power section a bit for low wattage. 

How do you feel about the results? Do you use it as your main amp? Can we hear a clip?!


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## thehoj (May 12, 2009)

This was the last recording I got http://vwtweaked.ca/music/bassflex feb0810.mp3
This was plugged directly in, no pedals, volume turned down for the first part, and cranked for the last part.

I sold the amp a while back though.. It was pretty good, but I found I was just using my Traynor ygm-2 a lot more, and I had the need for some new gear, so it went.

A CHB-100 would be pretty cool.
If you wanted lower wattage you could just pull out 2 of the power tubes since you're running fixed bias in that amp (2 opposite ones, so that you've still got one tube on either side of the phase inverter). You would just need to use a different OT secondary tap since you'd be doubling the impedance that the OT see's on the primary side, so your 8ohm tap becomes 16ohms, and your 4ohm tap becomes 8ohms.

If I were reworking one of these amps again, I'd probably drill out the 7-pin socket which is there for the 6c4 phase inverter, and put a 9-pin socket in there so that I could use a 12ax7 phase inverter in a configuration more like the 2204. Primarily just because I believe the 6c4 (single triode) is a lot lower gain than either triode in a 12ax7.. I could be wrong about that though.



the-patient said:


> Thanks a lot for the reply! I like the sounds of your idea, I'm thinking of adapting it into my CHB100, but maybe changing the power section a bit for low wattage.
> 
> How do you feel about the results? Do you use it as your main amp? Can we hear a clip?!


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## the-patient (May 19, 2009)

Thanks for the advice! I should be starting the project mid-April, early-May. I'll keep you updated!

Dylan



thehoj said:


> This was the last recording I got http://vwtweaked.ca/music/bassflex feb0810.mp3
> This was plugged directly in, no pedals, volume turned down for the first part, and cranked for the last part.
> 
> I sold the amp a while back though.. It was pretty good, but I found I was just using my Traynor ygm-2 a lot more, and I had the need for some new gear, so it went.
> ...


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## TubesRock (Apr 21, 2011)

*Question for Thehoj*

I have modified an old Chb35a following your circuits as posted back in 2/10 But I have a question about part of your power supply circuit. I hope this don't sound too stupid but... about the two 330k resistors going across the two 40MFD 450V caps, do they form a junction between those two caps along with the red/yellow xformer lead? it would seem like they would but a little clarity would make me feel better when it is time to apply the juice. I was also wondering if you have thought about adding tube reverb and tremolo to you circuit. I have a schematic based on fender circuitry that looks like it would work. Thanks for your hard work and inspiration. oh yea, Rock on!!


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## thehoj (May 12, 2009)

I'm not sure I know what you mean about forming a junction.. All I did is put one 330K resistor over each 40uf cap. So one of them is from the + to the - on the top 40uf cap, and the other one is also from the + to the - on the bottom 40uf cap.
They're just there to discharge the filter caps when the amp is shut off.. So that it's easier to work on.


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## TubesRock (Apr 21, 2011)

Thanks. If you don't mind another question, the .05 uf cap between pin 6 of V2B and pin 6 of V3 and, the .022 (mf?) cap between pin 1 of V1A and the Volume pot, are they electrolytic? and what voltage did you use? I will try to post some pics of my "creation". I hope my results can come close to what you got. I also hope that it doesn't go up in a fireball and mushroom cloud.  The pic I uploaded is before doing the Bassflex mods. At this point it still had a master Volume, no mid, and the gain was wicked crazy. I will get some new pics. I also posted a shot of a Crate Vintage Tube 30 I modded. I think it sounds great. Maybe I will try to get a clip up. Thanks again for your help.


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## thehoj (May 12, 2009)

Are we talking about this schematic?
http://home.westman.wave.ca/~vwtweaked/bogenrebuild/bogen2204 08july10 jp.jpg

I don't see the .05uf cap between pin 6 of V2B and pin 6 of V3.. The .022uf between pin 1 of v1a and volume pot is film. 
Anything in that low uf range is going to be film caps. I think 400v or more is fine for those.



TubesRock said:


> Thanks. If you don't mind another question, the .05 uf cap between pin 6 of V2B and pin 6 of V3 and, the .022 (mf?) cap between pin 1 of V1A and the Volume pot, are they electrolytic? and what voltage did you use? I will try to post some pics of my "creation". I hope my results can come close to what you got. I also hope that it doesn't go up in a fireball and mushroom cloud.  The pic I uploaded is before doing the Bassflex mods. At this point it still had a master Volume, no mid, and the gain was wicked crazy. I will get some new pics. I also posted a shot of a Crate Vintage Tube 30 I modded. I think it sounds great. Maybe I will try to get a clip up. Thanks again for your help.


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## TubesRock (Apr 21, 2011)

I was working off of an older schematic. I have that circuit built and pretty much ready to fire up. I like your new schematic and that is probably where I will be going next or possibly just scrapping my current build to follow your new design. I was also thinking about ways of adding another 12ax7 or two for reverb and tremolo. Have you given this idea any thought? This is the Bassflex schematic I was following.


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