Preamp jc 120

[Sensei Tim]

Unfortunately I don’t have a circuit available to test - and even if I did I made mine to use j201 jfets.

On Sun, Aug 11, 2019 at 8:12 AM Kirkus [via Guitar FX Layouts] <[hidden email]> wrote:

Sensei Tim if you could please share the voltages your transistors are biased at, it would really help me get an idea...

---

If you reply to this email, your message will be added to the discussion below:

http://guitar-fx-layouts.42897.x6.nabble.com/Preamp-jc-120-tp39378p47118.html

To unsubscribe from Preamp jc 120, click here.
NAML

--

[Kirkus]

Using a 31.5v power supply my readings are:
Q1: 19.7
Q2: 31.5
Q3: 19.9 (using 12k resistor instead of 15k)
Q4: 19 (again using 12k resistor)

Note that I used a 50k pot too for Q3 and Q4 and as I was sweeping, it made little difference to the volume before the signal was killed altogether, but I found the peak of the volume to be at 12k

I think it's safe to assume that the transistors are biased not that bad but I would like to remind you that they are not the 2sk117GR but the 2sk117BL with the only difference being the IDSS (no idea what this is) for example the GR is from 2 to 6.5mA while the bl is from 6 to 14.5 mA. Was told that it doesn't matter tho

[metall0r]

Hi,

i've built this one successfully and it sounds very good, thank you very much.

There's one thing i can not follow when having a look at the original schematic and your conversion. There is a 100nf ceramic and a 100uf electrolytic on row A and B between +30V/Ground, but i do not really understand why, as they're not part of the original schematic.

Could you please explain this to me? Would be nice to understand it fully.

Thank you in advance,
metall0r

[Blackboarcult]

They're decoupling caps to stabilize voltage and prevent ripple. If you have a good PSU, you can leave them out, but it's always good to have'em just in case...

[metall0r]

Got it, thank you very much for the quick reply.

[Blackboarcult]

Anytime

[Kirkus]

Anyone else gets clipping when going above 9-10 o'clock on the volume knob?

[Blackboarcult]

What voltage are you running it at? And what jfets are you using?

[Kirkus]

Using a 31.5v power supply and 2sk117BLs as I couldn't find the GRs

[metall0r]

I bought mine from a german store on ebay. They are shipping worldwide.

 

 
<https://www.ebay.de/itm/2SK117-GR-Japan-Transistor/310844793388?hash=item48
5fcb222c:m:m7rXE5nd0mcEO3bdPZfxEUg>

 

I don't have any issues with my boards. I'm using a Meanwell LRS-100-24 PSU.
Works fantastic.

 

Hope this helps

 

 

[Blackboarcult]

With that Voltage and those Jfets you should be Ok...
Just thinking, this circuit has no Gain-MV configuration, just a Vol control. Check if you are not overdriving the input of your poweramp, as this thing delivers a f**kton of clean output.

[Kirkus]

Don't get me wrong...it's loud even at 9 o'clock  I must crank my dr boogie and it's  black eye buffer/booster close to 3 o'clock  to match the jc120 volume...it just starts to clip beyond 10 o'clock

[Sensei Tim]

Are you using trimmers vs resistors to vias your jfets?

On Mon, Oct 14, 2019 at 9:21 AM Kirkus [via Guitar FX Layouts] <[hidden email]> wrote:

Don't get me wrong...it's loud even at 9 o'clock  I must crank my dr boogie and it's  black eye buffer/booster close to 3 o'clock  to match the jc120 volume...it just starts to clip beyond 10 o'clock

---

If you reply to this email, your message will be added to the discussion below:

http://guitar-fx-layouts.42897.x6.nabble.com/Preamp-jc-120-tp39378p47580.html

To unsubscribe from Preamp jc 120, click here.
NAML

--

[Kirkus]

I'm using the 15k resistors as per original schematic which should work as it's a copy and not modified in any way...Shouldn't it?

[Blackboarcult]

As Tim says! Check the drains to see if they're at half voltage (ideally), too high or too low (lower or increase biasing resistors)

[Kirkus]

Ok, I did some measuring today as instructed and my readings are Q1 19.8 volts Q3 16.2 and Q4 16.7
I believe I need to increase the value of the resistor going to Q1 as it might be driving the rest of the board too hard? PS is 31.5 volts
Thanks for all the help folks!

Blackboarcult wrote

As Tim says! Check the drains to see if they're at half voltage (ideally), too high or too low (lower or increase biasing resistors)

[Sensei Tim]

comments from August.  did you ever try re-biasing your circuit before you started swapping out transistors?

Sensei Tim wrote

I built the original circuit and had to swap drain resistors to get it to
work properly, so it's “verified”

Assuming that you don’t have any errors in your circuit the most common
things would be either 1) transistors aren’t biased properly or 2) the
transistors are fake/out of spec.

[Kirkus]

Hey Tim
I didn't swap transistors i just never found GRs and used BLs instead as i was told that it would make no difference...
I think i'll just throw it away and try and make a layout out of the schematic and have the pcb fabricated as i'm also having issues with the bass knob... I removed the bass pot and tested it with a multimeter and it works fine but when connected it works the other way around for some reason and not properly...Treble at zero Mid at zero bass to the maximum and I get no sound like if i had everything down to zero...

Sensei Tim wrote

comments from August.  did you ever try re-biasing your circuit before you started swapping out transistors?

Sensei Tim wrote

I built the original circuit and had to swap drain resistors to get it to
work properly, so it's “verified”

Assuming that you don’t have any errors in your circuit the most common
things would be either 1) transistors aren’t biased properly or 2) the
transistors are fake/out of spec.

[Willybomb]

I've built the first layout, voltages are pretty good (one is a little higher but not so much so), using 2sk30 iirc.

My volume control works like a distortion knob.  A couple of degrees turn and it sounds like a Blues Driver or something.  Running off 30v.

[solderholic_JPN]

Hello yiyorebel .
I made Preamp JC120→fortin33 by Neil mcNasty's layout of your thread.
It's cool sound!!
Thank you for your idea and Neil mcNasty's layout .