1940's Challenger CH 18 restore/mod

[Phyberoptic]

Picked up what I believe is a 1940's Bogen Challenger CH 18 tube amp for MIC and Phono powered by 2 6v6 valves and 6SF5 preamps.



I'ts in original condition with what appears to be original valves. There are a couple knobs that need replacing and the power cord is cut off. The unfortunate issue is that the canned electrolytic cap has leaked its guts into the chassis and messed up a number of components.



I received a schematic via request on this board for a similar Bogen CH 18 amp, and will trace out and modify the drawing to match what is there. Bogen did not have any archives of this amp unfortunately. What I'd like to do is restore this back to near original condition, getting rid of the death cap and putting in a three prong power cord. Then depending on the tone it produces mod it into accepting a balanced XLR mic input and replacing the phono input with a 1/4" input for guitar. I would think at that point replacing the 6SF5 valves with 12AX7 might be in order to increase the gain and make it a more usable harp/guitar amp. Along the way voicing the preamp stages for harp/guitar would happen also.

I suppose first steps after cleanup would be to test the capacitors for leakage and re-stuff the can with new electrolytics. I may try and keep the paper caps in by melting the beeswax, stuffing them with new caps and resealing them with new wax for the vintage look.

-=Phyber

[Leo_Gnardo]

I suppose first steps after cleanup would be to test the capacitors for leakage and re-stuff the can with new electrolytics. I may try and keep the paper caps in by melting the beeswax, stuffing them with new caps and resealing them with new wax for the vintage look.

You may test those filter caps (also cathode bypass caps, tone caps & interstage caps) from now 'til the cows come home - but in the end you're going to wind up replacing them all. I've been disappointed too many times by taking the path of "this oughta work . . . I hope!" but had 50+ year old caps fail, sometimes spectacularly, practically every time.

Instead of re stuffing old caps, have a look at Antique Electronics' offerings ( tubesandmore.com ) , a nice selection of can caps and "dynamite stick" electrolytics plus tons of other useful parts. I'm not an owner or employee, just another satisfied customer and you'll find plenty of us here on MEF. And welcome aboard! Good luck with your project. Those old "toaster" style amps are kool!

[DrGonz78]

Also, I came across another CH18 drawing that is not original and someone had traced it out apparently.

[Phyberoptic]

Also, I came across another CH18 drawing that is not original and someone had traced it out apparently.

Looks much closer, thanks. The model numbers for the TXs used is different however the outputs appear to be the same on them. Is there a specification chart for older transformers? And any guidance on how to test the OT out of circuit?

[Phyberoptic]

You may test those filter caps (also cathode bypass caps, tone caps & interstage caps) from now 'til the cows come home - but in the end you're going to wind up replacing them all. I've been disappointed too many times by taking the path of "this oughta work . . . I hope!" but had 50+ year old caps fail, sometimes spectacularly, practically every time.

Instead of re stuffing old caps, have a look at Antique Electronics' offerings ( tubesandmore.com ) , a nice selection of can caps and "dynamite stick" electrolytics plus tons of other useful parts. I'm not an owner or employee, just another satisfied customer and you'll find plenty of us here on MEF. And welcome aboard! Good luck with your project. Those old "toaster" style amps are kool!

Thanks for the welcome. In retrospect, I think that is sound advice. I'll order some 10mfd 600v caps and get started on cleanup. I should have replacements for the paper caps in my stash.

-=Phyber

[g1]

any guidance on how to test the OT out of circuit?

Output Transformers

[olddawg]

Honestly I wouldn't spend a minute trying to fix it. Some people seem to think these old PA amps have some kind of mojo. I've never played one I liked. They used to be plentiful when I was a kid. We called them "battery chargers", lol. BUT... if your transformers are good, you have a great case. $2 worth of wide aluminum stock will cover that whole control panel and you can install whatever control architecture you want. If it were me, I would get a turrent board and build up an amp based on whatever your B+ is and how much filament current you have. You can probably get a little more B+ with a solid state rectifier and more filament current by replacing the power indicator lamp with a neon or LED assembly. Also I suggest using new ceramic tube sockets. They are cheap and will save you a lot of grief.

