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Conversion Tube PA and Bass Tone
Conversion Tube PA and Bass Tone
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Old 25th November 2008, 06:35 AM   #1
bluehevy75 is offline bluehevy75  United States
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Join Date: Nov 2008
Default Conversion Tube PA and Bass Tone


I'm a first time poster and I hope this is the right place.

I recently bought an old tube PA amp. It has been converted with 1/4" input jacks and 1/4" output jacks. It is about 30 watts and uses a pair of 6L6G's.

My goal is to use it for bass. I know this is a little unusual I'm going to run it as one half of a bi-amp rig.

I will run my lows through an Ampeg B-15 and run my highs through this old tube PA head and a 2x12 cab I've got.

I had some questions about the controls. There is a single tone not that switches to four different settings. I was wondering if I could make some mods and change the sound of those settings.

I've included some pics. The tone control is there on the left/center side of the pic.

My main goal, I guess, is to cut out some bass without it getting crazy bright. I know that tone preferences are pretty subjective but with an amp this old and a switch like that I don't know where to start.

Anybody want to take a look and offer explanations/suggestions.


PS I was going to attach more pics but it doesn't seem to let me attach more than one.
Attached Images
File Type: jpg challenger 30 003.jpg (66.0 KB, 149 views)
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Old 25th November 2008, 06:27 PM   #2
Richard Ellis is offline Richard Ellis  Argentina
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Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
Hi Ya "bluehevy75" & welcome to the forums..
See if you can find a brand and or a model number somewhere on the chassis. With this info we all here can move forward and dig up a schematic.
From this schematic we can modify the circuitry all different ways.
Something I did see was someone has been inside & replaced some caps, also there appears to be a roasted cap in there (The blistered component).
____________________________________________Rick.. .....
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Old 25th November 2008, 07:31 PM   #3
bluehevy75 is offline bluehevy75  United States
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Join Date: Nov 2008
Thanks for the reply rick.

The brand is Challenger Amplifier Co. After doing some searching online it looks either to be a very early Bogen or a Bogen "spin-off" as the amp has many similar parts to a Bogen.

The model is CH30.

Random discussion boards think the amp is from the late 40's or early 50's. But there isn't a lot of info about it and no schematic taped to the bottom.

I had a fellow teacher (engineering) where I work look at it and he thought that it was just wax on that one cap but I'm definitely open to replacing it.

I know the guy before me put in 1/4" jacks (input and output) and beefed up the output transformer and grounded the plug. I don't know what else he did. I sent him an email asking but he hasn't replied yet.

Thanks for the help so far. I'm definitely down to tinker and learn. Reading schematics and making them is completely new to me so thanks for being patient. I looked for one online and couldn't find one yet.

Anyway the switch has four settings I'll describe the resistors the best I can. The capacitors I'll have to do when I get home from work.

1st Pin=1st Postion=Deep Bass
2nd Pin=2nd Position=Mellow
3rd Pin=3rd Position=Crisp (So far I like this setting best)
4th Pin=4th Position=Brilliant

1. There is a 22K resistor connecting the 1st and 4th position. That is, going from the "deep bass" to "brilliant" settings. It is curious to me that they are linked this way as the "brilliant" setting actually has a boat load of bass. Which is counter-intuitive to me as I assumed "brilliant" settings would have less bass than "crisp" seetings but that is not the case with this amp--or at least the way it is configured.

2. The first postion seems to have what is a .003 capaciter that goes to a ground.

3. The second position has a 65k resistor running to a gound.

4. The second position also has a capaciter that runs to a ground but I need to check it out when I get home.

5. The third position has a 68k resister running to a ground.

6. The fourth position has that big gnarly cap running to a tube.

I'd love any ideas but will try to do something more helpful when I get home tonight.

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Old 28th November 2008, 05:35 PM   #4
bluehevy75 is offline bluehevy75  United States
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Things are taking me a little longer than I'd like. I'm going to try to make a schematic of the amp before the weekend is out. So far I've just done a little bit of work learning to make/read schematics.

Attached is the Baxandall circuit I'd like to make and add to my amp. I think the values for caps and resistors are good as I stole them off one of Piazza's schematics for the b-15n (1962).

I'm not positive that the values going in and leaving the circuit are correct I'll need to check them against the amp. I'm going to try to make a schematic for the amp here in the next couple of days. I'm not sure whether I should use the computer to do it or not. I used the Eagle free-ware. I know that I don't the software that well but it still took me longer than I would have liked to draw that simple circuit. I also couldn't find the ground function.

Anyway, look for more before the weekend is out.
Attached Files
File Type: pdf baxandall circuit for bass.pdf (11.9 KB, 23 views)
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Old 1st December 2008, 10:38 AM   #5
bluehevy75 is offline bluehevy75  United States
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Join Date: Nov 2008
Well here we go. I drew up a schematic as best as I could for the head. I'm not sure that anyone will actually be able to read it but I tried. I attached the values for caps/res separately because I thought they would be unclear if I included them in the schematic.

Thanks everyone for the help on the tone circuit. I have changed the values accordingly.

So I guess my questions are:
1. Can I make mods to the existing tone circuit to cut out more bass?
2. Can I drop in the Baxandall circuit for more tone control in general?
3. Do I/can I/should I mod that phono circuit to make the amp more "useful"? Because I am never going to use that channel, jack, or volume control the way it is now.

I'd be interested in hearing what other kinds of questions or comments you have. See what you think. Thanks for your patience and thanks in advance for your time.


C1=ETR MET 103K 630V
C2= ETR MET 103K 630V
C3=4.7K OHMS 5%
C4= ETR MET 103K 630V
C5= ETR MET 103K 630V
C6=20 MFD 450 WVDC
C7=10 uf 450 v
C8=40uf 500v
C10= ETR MET 104K 630V
C11=.02 MFD 600 VDC
C13=50 MFD 50 WV
C14=10uf 450v
C15=.006 MFD 600 VDC
C16=.003 MFD 600 VDC
C17=.003 MFD 600 VDC

R1=2.2k +/- 10%
R2=4.7M +/- 10%
R3=4.7M +/- 10%
R4=1.5k +/- 10%
R5=100k +/- 10%
R6=220k +/- 10%
R7=220k +/- 10%
R8=33k +/- 10%
R9=6.8k +/- 10%
R10=470k +/- 10%
R11=2.2k +/- 10%
R12=100k +/-10%
R13=470k +/-10%
R14=220k +/-10%
R16=100k +/-10%
R18=22k +/- 10%
R19=Pot value? Markings=V202C 137350
R20= Pot value? Markings=V202C 137350
R21= Pot value? Markings=V202C 137350
R22=68k +/- 10%
R23=68k +/- 10%
R24=4.7k +/- 10%
R25=220k +/- 10%
Attached Files
File Type: pdf modified challenger 30.pdf (25.3 KB, 27 views)
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