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Old 17th May 2006, 02:43 AM   #1
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Default Another Bogen Conversion

Wow, been a while since I posted here. If anyone remembers me, my Bogen M60A is working great as a guitar amp. Tons of power, nice aggressive overdrive, and I actually love the Bax tone stack.

Anyway, I've got a buddy that needs a decent guitar amp, but doesn't have too much to spend, and I immediately offered to set him up with a PA convert. I'm thinking of doing a Bogen CHB-20A, they pop up on ebay from time to time. Here's a schematic.

http://richardstevens.home.mindsprin...gen_CHB20A.jpg

The whole aux circuit will be removed and the input jack will be placed where the aux volume knob was. I'm planning on keeping the preamp circuit the same (I like how my M60A sounds with the grid-leak biased 6EU7 preamp) with one exception. I want to remove the master volume that's currently there and place it later in the circuit. The problem is, the ECL86s cause this amp to have one of the strangest output sections I've seen. I don't even know what the purpose of V3A is, there'd no signal going to it as far as I can tell. Unless it's just there to provide drive for V3B? Anyway, where in this mess would be a good place to put the master volume?

There's a sitehere about a full-blown conversion to a Marshall "18 Watter" style amp, but I'd like to see what I can do with the existing Bogen circuit first.

Besides the jacks, tone stack, and other obviousness, is there anything else in particular you guys see that would contribute to "guitarifying" this thing?

Thanks.

-Darren
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Old 17th May 2006, 03:19 AM   #2
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Darren,

That looks like a paraphase type splitter to me. Check out Bonavolta's phase splitter page here.

Making R5 fixed and R13 a pot. would place the master volume control later in the circuit.
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Old 19th May 2006, 02:24 AM   #3
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Yep, looks like you're right about it being a paraphase splitter. Anyway, here's my first shot at something here. Photoshop works wonders on schematics. Anything I missed? I left the tone stack alone on purpose, the Bogen-style Baxandall is actually a nice change from the normal F/M/V style.

http://www.teamfc3s.org/forum/attach...&postid=530301

My concern now is that the ECL86 is getting scarce, and matched pairs are hard to come by. The amp runs the tubes a little hard to begin with, and I'd like to make the bias to each tube adjustable to eliminate the need for a matched pair. I know I'll have to put a 1 ohm resstor between the cathode and ground for each pentode stage, and I know I'll have to probably squeak another volt out if the bias supply so I can regulate it with trimmer pots before R18 and R19. Would dropping the value of R23 or R26 slightly give me a little room to mess with? What value of pots should I use? What bothers me is that the grid of V3A is fed from the main bias supply, so if I give it another volt there (this is before where I want the pots to go), how is it going to affect V3A?

-Darren
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Old 19th May 2006, 01:19 PM   #4
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I have a pair of Bogen CHB35s which might be a better place to start, with tubes that are in current production. No schematic, though - anybody got one?

tbavis(at)rochester(dot)rr(dot)com
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Old 19th May 2006, 06:54 PM   #5
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Here ya go.

http://www.schematicheaven.com/hifia...gen_chb35a.pdf

Looks like a fun little unit, much more versatile than the CHB-20A. I'll fire up photoshop tonight and play with some layouts. I like how the tone stack is between the 12AX7 stages.

You interested in selling one? There's a few '35As on ebay right now but it'd be nice to get one straight up without the hassle of bidding.
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Old 19th May 2006, 06:55 PM   #6
jleaman is offline jleaman  Belgium
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I just got a pair of bogen amps. They are on the way. Im going to follow this thread and see what i can make of it.
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Old 19th May 2006, 09:16 PM   #7
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What models? I converted an M60A not too long ago, there's a huge thread about it on here somewhere. You might find some useful info in there if you can get around my ignorant babbling .

The M60A wouldn't be my first choice if I was to do it again, due to there being tons of stuff in the original circuit, and it uses 8417s in the output. I still have to replace the power cord with a computer-style socket, move the master volume, and add a bias voltage doubler/adjustment circuit to run KT88s. I haven't removed much of the original circuitry, there's so much stuff in the amp that I have to many options . I need another simpler project to take my mind off of that one for a while . The MX series looks even worse to work with due to the sheer amount of inputs and wiring.

I'm gonna go download a bunch of PA amp schematics and see what I can do with them. I know I'll never get around to doing that many conversions (unless I can start selling them) but I've got nothing better to do this weekend .

-Darren
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Old 20th May 2006, 01:09 AM   #8
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I bought 'em cheap to fix and put on eBay... so maybe we can make a deal. Got them without tubes, but I'm pretty sure I have 'em all.

Has a cathodyne phase splitter, though - that has to go. Overdrive will be nasty - 12AX7 or AT7 long-tail pair is the way to go. Maybe a 6J6, since it's 7-pin socket...

I didn't know about the folder of PA amps at Schematic Heaven - I'll grab some copies. For Stromberg Carlson, there's only one place to go - my website: http://www.audiophool.cjb.net/MadeInRoch.html

And for that matter, I have a bunch of other Sams folders too - Bogen, Masco, etc. - if there are any in particular you need, I'll look.
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Old 20th May 2006, 03:28 AM   #9
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Looks like all the CHB models from the '35 up are pretty similar up until the output. 6J6 long-tail pair sounds like it'd work for probably all of them. It'll take a bit of creative redesigning, but it's be a lot easier than trying to mess with the paraphase of the '20A.

My M60A still has a cathodyne setup. The cathodyne is probably what's giving it its "unique" sound right now, and I don't want to change it. It's loud and obnoxious, and I'm loving it. I think it would be a major hassle to change. Then again, I do have the 9-pin input transformer sockets that would hold a 12AX7 nicely...

Anyway, I would probably want to do something different on a CHB, they're a much simpler amp in general and the effort would certainly be worth it.

When I see the guy again next week, I'll see how much he wants to spend. This has got me excited to build one for myself, but I've got no use for another amp right now.

By the way, in case you want to add the M60A schematic to your collection, here's my schematic. Courtesy of Rob Mercure on the Ampage forums:

http://www.teamfc3s.org/forum/attach...&postid=527497

-Darren
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Old 20th May 2006, 04:19 AM   #10
imix500 is offline imix500  United States
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Tom Bavis, got a schematic for a ch8? Thanks.
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