JFET newbie questions : overdrive pedal

[tepsamps]

Hello there,
i'm new to solid state, i began a overdrive kit based on jfet (J201). https://www.musikding.de/docs/musikd...exi_schalt.pdf

It does not work (no sound at all when engaged) and i started to debug. First i have very little voltage at the drain, the circuit is 9V, so i should have 4.5V ideally, and i have 2V on the first, 1V on the second, and 0.5V on the third. I saw on other JFET circuits that they were using trimmer to set the correct value for the drain resistors, here it's a simple resistor of 15k. If i use a diode tester the jfet seem to be ok and quite identical, so i should get at least the same voltage? No?
Thanks for advice on debugging this stompbox

[loudthud]

Please post the schematic (or a link) you are using. JFETs usually need some circuit changes to work properly, especially when only 9V is available.

Sorry, I didn't see the link you posted. Adjust the 15K resistors higher or adjust the 1Ks in the Source lower.

[Helmholtz]

It's a clone of a Wampler Plexi-Drive.

Drain voltages measured on an original:
T1: 4.6V
T2: 4.7V
T3: 4.9V

All source voltages around 0.3V.

More info tomorrow. Too late for me now.

[Helmholtz]

From your drain voltages I suspect you got out-of-spec fets. The fets draw way too much current and at least the third one is in saturation and can't amplify/pass signal.

Real J201s are rare and expensive. I've seen many J201 labeled fets that actually measured as (non desirable) J202 types.

Even real J201s have a parameter spread of a factor 5 and not all of them are well suited for 9V operated gain stages.

You may try to increase the values of the source resistors to lower drain current. Alternatively you could lower the drain resistors to increase the drain voltage.
But either method will lower achievable gain (and headroom).

Here's a method to measure the essential fet parameters Vp and Idss:
http://www.runoffgroove.com/fetzervalve.html#5

[J M Fahey]

IF they sold you cheap generic (switching) Fets such as J112 labelled as more expensive J201 they will be way underbiased there.

Try using 4k7 biasing resistors, in R3-6-10 positions.

If gain is too low bypass R3 and R10 with 1uF caps.

[Mick Bailey]

My own experience of J201 circuits is that even if the biasing is not optimal and fixed value resistors are used, the circuit will work just fine. I bought all of my J201s when they were current production though, in batches of 100 and all pretty closely matched for characteristics - I've never needed to change any values of published circuits nor needed to use trimmers. Maybe with the price increasing and the fact that they've been out of production for a good number of years has resulted in substandard or fake parts circulating. These days that's always a consideration. Here's an interesting video; https://www.youtube.com/watch?v=attfplzdDEM

Edit; the SMD versions in current production are reported to be good as well as having consistent characteristics. You do need a TO-92 adapter board to use them, though.

[J M Fahey]

True, but sadly I suspect fakes here , *specially* if thru hole TO92 types, thatīs why I straight suggest a way to make them "usable".

Of course using SMD types with adapters is the proper way to get full original performance.

[Mick Bailey]

The guy in the video got re-labelled BJTs. It would be worth checking if the ones in the kit are the same

[tepsamps]

Thank you so much for your replies and sorry for the late reply. Yes it seems mine are the same than in the video. They behave just like 2 diodes. I did not buy them on ebay, but at this store
The kit was given with the smd j201 but i did not dare putting my solder iron on so small things so i put my own J201's with 3 pin sockets that i had to remove to finally solder the smd's on the pcb. And then my 9v adapter died (cry) I can't wait to hear Jfet's tone...