NPAD - Bogen Challenger CHB50

[W.L.Weller]

I'm starting off on the wrong foot by waiting to post photos, I know. A little googling and youtubing seems to suggest that these are pretty decent guitar amps "stock".


http://makearadio.com/schematics/index.php

Half of a 6EU7 as the input stage, a Baxandall tone stack using a 12AX7, a cathodyne phase inverter using a 6C4 and a pair of push-pull 6L6.

I'm going to order filter caps and check the coupling caps for DC leakage, as well as the rectification diodes; anything else I should check prior to firing it up? These use a voltage-doubler power supply, someone else noted that the 100μF capacitors should be the best you can get:

https://organforum.com/forums/forum...s-accessories/50213-bogen-chb-50-amp-question

<<This amp uses a voltage-doubler B+ power supply, so use the highest-quality electrolytic capacitors you can find for the 100uF caps in the doubler circuit. They get charged and discharged more on each rectification cycle than standard reservoir capacitors.>>

And a few posts from back in the mists of time here:

https://www.tdpri.com/threads/bogen-chb-50-conversiin-suggestions.366277/

https://www.tdpri.com/threads/building-a-bf-clone-from-a-bogen-pa-head.379455/

Anyway, swapping the input and output connections for 1/4" jacks is my first order of business (because I have those parts in stock.) And I don't think I have the appetite for replacing the PT, so I'm going to service it and see how it sounds "as-is." Maybe I'll try swapping the 6C4 for a 6J6 if only because I have a dozen 6J6, but I haven't looked at the pinout yet. Plus, NOS 6C4 are only a couple bucks, unlike the rest of the tubes this amp uses. Pics to come.

 

[archetype]

And I don't think I have the appetite for replacing the PT, so I'm going to service it and see how it sounds "as-is."

You may already have this in mind, but... Unless it's known to be operable, I'd check continuity on all windings of both transformers before I put a minute or a penny into anything else. If one of the transformers is dead, then it's decision time.

I'd make it run before I did any mods, IMO.

There's a Bogen CHA-33 and replacement parts in my basement, calling my name.

 

[oldlefty]

I think we had one of those that we used for our mics back in the 60s?

 

[W.L.Weller]

You may already have this in mind, but... Unless it's known to be operable, I'd check continuity on all windings of both transformers before I put a minute or a penny into anything else. If one of the transformers is dead, then it's decision time.

I had a few minutes of catastrophizing about the money I'd wasted on another pile of garbage when the continuity test didn't "beep" on the primary side of the OT, but then I thought about what an OT is, and how it works, and switched the meter to ohms.

The OT primary measures about 220 ohms, and 110 from each side to the center tap. So based on the barest of investigation, I think I'm going to plug it into a lightbulb limiter and alligator-clip in a signal & load. The electrolytic capacitors measure within tolerances and the fuse is the right value, so in my mind the worst that can happen is that I have to switch it off quickly if the limiter gets bright. I'm sure I'm overlooking other failure modes, but hearing it pass sound before spending $$ on caps and time on replacing jacks etc. is a solid bit of advice @archetype, thank you.

I think we had one of those that we used for our mics back in the 60s?

Yep, it's made for 2 mic inputs, or a mic and a turntable. How'd it sound?

 

[archetype]

I had a few minutes of catastrophizing about the money I'd wasted on another pile of garbage when the continuity test didn't "beep" on the primary side of the OT, but then I thought about what an OT is, and how it works, and switched the meter to ohms.

The OT primary measures about 220 ohms, and 110 from each side to the center tap. So based on the barest of investigation, I think I'm going to plug it into a lightbulb limiter and alligator-clip in a signal & load. The electrolytic capacitors measure within tolerances and the fuse is the right value, so in my mind the worst that can happen is that I have to switch it off quickly if the limiter gets bright. I'm sure I'm overlooking other failure modes, but hearing it pass sound before spending $$ on caps and time on replacing jacks etc. is a solid bit of advice @archetype, thank you.

Yep, it's made for 2 mic inputs, or a mic and a turntable. How'd it sound?

Groovy. Carry on.

 

[W.L.Weller]

It verks! It's verking!

Alligator-clipped in the signal generator and hooked up a speaker. Stunningly quiet for an amp that's apparently been sitting since the late 80s!
Almost wish I had a harp mic with a screw-on Amphenol connection. Time to shop for parts!

 

[oldlefty]

Congratulations! Get to it.

Yep, it's made for 2 mic inputs, or a mic and a turntable. How'd it sound?

Well, it was 55 years ago, we were in high school, and it made our voices louder. 'Nuff said.