Preamp jc 120

[alltrax74]

I replaced the J201 with the 2sk30 i got in my mailbox yesterday, the diffence is quite important i like it much better now.
It works very well at 9v but more headroom would be nice so i think it's worth addind a charge pump

[moda]

Hey Alltrax, good to know. Much appreciated. I have changed my mind from the tiny terror to a couple of boards from ebay. Chip is a td2030a. They used them in the old solid state 10w marshalls.
Whole board off ebay is about 6 euros. Bought a couple, see how it goes....

[moda]

Also runs off 19v. Hopefully this will give me enough headroom

[alltrax74]

Got 1 in my mail box today, I will give it a go with the JC120 in front

[moda]

Look forward to hearing your results. My boards dont get here until 3 weeks.
I built a couple of glam chourus boards. One is boxed that i love. Might pop the other in with the preamp. Bunch of different voltages going on but should be able to make it work!

[alltrax74]

Well I plugged it yesterday and was a bit disapointed, this board sounds way too dark for my taste

[Neil mcNasty]

If you replace the two 1.5nf with 470pf it should get brighter.
The other JFet variation of the JC120 has smaller value there (edit: HIGHER value actually... 3.3nf...)
There is also two versions that uses opamps instead of JFets...

Regarding it being dark in tone: it is a Jazz amp, so that is somewhat expected, even though I do remember it to be quite open and clear in the highs as well...
Maybe the Power Amp section comes into play here and brightens things up...?

[moda]

Well i got my little 18w power amp boards from ebay today. Hopefully be able to give a second opinion on this preamp in a few days.

[spacefuzz74]

Anyone have any updates on what JFET they like best for this?

I found some 2SK117GR's from various sources, but I can't tell if they are legit.  Has anyone had any issues with them?

In this thread 2sk30's are also mentioned...any info is appreciated!

[Sensei Tim]

Has anyone used anything other than the 15k resistors to boss the jfets?

[Sensei Tim]

Ok so this is the second version of this preamp I’ve built and I’ve had nothing but problems.

I get this whining buzz in the background.  

I’ve tried origina transistors as well as j201

I’ve added trimmers innplace of the 15k resistors

Q1 drain sounds normal.  After that the tone of my 1k test tone changes...like there’s some kind off other tone superimposed.  The tone stack sounds good, but the signal coming from vol 2 is super quiet.  

[Sensei Tim]

had some downtime today and did a layout with trimmers instead of fixed bias resistors.  I think everything checks out and should be identical to Neil's original layout.

[spacefuzz74]

<quote author="Sensei Tim">
had some downtime today and did a layout with trimmers instead of fixed bias resistors.  I think everything checks out and should be identical to Neil's original layout.

ST,

With 24V input, what voltages should I be looking for when biasing the JFETS?  Thanks!

[Sensei Tim]

No idea, it depends on the transistors.  I'm guessing somewhere around 1/2 of the supply voltage is a good start?

[Kirkus]

Do i need to connect the pots too in order to make it work folks? Cause the circuit doesn't work atm and i'm troubleshooting it. Please help!!

[Blackboarcult]

Aye, won't work with pots unconnected. Wire it all as per layout

[Kirkus]

A life saver my friend!!! I was about to spend more hours on tryin to figure out why it's not working
If u remember my request for the second channel of he jc120 i decided to give this version a shot and combine it with a stereo chorus so no hurries for my request :P

[Blackboarcult]

Good to know, mate, as I'm flooded with other projects right now. I'll eventually lay it out, though ;-)

[Kirkus]

Still struggling to make it work :/ There are no unwanted bridges on the board but all i get is noise and it's driving me crazy!!
Not even using a bypass switch so i have less room for error.
Ground to dc jack and input jack
Input and output to the corresponding jacks
All pots connected as per layout including the bright and the hi lo gain switch
Please help!

[Sensei Tim]

which layout are you using?