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Old 20th August 2009, 01:09 PM   #1
thehoj is offline thehoj  Canada
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Default Converting Bogen chb-35a to more guitar friendly amp (newbie inside)

I've got a CHB-35A bogen PA amplifier, and I'm using it at the moment with my guitar, running into one of the aux inputs.

Here's a schematic of the amp:
http://schematicheaven.com/hifiamps/bogen_chb35a.pdf

As you can see from the schematic I'm basically bypassing v1 all together. Additionally I don't feel like I have proper control over the tone for this amp, as I'm sure it's set up for the vocal range.

It does sound pretty nice, but I feel like I could do so much more with it.

It was recommended to me recently to replace all the filter caps as that could be affecting tone quite a bit, so I'll be doing that shortly. In addition though I'm tempted to try rewiring and changing some components to utilize the first gain stage, and also have more control over my tone.

I found some basic schematics for a gain stage and tmb control on weber's site, and just used that in conjunction with the bogen schematics. I figured I would rewire v1 as a 12ax7 since they're so much more common, and if I'm doing some rewiring anyways, it makes sense.

Anyways, here's what I came up with. Would this make sense? Am I missing anything with this idea?
http://vwtweaked.ca/images/bogen%20modified%20idea.jpg

I'm just looking at it again, and I probably shouldn't have that line coming from the green wire on the output transformer anymore should I.. I would probably disconnect any of the other unnecessary connections from that output transformer as well.

It was also suggested to me to make my conversion schematic based on the ampeg sb-12 7868 because of them having the same power tubes.
Does that actually matter for the preamp section? Is the preamp section I've come up with a decent one, or should I come up with a modified schematic based on the sb-12?

Thanks for any help. I'm somewhat of a newbie to amps, but I have experience in soldering, and some electrical training (from years ago) so I have a basic understanding of the components in this amp, and how they interact with eachother. I don't however have any real experience with tubes.
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Old 20th August 2009, 05:15 PM   #2
TheGimp is offline TheGimp  United States
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I'd add an input resistor of at least 47K into the screen of the first tube.

Also, you could rewire the tone section like a tonestack from Fender, Marshall or which ever amps you like the best.

Here is a late model fender schematic where they had standardized on their tonestack. I think it varied a good bit till the mid or late 60s.

http://www.schematicheaven.com/fende...ar_pro_rev.pdf

This is what I'm doing to a Tranor YVM-1 for a friend.

Replace all the filter caps with 500V ones since I measured 465V on the first one.
Add a bleader resistor (270K 2W) on the caps to discharge them when power is removed. SAFETY ISSUE.
Replace any other Al caps that I find at the same time.
Replace one of the input caps with 68K resistors (CC 1/2W).
I'll let him try it after replacing the caps and converting one input channel. When he agrees, I'll convert the rest.
Then I'm rewiring the tone controls with a standard Fender ToneStack.
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Old 20th August 2009, 09:54 PM   #3
thehoj is offline thehoj  Canada
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If I do decide to somewhat replicate the ampeg ( http://www.schematicheaven.com/ampeg..._portaflex.pdf ) does this schematic make sense? I attempted to modify the chb schematic slightly to make the tone controls similar to the sb-12 on v1, also convert v1 to a 12ax7, and then to v2 and still keep v3 feeding into the power tubes.

http://vwtweaked.ca/images/bogen%20a...onv%20idea.jpg

Does my schematic make sense? Is it valid? Did I do anything wrong here?
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Old 21st August 2009, 08:05 PM   #4
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Good project!
You have probably already come across these links:
http://www.schematicheaven.com/hifi.htm
http://www.netads.com/~meo/Guitar/Amps/PA2Guitar/
http://www.vacuumtubecollective.com/...rttoguitar.pdf
I built a Portaflex B-15 clone from a Bogen and it worked well.
John
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Old 21st August 2009, 08:06 PM   #5
thehoj is offline thehoj  Canada
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Hadn't seen the last link before, thanks!
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Old 21st August 2009, 08:59 PM   #6
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First of all I would rewire a phase splitter, to more guitar-friendly one, like in last Fender tube amps.
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Old 21st August 2009, 09:57 PM   #7
thehoj is offline thehoj  Canada
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Please forgive my lack of knowledge in this area,
I did some research on the phase splitter, and I'm not exactly sure how/where this would be implemented in the original bogen circuit..
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Old 22nd August 2009, 02:54 PM   #8
thehoj is offline thehoj  Canada
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So I've come up with a new schematic (that should be a bit more simple) that works with the mic 1 input I've already got, and uses the 6eu7 that's there. But I change caps, and rewire the tone controls between v2a and v2b to match the ampeg.

In the schematic I'm linking, I've outlined the caps that I will change the value of and replace with new caps in red (do these new values appear to be more targeted towards guitar tone?), I've outlined other caps in green which I will simply replace but keep the same value, and then I've outlined some sections in purple that I'm slightly confused about (should the pots be 1M?), I don't quite understand what they do, and whether I should replace those or not, and what kind of bearing they have on tone, or overall operation of the amp.
Should I have the 150 cap before the mic1 volume? What does that do?

Also, the schematic says that all caps in whole values are MMFD.. Is this the same a pF?

Am I cascading v1a into v1b properly? And is it okay to leave the connections to REM #1 and #2 disconnected? I don't understand what these were for..

Finally, how much of a bearing do resistor values have on tone?

Any suggestions/input on this schematic are appreciated, as in, are the adjustments I'm suggesting more targeted toward guitar tone? I don't have any electrical components right now, so I need to make an order for parts, and I'd rather not get a whole ton of extra parts that I won't need in the end.

http://vwtweaked.ca/images/bogen%20s...mod%20rev2.jpg

Last edited by thehoj; 23rd August 2009 at 06:41 PM.
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Old 23rd August 2009, 07:13 PM   #9
thehoj is offline thehoj  Canada
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I'd like to figure out if technically I've cascaded all the gain stages together correctly, and just make sure that I haven't done anything I shouldn't have,
But I'm also struggling with a different issue now..

I'm trying to determine whether I've got the correct values for my tone controls.
I'm coming around to the idea that I might like TMB controls, and so the classic fender tone controls are looking interesting to me.

http://vwtweaked.ca/images/bogen%20s...mod%20rev3.jpg

I've been playing around with a tone stack calculator, and it bases component values on your source impedance to that circuit. How much of the prior circuitry play into that source impedance value?

I understand that I need to include capacitor reactance and resistor resistance in this impedance value, but does all of the prior circuitry up to that point play into that value (tubes included)?
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