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Practice Amp/Bogen Challenger Restoration Tube Amp Build

Thought that I would start a thread on a custom low watt practice tube amp build that I have in progress with my guitarist Scott (aka Roctal-Amps on Talkbass). He's the one that built my large custom tube amp as you can see here.

http://www.talkbass.com/forum/f15/custom-tube-amp-build-thread-736187/



Here is a little history behind the build. It all started from an ad that I ran across on craigslist for an old Unicord/Univox solid state bass amp. Here is a link to the same amp that I bought when the seller was trying to identify it.

http://www.talkbass.com/forum/f15/need-help-identifying-amp-872589/



The seller also threw in an old beat up Bogen Challenger CHA-33 PA tube amp. I knew it need some work but figured that Scott would be into it so I picked it up as well.

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Well needless to say after getting it home and opening it up we quickly determined that it would not be worth saving. As you can see in the pictures below it had a pretty serious overheating problem in the past.

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So we came up with the idea of building a small practice bass tube amp using this chassis. We are shooting for something in the 20 - 30 watts range with a CD/MP3 player input. It is also going to be designed to handle the output of a low B-string as well. The idea is to keep the vintage look of the Bogen Challenger chassis but to use a completely different circuit based on Scott's layout with Octal tubes (I'll let him comment on all the technical details). All of the parts will be coming from his collection of "junk" that he has picked up over the years of working on Tube Amps.



Here is the chassis all stripped down

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Starting to get some new parts put into it and some fresh paint on the chassis. The layout of the knobs is going to be different compared to the old layout so that's why we painted over the existing labels.

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That's all I have for now. I'll keep updating this post as I get more pics and info.
 
Both the original transformers were trashed so they are getting upgraded. The output transformer is a Hammond 16xx series (20-watt, 6.6K) and the power transformer is from old HP signal generator. It is huge but it is the only one I had would fit the cut out in the chassis. The output stage should make around 25 watts or so with cathode biasing. The output transformer is rated 20 watts with a bandwidth of 30hz to 20khz for home audio use, probably around 30 watts ro more for musical instrument duty.

The basic circuit is

Bass In -> 6sl7 \
6sl7->6J5 (phase inverter) ->6l6GC PP
Line In -> 6sl7 /

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