Bogen CHB 100 build

A while ago I got the bug to build my own amp, and shortly thereafter purchased a Bogen CHB-100. With my interest in schematic reading I thought it wouldn't be too terribly difficult but now I think it might be otherwise. I've been reading up alot on amp layout and design; I didn't realize how rusty I am with this stuff. Since this is my first build, I'm keeping it simple and doing as little heavy lifting as I can and doing some copy-and-paste engineering. I've come across this build along with schematic and audio clip which is very nice and something I'm looking to replicate, except for the fact it's for a Bogen CHB 35a.

http://www.guitarscanada.com/amps-cabs/31000-converted-bogen-chb-35a-pa-amp-guitar-amp-fitted-into.html

Here is the schematic presented in the link and then the schematic for the CHB 100.

http://vwtweaked.ca/images/Bogen%20BassFlex%2008feb10.jpg

http://uglyampking.brinkster.net/Images/CHB100.jpg

Seeing as the 100 just uses two more 7868's, can I essentially just compensate for the extra tubes in the circuit? I know there's alot more work on my end, such as switching out the 6EU7 for a 12AX7 and looking at voltages across the circuit (at quick glance I can already see there's 420v across the 7868's in the 100 and 430 across the BassFlex circuit), but I just wanted to ensure I'm on the right path or at least headed in the right direction.

Here's a few clips of some other hot rodded Bogen's I've come across. This CHB-50 seems to be heavily modded to my untrained eyes.

http://www.youtube.com/watch?v=Qe4zW0zrYcM&feature=plcp

And there's this one on eBay right now that sounds kinda fuzzy.

http://www.youtube.com/watch?v=GwnNnEKwxcM&feature=plcp

http://www.ebay.com/itm/VIDEO-CLIP-ADDED-Vintage-Bogen-CHB-100-Tube-Guitar-Amp-Modified-Tweed-EXC-/150824984534?pt=Vintage_Electronics_R2&hash=item231dde9fd6

Thanks for the schematic, Radana....saves me having to find it. I bought one of these recently. I will probably keep it as a P.A. since I have a lot of guitar amps....and I don't have one single tube P.A. Plus, I owned one of these...maybe this very one...back when I was young.

FWIW, there is no firm reason why one would have to change that 6EU7 to a 12A_7 except for maybe tube availability. There are some great Gibson amps that use 6EU7's in the preamp.

Hey guys, here's a delayed update:

I put this on the back burner for awhile, but finally got around to tearing into it yesterday. At first I was just going to try and get away with just a cap replacement and modify the circuit like others have done, but opening it up yesterday revealed not only some strange discharge from some of the capacitors (you can see it a bit in the pictures below) but I saw a few cracks in a few of the resistors. Now I'm thinking if I should just pull the transformers from this and build a circuit from scratch.

http://img802.imageshack.us/img802/5145/img0214m.jpg

http://img89.imageshack.us/img89/3595/img0216pl.jpg

http://img843.imageshack.us/img843/8722/img0218wg.jpg

So I'm thinking if I am just gonna pull the iron from this and build one from scratch, I need to test them. Should I replace the chord with a 3-prong beforehand, and can I test them on the board as is or should I pull them out? I am thinking too to just build a simple circuit, such as a 5E3. I know these trannys are 100w, but after reading how Martin 112 and Dearmond 15's are essentially 5E3's with bigger trannys and a few other threads about oversized iron in heads, I'm thinking this is doable--kind of like putting a tractor engine in a VW--but doable. I've got a lot of more research to do, but didn't know if anyone had any suggestions.

you really need to consider a few things before going further. one is transformer impedance, ie are those transformers going to translate over to a different amp? the next is the power transformer, what kind of voltages can you expect and how are you going to compensate if necessary? are you ready to make those changes/compromises?

the bogens have baxandall tone stacks so that is yet another issue to consider.

my honest opinion of your situation is a recap, change of input/output for guitar, and give it a try. if you can't bond with that sound, try modification of the existing circuit. if you just can't live with modification AND you can find a reasonable conversion to a different circuit only then I'd say gut it.

its not very often that this is the case. I've seen more than my share of gut and convert amps that are more like jigsaw puzzles of misguided compromise. "I didn't have this so I used that" and "but I like my results" aren't terribly elegant statements to make about your work.

you also have to consider the cost of time and money of conversion vs buying a kit or a different amp. if you're going to gut it and keep the transformers all you're really saving on is maybe the cost of transformers.

I've worked on 4 or 5 of these Bogen Challenger amps, and would agree with Smokingman. The voltages on the PT may be problematic for a 5E3 and you're talking a lot of work with a number of challenges. I've found that these amps don't sound bad if you just recap and refurb. Once you complete that it's a good mod platform to tweak the tone to your liking. Stock they sound a little sterile, but with pedals can sound great.