[trobbins]

Phyber, imho any old PA amp is worth tweaking in to a guitar amp if you have the interest and would enjoy the effort.

The process is not like a kit of parts and an instruction list, but you learn oh so much more with the journey. If you were keen then I suggest you need to assess what you've got, what you can tweak, and then do a restoration. Starting with a clean amp that doesn't immediately need de-rusting and new hardware is a bonus, as it then mainly involves circuit changes and safety.

There are many forum threads on each and every topic you are likely to come across, such as OT testing. And a few articles are around on restoration such as:
https://www.dalmura.com.au/static/Re...0PA%20amps.pdf

As more people restore old PA amps, then more details are easier to come by (such as adding photos and links and test results to a forum thread like this) - google will be the friend of many restorers in years to come.

Ciao, Tim

[Enzo]

Old PA systems are amps in a box. I don't expect it to sound good, what I expect from it is a partly built platform for my own circuits.

The typical push pull power stage doesn't need any redesign. I might change a few parts, but for most of us the power stage is prewired and ready to go. The preamp is what we change. We might tweak the existing circuit, or rip it out and put in our own preamp design. In any case, the tube heaters are already wired, and the B+ circuit is ready to go.

I can do the exact same thing by ripping into say an old Traynor head, or any old tube head. I see little difference.

[Phyberoptic]

Old PA systems are amps in a box. I don't expect it to sound good, what I expect from it is a partly built platform for my own circuits.

The typical push pull power stage doesn't need any redesign. I might change a few parts, but for most of us the power stage is prewired and ready to go. The preamp is what we change. We might tweak the existing circuit, or rip it out and put in our own preamp design. In any case, the tube heaters are already wired, and the B+ circuit is ready to go.

I can do the exact same thing by ripping into say an old Traynor head, or any old tube head. I see little difference.

Yes, that's likely the route I'll take. I have a order in for the caps I need and will be cleaning up any burnt or shorted resistors and wires. I'll first restore it to stock and see what it sounds like, then redo the preamps circuits to taste.

[Chuck H]

I wouldn't even trouble with restoring the original circuit and then later trying to touch up the tone with circuit changes. What you'll end up with is an amp with no known, popular mods to research, an odd, possibly prohibitive layout for a high gain amp (which guitar amps typically are, even vintage amps) that uses old tubes that are often only available used on *bay. Blech!

You can expect to replace ALL the electrolytic caps anyway. You can expect to replace some of the film caps and drifted resistors as well. Why put time into the existing design only to have it backing into a corner later when you want to change it? You have a PT, an OT, a cool looking box and whatever pots and switches might be useful in a redesign. Build yourself a guitar amp. You can do anything from a Deluxe (BF, non reverb or tweed type) to a Dr. Z Z28. Do that and NOW you have a platform you can start on.

OTOH I read that Terry Kath (Chicago-25 or 6 to 4) used Bogen amps often. Even on stage. Modified with little more than 1/4" ins and outs. So YMMV

[Handlebar]

Dr. Gonz, I just took in a CH18 for hum. No work done yet. Anticipating under-board can replacement, then go hunting with my scope on its isolation transformer. I saw your post about a non-original drawing on 9-27-17 and "Attached Files" but it's not clickable.
I'd be grateful if you could email me this, or repost somewhere from whence it could be downloaded.
Many thanks,
Barney KL7HNY

[g1]

Here it is:

[DrGonz78]

Weird thing is that when I go down to that original post and click it, the file downloads correctly on my computer.

[g1]

Weird thing is that when I go down to that original post and click it, the file downloads correctly on my computer.

Hmm. Me too. Not sure if I tried the old one before I resized and re-posted. I thought I did.