Echoplex Preamp: analysis y mods.

[Plexi]

Hi everybody in the DIY community!

I spent the sunday analyzing this wonderful preamp.

I used this layout: https://3.bp.blogspot.com/-7j5VMPo4x8I/V_TluXFrWjI/AAAAAAAAESI/Cmcv3bAxWcoIbBWk3n7cZ8W-bmmZztzwQCK4B/s1600/Echoplex%2BPreamp.png

Notation:
- R4: for bias, if is needed (not my case).
- Go from Source, to a 10uf cap, then to ground if you want some gain. Play with the cap for more treble or bass.
- Use suitable for >25v caps, to run with 18v.
- Jumped R6 for more increased volume output: sorry 'mojo'..

I'll remove R5 too, and put a A500K at the output as a voltage divider/master volume.


That way I'll have more volume and usefull master, instead the 500k pot in parallel with R5 (220k) to ground.

I gess it's almost the same, and what R5 and R6 are doing in a fixed position.

Am I killing too much mojo?
Experiencies? Recomendations?

[GibsonGM]

Not sure what you are up to, Plexi...you'll know if you kill the mojo - the tone will stink 

That is close to a Tillman preamp - you can look that up on a search engine and find out a lot more about it!!   Yes, it is a nice preamp, probably very clean at 18V, too! 

You can put a pot in series with the bypass cap at the source, and adjust gain/fat that way, too

[davent]

Have you seen the Fatpants  Booster project at Madbean? Based on the Echoplex and includes a charge pump to get you 18v.

http://www.madbeanpedals.com/projects/FatPants/docs/Fatpants2013.pdf

dave

[Plexi]

Have you seen the Fatpants  Booster project at Madbean? Based on the Echoplex and includes a charge pump to get you 18v.

http://www.madbeanpedals.com/projects/FatPants/docs/Fatpants2013.pdf

dave

Yes! I saw that project!
Looks great, like the EP Booster (a big disappointed to me: even when I tweaked all the caps and values to get more of the EQ options).
I saw that the oter jfet is acting like buffer (am I right?), similar to EP Booster.

That's why I went for the simplest one now: and didn't get dissapointed.

BTW: I noticed in the Dunlop RI that there's some values of the first EP, not the 3rd version.

[Plexi]

Please thread: don't die 


What about use A100K instead A500k at the end?
I saw that in every schematic or layout they use 500k: something with the impedance, etc??

I tried the last one, and almost the 50% of the turn is useless

[BubbaFet]

Friend, you are missing out on lots of JFET knowledge nuggets if you have not yet used this forum's SEARCH feature. The answer you seek is in there somewhere, I promise!   

[Plexi]

Friend, you are missing out on lots of JFET knowledge nuggets if you have not yet used this forum's SEARCH feature. The answer you seek is in there somewhere, I promise!   

I did! thanks!
Any thread to suggest? 

[BubbaFet]

Well my favorite, and I mean this in all sincerity, what did it for best for me, was reading every post that had the word 'jfet' in it, (there is a FINITE number of them, don't despair, it's doable), and looking for bits of information that I didn't know before, and trying things out. Often someone has posted some insight or observation that nobody else commented on, that isn't widely known, or has not had much attention. That, to me, is where one can find a diamond,  mixed in with all the gold. And diamonds are in there, some only recently brought to the surface of our collective awareness! These truly are the best of times.

So I know that I'm being obtuse, but I am honestly trying to be helpful, and I apologize if I am not.   

[Plexi]

That's almost the same process I did 
I prefer to use google, and "site:" prefix 

[BubbaFet]

Cool  . I use the method you describe as well, along with this site's 'advanced' search function. It might be Google powered, I don't know, but I felt that it provided a better 'response' to my inquiry, or perhaps just a different perspective. Different types of AI, I suppose 
Play on!

[Plexi]

Hi everyone...
Here's another update of Plexi's diary of a Jfet (¿)

Today I built this beautyful preamp with a J201.
To my desception, it sounded awful: thin, low volume... and without the body and compression that the 2n5457 gives.

Looking the Tillman's preamp schematic, J201's preamp for excelence, I found out they use lower value from Source to ground.
So...I replace the 3k3 resistor to ground for a 1k5, leave the 22k to Drain (if I reduce it, it'll lost headroom)... and voilà!!!

That's my recomendation.

[Plexi]

Hi everyone...
Here's another update of Plexi's diary of a Jfet (¿)

Hi everyone ...
Here's another update of Plexi's diary of Jfet (?)

I was laying around the MXR RI, and found out that they're based on the first version of the echoplex.

Anyone knows which JFET they use?
As I can note, the resistors are the same as in the schematic (for bias reasons)..

[robthequiet]

According to my notes, the first version used tubes. Don't have the exact tube part, unfortunately.

The EP-3 which is most popular apparently used TIS58, although  my print is kinda fuzzy.

[robthequiet]

They have a circuit out there that is called "Groupmaster" -- I think it precedes EP-1, but not sure. Same preamp as what you have but Source R is 10K. Still TIS58 FET.

[Plexi]

My bad, I mean the late EP3 version.
I post an schematic with the differences in my first post.

[robthequiet]

Here is a link to the TIS558 datasheet, might be helpful

[Plexi]

Hi everyone ...
Here's another update of Plexi's diary of Jfet (?)

I was laying around the MXR RI, and found out that they're based on the first version of the echoplex.

Anyone knows which JFET they use?
As I can note, the resistors are the same as in the schematic (for bias reasons)..

[Plexi]

What do you people think?

[robthequiet]

Howard over at Catalinbread has some great design chops.

[Plexi]

Howard over at Catalinbread has some great design chops.

And always share schematics