Preamp jc 120

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Preamp jc 120

yiyorebel
Hello everyone, I needed to know if it is possible to do this preamp (schematic attachment), the idea is in pedal format, the schematic corresponds to the clean channel, I love the clean sound of this amplifier, but I do not have the money to buy one.
I have seen in the forum that they have made several preamp, like the bassman, sunn, orange tinny terror, but I have not seen the ac-30. Well returning to my request, I hope you can do this poreamp JC 120.
Best regards, thank you very much
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Re: Preamp jc 120

Neil mcNasty
This post was updated on .
Just looked over this layout one last time and think it is safe to build.
It looks correct according to the schematic above, but let me know if you spot any errors, and I will correct them.

I've added a switch at the input (Labeled: "Gain SW") so that you can toggle between "High" and "Low" input settings, instead of having two input jacks.

I have not had the time to build and verify it yet, but I will definitely build this and trying to add it to my Roland Bolt 60 amp that is lacking the whole preamp/reverb section.

Good luck with your build!
Looking forward to hear about how it turns out.
Cheers!

EDIT: Shaved off one row & updated layout to match transistor orientation. Watch pinout if using optional JFets...! Also added missing info: "Volume 1 to Ground"

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Re: Preamp jc 120

yiyorebel
Wow Neil, thank you very much, I'm going to get down to work and see how it turns out, I'll be reporting any details.
regards
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Re: Preamp jc 120

Sphere80
Thanks, Neil! And thanks,  yiyorebel  for  proposing this. It would be great to have  one of the classic clean amps available for recording.

What would the best, or more accurate use be? as a tone coloring pedal, or using it truly as a preamp, direct into a (solid state?) power amp?  Are there any level /impedence issues to  be concerned with , or just plug n play?

 And thinking ahead, what chorus pedal would one want to complete the classic sound?  
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Re: Preamp jc 120

Neil mcNasty
Since this layout is identical to the actual preamp section from the amp, it should work fine running it directly into the power amp section (that's the way I intend to use it).

If you are building it as a pedal, you can replace the 100K resistor going to ground at the output, with a 100K potentiometer, so that you get a Master Volume control.
I guess that the Volume control has it's sweet spots, as it is pushing the two last gain-stages/transistors, and therefore it can also be regarded as a Gain control.

Regarding which Chorus pedal?: Sorry don't have a clue, but it could be interesting to have a look at the schematic, and see if the Chorus could be turned into a pedal as well...
I'll have a look, but I can not promise that I'm the right guy for the task, since I'm mainly used to make veros of simpler stuff like preamps and dirt boxes...
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Re: Preamp jc 120

Sphere80
Seems the chorus section was similar to the  Boss CE-1.  I have a digital model of one in one of my  multi FX things.  I would experiment with anything available , of course, and was partly joking since the chorus was known to be part of the appeal.

Are there more easily available substitutes for the 2sk117 ?
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Re: Preamp jc 120

yiyorebel
Hello Sphere80, I'm going to use the 2n3819, you have to turn them 180 degrees, the circuit is powered by 30v, will have to change some component if it is fed with less voltage ?.
I have a preamp twin reverb that uses 2n3819 and I use it up with 9v, of course 12v walks better. Maybe this jc120 works well with the 2n3819 and 12v. Just finish it and try to give details.
Best regards
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Re: Preamp jc 120

Royboy
In reply to this post by Neil mcNasty
This preamp could be just what I'm looking for but doesit need 30 volts to power it. Could I use 18 volts
so that I can use 9 volts and one of Mad Bean's charge pumps to give 18 volts
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Re: Preamp jc 120

Neil mcNasty
It will probably work fine on 18v as well.
But with less headroom...
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Re: Preamp jc 120

alltrax74
I guess as usual those 15k resistors from power supply to drain are worth socketing for correctly biasing the fets
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Re: Preamp jc 120

Neil mcNasty
Yes! Good idea to socket the 15k's for biasing...
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Re: Preamp jc 120

Royboy
In reply to this post by Neil mcNasty
I have another problem. I am 73 years old so I have a lot of problems.
There is a portable guitar preamp in Instructables which is a Blackface Preamp combined with an ROG Tonemender. I have been trying to convert it onto veroboard  but I am having trouble
Can you help me please?



I tried to insert an image but this is all I got
I think i have been nobbled by nabble
Us old blokes in Australia are behind the rest of the world
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Re: Preamp jc 120

alltrax74
In reply to this post by Neil mcNasty
I built it yesterday, it seems to work, I didn't take time to really test it it was late.
I replaced the output 100k resistor with a MV and used J201 (G & S legs need to be twisted it's not the same pinout as the 2sk).
One thing : volume pot does not seem to have any effect, but i'll have to check that back. I used only 1 input and socketed the resistor, I only tried the 33k so far.
I will spend more time on it this week-end and will report back
Cheers

Royboy sorry I can't help you I suck at drawing layouts. You should create your own request
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Re: Preamp jc 120

alltrax74
2sk30A seem to be an accurate replacement, same pinout and easy to source
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Re: Preamp jc 120

Neil mcNasty
In reply to this post by Royboy
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Re: Preamp jc 120

alltrax74
In reply to this post by alltrax74
I played a little more through it
Bass pot is backward and I confirm volume pot has no really audible effect.
As it it, it sounds clean / limit breakup but not in a very musical way, a bit harsh (68k input)
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Re: Preamp jc 120

alltrax74
Volume 1 to ground was missing, verified
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Re: Preamp jc 120

Neil mcNasty
I see now that I forgot to add info about Volume 1 going to ground...
Sorry about that!
Layout has been updated!
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Re: Preamp jc 120

moda
Might put this in front of the tiny giant.....
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Re: Preamp jc 120

alltrax74
I'm thinking of building a serie of amps based on the Tiny giant power section
The range could be :
- Blackface (black 65 or Jan Ray)
- Plexi (Barber direct drive)
- Brown sound (BE-OD)
- Modern High Gain (Baber dirty bomb, slo100, Ubershall...)
...
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