Hi, I just acquired a simple chassis that has a SE setup, one 12ax7 and one 7868. It has the OT & PT as well. Its a Bogen 10A unit. Just wondering if there are any schematics floating around for a Champ or SE build that uses the 7868 tube. I fired it up as is and it works but its strange as you can imagine with the way its currently setup. Plan is to pull out the components and make a little ' Eagle ' type amp.
Cheers Ange
7868 Champ project
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Stevem
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Re: 7868 Champ project
You should not need to do anything to the output stage except replace the cathode bypass cap due to its age, or add a 50 to 100 uf cap @ 50 volts if it does not have one, then you can just employ the preamp circuit for whatever Champ you want!
You could also change the output tube socket over to a octal type and use a 6l6gc tube, but any other output tube will pull too much heater current.
You could also change the output tube socket over to a octal type and use a 6l6gc tube, but any other output tube will pull too much heater current.
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: 7868 Champ project
7868 is just a compaction 7591 which is often rewired and sub'd with 6L6s which depending on the voltage is often swapped with 6V6s, so go for it. Ampeg used the 7868/7591 in guitar amps so you can check some of their schematics but they never used those in an SE.
Not sure what the eagle looks like but the secret to low powered amps IMO is KISS - one vol knob is actually all you need, in a 5C1 you can even skip the vol pot
A tone can be skipped like a 5F1 but if you insist a pot simply grounding out treble wont hurt much like a 5F2A, but you'll never use it. If the eagle has a tone stack you might want to consider simplifying it, but then maybe the eagle kills as is so I can't really say.
IIRC the 7591 is easier to drive than a 6L6 so you might have to tweak. Maybe a AC4 circuit might work for it. It can also handle more voltage than a 6V6 or EL84 but I think the eagle was made for 6L6/6550/KT88 so even better, but your plate voltage will depend on your PT if you're keeping it. The BF Champ power string is probably better than a 5F1 if your are running it hotter.
Not sure what the eagle looks like but the secret to low powered amps IMO is KISS - one vol knob is actually all you need, in a 5C1 you can even skip the vol pot
A tone can be skipped like a 5F1 but if you insist a pot simply grounding out treble wont hurt much like a 5F2A, but you'll never use it. If the eagle has a tone stack you might want to consider simplifying it, but then maybe the eagle kills as is so I can't really say.IIRC the 7591 is easier to drive than a 6L6 so you might have to tweak. Maybe a AC4 circuit might work for it. It can also handle more voltage than a 6V6 or EL84 but I think the eagle was made for 6L6/6550/KT88 so even better, but your plate voltage will depend on your PT if you're keeping it. The BF Champ power string is probably better than a 5F1 if your are running it hotter.
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potatofarmer
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Re: 7868 Champ project
Is it a Challenger CHB-10A?
If so, the schematic can be found here: http://makearadio.com/schematics/images ... b10a-4.jpg
Or other Bogen schematics here:
http://makearadio.com/schematics/
The preamp is weird, at least for guitar world. Grid-leak biased first stage, fixed-bias second stage... the power amp looks fine though.
IMO, put any sort of preamp in it you'd like and it should have no trouble driving the 7868. They're a little harder to drive than 6V6s but easier than 6L6s.
Gotta love that Bogen voltage doubler power supply.
If so, the schematic can be found here: http://makearadio.com/schematics/images ... b10a-4.jpg
Or other Bogen schematics here:
http://makearadio.com/schematics/
The preamp is weird, at least for guitar world. Grid-leak biased first stage, fixed-bias second stage... the power amp looks fine though.
IMO, put any sort of preamp in it you'd like and it should have no trouble driving the 7868. They're a little harder to drive than 6V6s but easier than 6L6s.
Gotta love that Bogen voltage doubler power supply.