Ultra-noob help: Where to start on a Bogen CHB 20A

[j_nition]

hey guys, new to the forum here and would like to see if I can shake the bushes a little here to learn a few things about a little amp I just picked up.


So this little Bogen Challenger 20A shows up on the floor of the garage at work last week from a load of things my boss pulled out of an old barn where his barbershop group rehearses. Thinking its obviously being staged for the dump, I offer him a few bucks for it but he says I can have it for free. My initial interest was that it being all tube, there might be some opportunity to somehow modify it into a guitar amp. Well, low and behold with a quick google search, I find that my intuition served me well as more than a few of you here on the forum have been doing this exact thing for a while now.

just to clarify, I'm a musician and not an electrician. I couldnt tell you the difference between a transformer and a capacitor but I do know great tone when I hear it and have a decent knack for coaxing it out of unlikely gear.

basicly, where I'd like to start with this project is figuring out how to get the weird amphenol mic plug converted to a 1/4" jack and decoding the process of how to wire the speaker outputs to a 12" cab to see where the tone starts in its raw state... and maybe learn a little more about how to read a schematic and fine tune it from there.

Anyone interested in pointing a noob in the right direction? From a quick browse of the current threads here, it looks like this is the right place!

Thanks!

[Chuck H]

First things first. You shouldn't just change the jacks and plug your guitar in. Tube amps contain wear parts and that amp is old. What's more, some parts inside a tube amp will age WORSE if the amp isn't in use. I would think that your Bogen PA is in need of new electrolytic capacitors. They're not hard to identify or replace. Then there is the condition of the tubes. If none are appearently bad you can just move foreward until you get to hear them under actual playing conditions. After that you could change out the phono plugs for phone plugs on the input and output. Take some time to learn safety procedures when working on tube amps. These aren't your average appliances. There are potentially LETHAL voltages inside EVEN WITH THE AMP UNPLUGGED FROM THE WALL. There are procedures that render these amps safe to work on. Google it.

Terry Kath from Chicago (the band) did use Bogen PA's as part of his setup. The circuits in these amps aren't far from what you might see in vintage guitar amps from what I've heard. So, do some google searching, or search here at this forum, and find out about replacing electrolytic capacitors. You'll want to replace them all. Then look into bias requirements for the tube type in your amp. Ask Q's here for setting the bias. Then you can swap out the jacks and see how it sounds for guitar.

Chuck

[j_nition]

ok now we're cooking! so replacing the caps is where I should start. I assume it would require just matching up the values I see with new ones... sounds simple.

-The tubes look fine but obviously that doesnt count for much. I have a cheap multimeter... does it require more than that to test them?

-as for the jack swap, is it as simple as moving the tip and ring leads from the weird screw-on mic jack over to a 1/4" jack?

-do I have to rewire the speaker terminal to a 1/4" jack too or can I hardwire a speaker with the existing terminal strip? if so... how is this done?

thanks guys!

[Chuck H]

ok now we're cooking! so replacing the caps is where I should start. I assume it would require just matching up the values I see with new ones... sounds simple.

Just the electrolytic caps. You can ignore the film caps as they last virtually forever. The electrolytic caps are the big cans in the power supply and the smaller ones throughout that look like a tube with a groove on one end. They are usually in whole uf values like 22uf, 47uf, 100uf, where the film caps will be valued in fractions of uf's like .1uf, .02uf, etc. Google up "electrolytic" and "can capacitor" to see what you'll be looking for. The "can" type is likely what is in your power supply and is actually more than one capacitor in the same can, thus the name. I can't say for sure what's in your amp because I've never opened one.

The tubes look fine but obviously that doesnt count for much. I have a cheap multimeter... does it require more than that to test them?

Yes. And no. In the circuit you can use your meter to test a tubes performance and get an idea if it's grossly bad. But there are finite measurements that a tube tester is better for. But don't bother with a tube tester because we don't need to know the finite measurements to know if a tube is performing. What you can tell with your meter combined with listening to the amp will be fine.

as for the jack swap, is it as simple as moving the tip and ring leads from the weird screw-on mic jack over to a 1/4" jack?

Yes. If it isn't clearly marked on the amp you can tell with your meter. The ring lead It will be the one that is grounded to the chassis. The tip lead will go to a tube grid (via some components)

do I have to rewire the speaker terminal to a 1/4" jack too or can I hardwire a speaker with the existing terminal strip? if so... how is this done?

Hopefully the speaker connections of that amp are clear enough to show +/- leads. So sure, you can just wire it up like you would any home audio speaker. I favor adding a female jack for later convenience. Removing a male jack from a speaker cable that I will eventually want to repair seems like more trouble than adding a female jack in the first place.

[j_nition]

thanks for the great starting points...


....actually I wish the speaker wiring was easier to figure out

-theres a terminal strip with 6 screw labeled"

G C 4 6 8 16 25v

- It says "class 2 wiring acceptable except 70v requires class 1"

-there's a flying lead coming out of the chassis

-and lastly it there are two 3 pin sockets labeled

PIN 1:GND PIN 2:IMP SEL PIN 3:70v


any ideas?

[Chuck H]

I have some ideas. But to be certain I'll need to look at a schem. I'll let you do the Googling on
that. Good luck. I know some of the Bogen stuff is out there.

Enzo is pretty slick with this sort of thing. He may see that his name was referenced and pop in if he understands the markings.

[olive2sing]

-theres a terminal strip with 6 screw labeled"

G C 4 6 8 16 25v

The G terminal is ground and normally has a jumper to the C or common. If the jumper is not there I would put it there. Common and ground would connect to the sleeve of say a 1/4" speaker jack. From the tip you could run a jumper out to the terminal strip and use it to select 4,6,8,or 16 ohm speaker load. For a guitar amp you can just ignore the 25V or 70V terminals(70V normally has a separate common) The jumper wire may be there already and go to a 3 pin speaker terminal. I used a lot of bogens in the early 70s and I'm still playing my guitar through one I've had since 73, thanks to the folks on this forum.

[j_nition]

oh man thanks! thats a great tip on the speaker wiring. This forum is really a Godsend!

[Casey4s]

Do you have a schematic for that amp?

[j_nition]

Do you have a schematic for that amp?

I found this but it all greek to me.

http://schematicheaven.com/hifiamps/bogen_cha-20.pdf