New member working on a Bogen Challenger CH-8 upgrade/overhaul

[BrokeGunner]

Hi all,

I'm new here and fairly new to guitar amplifiers. I have been into ham radio for over 40 years (I'm 58) and my main hobby is restoring old tube receivers and transmitters. The plan is to repair them and then use them on the air for a while and find them a new home but somehow I don't sell many and I keep stacking the pile of radios higher and higher ;)

I have been interested in getting some experience with guitar amps so I bought a MOD 102+ kit and I built that up last year for my brother who is a guitar player. That went well so then I found this Bogen CH8 and bought it with no tubes and not working for $100. I had all the tubes for it in my stash so I replaced the dripping wax caps and installed a 3 wire cord and got it working. The original line up is a 6AV6 on the phono input, a 6SJ7 on the PA input and a 6L6 PT with a 5Y3. I didn't bother hooking up the 6AV6; I was just running the 6SJ7 preamp with the original tone control which is a 4 position switch. I had a friend play it and he liked it but it didn't have much volume and we just kept the tone switch in one position that sounded best.



My plan was kind of like an old car; getting it running on the original points ignition and the 1 BBL carb and then once it is running and I know the bottom end is good I will install some performance parts and get it running right. So that's what I am doing now, getting some better performance.

I have studied a lot of schematics and I like the 5F2A circuit but I am hoping to make 2 changes:

1. I would like to include the tone circuit (should I call it a tone stack?) from the Champ AA764.

2. I am planning to include the 6SJ7 as a second preamp.

I have already opened up the amp and gutted it. I removed the 7 pin for the 6AV6 and installed a 9 pin for the 12AX7 (I have a bunch of 12AT7 if that is ok as 1st preamp with the 6SJ7 as second preamp).

I have already installed the dual input jacks and wired that up, and replaced all of the heater wiring because it was not twisted originally. I have a HUGE supply of Allen Bradley Type J pots so I installed a pair of 250K for the two tone controls and then a 1M pot for volume.

My question at this point here is just a sanity check on my plan. I have no experience with gain levels so I don't know if having 2 stages of a 12AX7 feeding into a 6SJ7 will be too much? I've read https://el34world.com/Forum/index.php?topic=27752.0 dozens of times and it seems to be describing the same circuit I am working on.

On the AA764 schematic the volume pot is between the tone stack and the 2nd half of the 12AX7. On the Bogen schematic it is before the grid on the 6L6 so I don't know where is the best place to adjust volume?

I hope I didn't type too much for my first post!! The photos attached are of the CH8 after I got it running based on the original design. I didn't have a can cap so I disconnected the can and used a terminal strip to mount discrete radial caps.

Thanks,

Brian in NorCal

[Platefire]

Hi Brian
Welcome! Nice looking Bogen there. Looks like you got a good start replacing that 6AV6 with a 12AX7. Never had a amp with a 6SJ7, so I can't tell you much about them. I was big into converting PA amps, reel to reel's and mono Hi Fi amps to guitar. Kind of slowed down because like you, they stacking high toward the ceiling:>)   I just finished a Voice of Music 8810 amp converting to a 5F2A as of this afternoon. This I rebuilt for a guy in Parma OH. At least I won't have to add it to my stack after I ship it. You will find plenty of help here. I've been doing this for24 years and still need plenty of help. Platefire

[Platefire]

Just thinking, I think you could make a 6L6 Champ out of it using the AA174 design as a pattern. Not sure how the 6SJ7 could fit in that picture because basic Champ design just has one 12AX7, One 6V6 and in your case a 6L6 and 5Y3 rectifier. You have pot location holes in chassis for the Vol/Treble/Bass and leave the last knob as is for on/off switch. You would probably have to gut a lot of the existing wiring/components you presently have in there to build it. Just a thought!

[BrokeGunner]

Just thinking, I think you could make a 6L6 Champ out of it using the AA174 design as a pattern. Not sure how the 6SJ7 could fit in that picture because basic Champ design just has one 12AX7, One 6V6 and in your case a 6L6 and 5Y3 rectifier. You have pot location holes in chassis for the Vol/Treble/Bass and leave the last knob as is for on/off switch. You would probably have to gut a lot of the existing wiring/components you presently have in there to build it. Just a thought!

I'm thinking the "AA174" is a typo as I can't find anything on it, other than an Alaska Airlines flight lol

I did mention the AA764 as one that I am looking at, so yes I agree the Champ has been helpful to study and in the thread I linked to in my original post, there were some replies with sample schematics suggesting this can be done. I'm going to see what I can get going and will report back on my progress. Thanks!!

[Platefire]

Yes, my mistake! AA764 is correct

[AlNewman]

You may want to set up your power section and see what types of voltages you get on your 6l6, and that could help determine what you can do with your pre amp.