Simple circuit but...

[Smash]

Hello everybody!

Fresh new here and in the world of DIY pedals!

This site is just amazing! I've already built a Euphoria OD, Pharao Fuzz and a GuitarPCB's Cpt Munch and they all work great! Very happy with them!

So I thought I could make a booster without major problems as they are fairly simple. So I chose the Echoplex with the gain pot: http://tagboardeffects.blogspot.fr/2014/05/a-pair-o-preamps-echoplex-and-tillman.html

Great, couple of components and voilà. BUT, when I turned on with knob all the way down, it cuts the signal off completely and all the way up, I kind of have the same volume as the dry signal. So measuring the voltage of the 2N5457 I have:
D - 1.45V
S - 1V
G - 0.1V

Already swap the transistor, the 100nF cap and verified all the connections. All resistors seem to have the right values... I really don't know what to look for...
Something really obvious I'm missing?

Thank you for the help... sure somebody lot smarter than me will have something

[boratto]

try to put another 2N5457

[Smash]

Hey!

Thanks for the answer...
Yeah... I did that... voltages were similar...

[Frank_NH]

If you look at the circuit, it's just a simple common source jfet amplifier like this:

Common Source Jfet Circuit

The boost is created by switching in the 10 uF cap in parallel with the source resistor.  (Note - you mentioned a 100 nF cap - I assume you meant 10 nF).

From your voltages, the drain voltage is too low - you should be getting something close to 4.5V (half  your source voltage, assumed to be 9V).

You can do two things here:

(1) Keep replacing the jfet until you find one which biases properly at the drain --> 4.5V.  BTW it should be a j201.
(2) Install a 25K or 50K trimmer resistor in place of the 6.8K resistor and adjust it until you get 4.5V on your jfet's drain.

Let me know if either of these work for you.  

[Smash]

Thank you for the tips Frank!

But actually I think you're refering to the Tillman's...
I built the Echoplex

But I'm guessing I have to look for the same things... like replace the 22k in my case(?) till I get 4.5V?

[Frank_NH]

Whoops - sorry 'about that.

But my comment about the drain voltage being about 4.5 V should still apply.  And, I'd try a j201 (and other jfets) in your existing circuit to see if they bias correctly.  If you have one, put a transistor socket in your build to make it easier to replace the jfet.  You could alternatively socket the 22K drain resistor and swap that out with your existing jfet in place.  Good luck - you should be able to get this to work.

[Smash]

OK! Thank you!
I'll give it a try when I get home and I let you know if it works!

[Smash]

Hello again Frank...

Well I tried to change the drain resistor and I went with a 10k. Now I have:
D- 5V
S- 1V
G- 0V

I can hear a little bit of increase in volume but something tells me that I should have a lot more... and I got the pot all the way up...
An annoying thing is that I don't have any socket here... so solder... cross fingers... desolder...

[Frank_NH]

From further research here is the original schematic (without volume control):

EP3 preamp schematic

You'll notice that the supply voltage is 18V!  Hence a 9V supply for the resistors specified likely won't provide the appropriate bias you need here.

So what can you do?

(1) Use a 18V supply with the original resistors as prescribed.  This is probably the only way to get the right sound if you're going for an authentic EP3 preamp.

(2) If you need more oomph at 9V, try eliminating the 100K resistor at the output (replace with a jumper).

You can do some other things like increase the 22 nF cap on the right hand side to a 10 uF electro (with the - side to ground) - that'll get you more gain.  But then you start departing from the original EP3 design, and may as well build something else, like the EQD Speaker Cranker (a great circuit BTW!).

Good luck.

[Smash]

Hey, thank you again for taking the time to point the right direction!
I think I can do some experimenting here with all these tips!

I'll let you know how it goes...

[Smash]

Hello again!

Yep, I've swapped the 100k  from the output for a 500 and the effect is a lot more noticeable. The circuit is very warm and works well with single coils.
I also saw some reviews and I notice that the original does not add "gain", it just "color" the sound so I think it's normal.

Thank you for all the help!

Well, after that, I'm gonna have to build a real clean boost.

Any suggestions?