The CHB100 is a really hard one to start on though (given you're "rusty"). The CHB35 is a much better choice because there is less to do and much more room to work in. My advice would be to look for a CHB35 (CHB50 is also a good one and it's a 6L6 based amp) carcass and start with that one, and save the 100 for the next round.

Here's one of my projects - CHB50

Thanks SmokingMan. I'm looking up parts to order right now. Bdgregory, yes, I am "rusty" as well as "stubborn". :lol: After looking back I've come back to my original plan of just following the BassFlex schem.

I'm basing most of my thinking on Kirchoff's Law, so correct me if I'm wrong anywhere:

The basic architecture of all the Bogen CHB's is the same, with primary differences laying in the number of tubes (the same types remain) and respective resistor and cap values. Another reason I think this might be possible is I've been looking at amps with oversized iron and think that I can achieve "the same circuit architecture with larger transformers, creating more head room."

I can follow the BassFlex schem with the CHB-100 as long as adjust it to compensate the voltages. For example, on the chb-100, it shows on the OT transformer a voltage of 450v; on the BassFlex, its 487v. Is this just a matter of putting appropriate valued resistors in line to bring up/down the voltage?

Hey folks, so after assessing everything, I'm going to test the transformers tonight or tomorrow, and if they're alive and well I'm going to pull everything out and rebuild it and put it point to point on a pcb board. I found an updated schematic and final analysis by the builder of the Bassflex schem here along with his written conclusions.

Bogen 2204 (http://www.guitarscanada.com/amp-building-technical-repair/34847-bogen-chb-35a-rebuilt-millionth-time-its-staying-way.html)

Bogen Conversion Notes (http://mysite.verizon.net/vzepua0l/id10.html)

In the first link, someone mentions also using a Bogen CHB 100 and the builder states:

“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 mention of the OT tap is confusing me thought, as I also did a comparison of tube voltages via schematics (see below). They both are within spec, but the 2204 circuit has a pretty high plate voltage. Again, just knowing Kirchoff's Law, will adding resistance lower this?

Bogen CHB-100 (http://img843.imageshack.us/img843/8757/bogenchb100b6.jpg)

Bogen CHB-35a/2204 (http://img24.imageshack.us/img24/4462/bogen220408july10jp.jpg)

7868 tube data (http://www.tubebbs.com/tubedata/sheets/049/7/7868.pdf)

I'm off to research lowering that plate voltage and seeing if an EL34 or other tube could handle it better.

You can look at the output transformer needing to be balanced, say it had 4k load with four tubes and 8 ohms on the secondary. Well if you pull a set of tubes you now have 8k on one side, you need to bump up the output side in turn. You could put a 16 ohm speaker on the 8 ohm tap and that would do it. Or if you wanted to keep the same 8 ohm speaker you have to change the turns ratio between the primary and the secondary. You can do this by hooking up the 8 ohm speaker to the 16 ohm tap.

EL34 or 6L6 I would think,

You can look at the output transformer needing to be balanced, say it had 4k load with four tubes and 8 ohms on the secondary. Well if you pull a set of tubes you now have 8k on one side, you need to bump up the output side in turn. You could put a 16 ohm speaker on the 8 ohm tap and that would do it. Or if you wanted to keep the same 8 ohm speaker you have to change the turns ratio between the primary and the secondary. You can do this by hooking up the 8 ohm speaker to the 16 ohm tap.

EL34 or 6L6 I would think,

So by that logic, if 8k is standard, I could put it on the 4k?

So by that logic, if 8k is standard, I could put it on the 4k?

No. If 8k is the proper impedance for a quad of tubes (I just used 8k as an example, just a number) if you remove two tubes you have half the available power to drive them and the impedance has to be cut in half, so you would want to load two tubes with 16k.

I have a CH 100 in the garage that needs some TLC. I was thinking of turning it into a bass amp. thanks for posting the links to schematics and the like.

Well I didn't get around to evaluating anything, had one of those days where you run around and don't get anything done. I did however come to a conclusion:

I've always had this difficulty wrapping my mind around the relationship between tubes and transformers; I kept thinking the type and size of tubes used was predicated on what size your transformers where. If you had a 100w tranny, you needed a specific amount of particular tubes (four 7868s, two el34s). But now I'm slowly realizing that regardless of the wattage rating of the transformer, B+ is basically going to be the same, whether its 15w or 150w. If you want a 100w tranny to power a circuit with just a 12AX7 and a 6v6, that's fine because that tranny is overkill and rated for way above what'll be used.
I also learned that the 100w rating of the transformer doesn't mean its rated to work in that specific size of a circuit, but rather rated to operate to that limit until clipping occurs. I'm currently reading how four 7868's have the same amount of resistance of two el34's, so I'm looking to see how that plays in here. While perhaps I'm not the most productive or entertaining person to read, at least this will hopefully turn into something good to read for someone starting out with no clue on starting something like this.