Marshall plexi Jfet preamp

[fuzzo]

Hi guys,

I drew a Jfet preamp simulation of Marshall pelxi (which one with 2 Normal and bright chanel).




I would know your opinion about it.

the original schematic :



Also I'm looking for a sixth control  so If you have any idea let me know.


Actually I thought to make a Orange OR120 preamp with the same idea (Jfet with 24V power supply) but I don't magane to choose one and make it. So maybe you can help me

[Ripthorn]

Looks fine to me, have you considered a presence control or contour?

[fuzzo]

have you considered a presence control or contour?

yeah but I don't know how add it. The presence control plays with the power tube and there's no power tube here.  the contour control is the control that can boost, flat or scoup mids ?

[petemoore]

  Presence control comes from the feedback loop from the output section of the amplifer {IIUC.
 What is shown should provide good tone control, and clean boost.
 If it does distort, it will probably do so because a Jfet is misbiased or runs out of headroom @9Vdc. That preamp, in tube form is clean sounding.
 In fact, I rarely got a hint of distortion from my 100w Plexis.
 The only time they distorted [and sounded good] was when I blasted into 8 speakers in 2 cabinets. Otherwise the amp was incredibly loud and clean.
 The distortion was produced in the output tubes/speakers mostly I believe.

[anchovie]

A quick and easy sixth control would be to replace the 33k slope resistor in the tone stack with a 100K or 250K pot.

[BAARON]

Have you tried it out to see what it sounded like yet?  I built something similar last year and found that the bass response ended up being way too weak.

[petemoore]

  Jfets can have nice distortion when pushed to amplify non-linearly.
  Otherwise it looks like it could be a nice clean preamp.
  What V+ is of course will vary how cleanly [especially the later] stages amplify.
 

[fuzzo]

A quick and easy sixth control would be to replace the 33k slope resistor in the tone stack with a 100K or 250K pot.

why not. I change that resistor in tonestack calculator. I'll post the graph.

Have you tried it out to see what it sounded like yet?

Nope and it's why I don't magane to choose this kind of JFET preamp. I don't know what kind of sound will produce.

If it does distort, it will probably do so because a Jfet is misbiased or runs out of headroom @9Vdc. That preamp, in tube form is clean sounding.
 In fact, I rarely got a hint of distortion from my 100w Plexis.
 The only time they distorted [and sounded good] was when I blasted into 8 speakers in 2 cabinets. Otherwise the amp was incredibly loud and clean.
 The distortion was produced in the output tubes/speakers mostly I believe.

Otherwise it looks like it could be a nice clean preamp.
  What V+ is of course will vary how cleanly [especially the later] stages amplify.

Actually this projet won't be used like a distortion pedal but like a regular preamp plug into a power amp. I'll use a 24V power supply (maybe less) with a good transformer to have a better dynamic than with 9V. So I would a sound go to a dirty clean (I don't like "pure clean" sound) to a cool fat overdrive.

[petemoore]

  Dirty clean...
  It'll do some minor non-linearity anyway...Jfets can sound smooth like 1rst input tube IME.
  But could sooner run out of headroom than a preamp tube with greater supply voltage.
  Perhaps boost or attenuate frequencies near the front end....figure out what distortion levels this produces when the later transistors re-multiply, these seriesed amplification factors...multiply quickly what is input down the line, and the last or later transistor could easily be made to distort by it [BB and see, I'm gessin'].
  Tone control helps re-tame-down the signal level, the passive losses 'suck juice' out of the AC signal.
  Maybe bypass a 'middle-ish' or early gain stage to make 'clean' become dirty, bypassing the TC of course bumps up the signal to whatever is after it.
  Anyway considering the headroom Vs. power supply voltage of the preamps, considering also to tame down early stages or boost them up...depends on what kinda sounds it makes !
 

[yertle]

I think it would be cool to give both the normal and treble input a volume control (and remove the switch) so you could blend between them instead of just switching between them. Btw, I doubt that the source follower with the 100k source resistor will work, try something in the 1k-10k range, could be wrong though. It's probably also a good idea to have a recovery stage after the tonestack.

[fuzzo]

I think it would be cool to give both the normal and treble input a volume control (and remove the switch) so you could blend between them instead of just switching between them.

I thought about it , too . Instead of have two input (normal and bright) maybe I can wire a swtich for Birght/Normal/both. Or yes, two volumes to blend both.

[slideman82]

This was my first personal attemp to build some Jfet Distortion, but I never got it working properly, I mean, it broke really bad, sounded ugly... so I left it...

[CynicalMan]

This was my first personal attemp to build some Jfet Distortion, but I never got it working properly, I mean, it broke really bad, sounded ugly... so I left it...

Did you remember to adjust the trimpots for 1/2 supply voltage on the drains?
If you don't get that right, it'll sound really ugly.

[JDoyle]

At a quick glance, and leaving the whole drain-trim debate in the ether, I would completely eliminate the 470k series resistor on the gate of post-drive control JFET - all it is doing, really, is adding noise.

[brett]

+1 for ditching that 470k resistor.
Though if you are super cautious, you might put a 47k there for RFI protection.

[slideman82]

This was my first personal attemp to build some Jfet Distortion, but I never got it working properly, I mean, it broke really bad, sounded ugly... so I left it...

Did you remember to adjust the trimpots for 1/2 supply voltage on the drains?
If you don't get that right, it'll sound really ugly.

I'm not a newbie, bias is everything to me! But I don't know why it sounded so bad... oh, I've just remembered I have some sort of plexi/jcm800 based Jfet distortion and sounds great!  I had forgotten that!

[fuzzo]

Hi,

I drew a version of plexi preamp with a "presence control" taken from "diefet distortion pedal" to have a sixth control.

I would completely eliminate the 470k series resistor on the gate of post-drive control JFET

Why eliminate that resistor ? what does it in the regular circuit  ?

[grolschie]

This was my first personal attemp to build some Jfet Distortion, but I never got it working properly, I mean, it broke really bad, sounded ugly... so I left it...

Did you remember to adjust the trimpots for 1/2 supply voltage on the drains?
If you don't get that right, it'll sound really ugly.

My ROG Eighteen sounds quite ugly on the bass notes when using high output humbuckers. Singles are fine.Tried adjusting BIAS to 4.5-4.6v on each. Will the plexi circuit do the same?

[slideman82]

The one I got working sounds like AC DC rockin' in 78!

[fuzzo]

I could add a gain stage after the volume control to have more gain

Any opinion about the presence control in the schematic posted ?