Diefet problem - low volume and gain, sounds thin

[Frank_NH]

Excellent!  Glad to hear it's working.  Hopefully, others will read this thread and remember to test their JFETs.   It doesn't take that long to do with a JFET testing rig, and it also makes it easier to dial in the bias knowing Idss and Vp.  The only problem is that you have to label each JFET can keep a log of the data...sort of like having a fine wine collection.  Hmmm... a Fairchild J201, 1977 - that was a great year for JFETs.  Firm but dry, with no bitter aftertaste...  

[marcus262]

ha ha ha, that's funny.
Got to ensure that they're stored at the right humidity and temperature.

I just think they were fake because 3 of them were above the allowed spec tolerance.

[motterpaul]

Congrats, I am really glad you sorted it out. And I know that a bad part is one of the hardest things to find, especially if they "kind of" work.

BTW: while it is not a brand new problem, this Jfet issue is getting worse for a lot of us as we start to run out.

Like I said - I also had a similar issue, and it took me a while to find it because the Jfet was working for a few months. It was that sudden drop in gain that turned out to be the best indicator that the FET was the culprit - a good thing for all of us to keep in mind, I believe.

By the way - on biasing this pedal.  I found it can be very amp specific. What I biased on my smaller combo sounded completely different on my Marshall (especially the low end). In the end I turned them all back to 4.5 just to be on the safe side - but yeah, this is one circuit where you can have a lot of fun trying different settings, and if it is for a specific rig there is no harm in using different bias settings.

[marcus262]

thanks :)

yes, I saw they stopped producing them.

I did a little search on biasing and everyone recommends half supply voltage on drain, but I also saw the article on fetzer valve which uses a formula and you get over half power supply, did anyone try that?

[Beaker]

There isn't an easy one world answer on this, but at the same time it's not too difficult.

The basic answer is too bias at half the supply voltage i.e. 4.5V for a 9V circuit, and 9V for a 18V circuit. This should be about right for most circuits.

However, some circuits sound better biased hotter or colder, for example 6.0V. Usually it will tell you on the build notes, or it will be mentioned in the comments.

Again, it is not set in stone, and can vary with the JFETs you are using - build a pedal with one batch of JFETs and it sounds best with the bias set at 5.5V, build another using another batch of JFETs and it might sound better at 6.0V.

What sounds best is subjective - you might prefer a hotter, more driven sound, or you might like it cleaner, so adjust the bias to suit.

In conclusion then, set the bias a 4.5V to get everything running ok, then fine tune it by ear until it sounds best to you.

[marcus262]

ok,
thanks.

will try by ear.

[Frank_NH]

To follow up on Beaker's comment, if you bias slight off 1/2 supply, then when the signal is large enough to begin clipping, it will clip asymmetrically.  Asymmetric clipping produces even harmonics, which are supposed to sound more pleasing, but then that depends on your preference.

In the world of signal amplification, most designers don't want their amplifiers to clip or distort, so biasing the amp so as not to produce either is of importance.  It's only us guitar players who want to mangle, compress, distort, and twist our amplified signals...  

[rocket88]

To add to beaker and franks comment. The idea of this pedal and many amp-in-a-box pedals is that you're using FETs to replace the tubes, which they were created to replace and mimic tubes. So, taking that into account, not saying they sound exactly like tubes or anything, but running with this understanding think about biasing them as if you were biasing the tubes in your amp. By running the tubes hot or cold will affect how and when they distort, as well as the amount and type of distortion you hear. Well the same thing applies here. As beaker mentioned, some people like running them a little hot, so a little cold, but the idea of 1/2 voltage bias is kindof a starting point for tweaking as this is what voltage they're "supposed" to be at.

Btw, frank when you think about it, not only do we musicians like the distortion from tubes, but amp designers have to set the to distort. It's the complete opposite of what they would normally do. Can you just imagine the first guitar player to walk into a shop and ask for the tech to work on his amp to make it distort and distort in a controlled predictable way? The tech must have thought he was crazy.

[marcus262]

I biased by ear and settled on 9.5 - 9.6 V for 18V supply.
So would that clip more asymmetrically and produce more even order harmonics?

[rocket88]

not sure if that's enough to produce asymmetrical distortion and even order harmonics, but if it sounds good that way i would definitely keep it that way, or write down the voltages you like and increase the voltage and give a listen.

[Lobbetto]

Unfortunately, I also had problems with the J201 sold by banzai.
I am comforted to know that in fact the problem was not just mine.
Now I know that I have to avoid buying from them J201.

I'd like to know where to buy J201 to stay safe.

[Travis]

Most of us are buying the SMD J201 from chromosphere's shop I think. I haven't seen any reports of issues with those, and they've worked very well for me.

I think smallbear is always safe if you're in the US. They have through-hole J201 for $0.55 each. I've always gotten good parts from them and I'm sure Steve would take care of you in the unlikely event of an issue

[Sensei Tim]

I don't want to sound like a broken record but... Smd with through hole adapters. No fuss, no muss.

On Friday, April 8, 2016, Travis [via Guitar FX Layouts] <[hidden email]> wrote:

Most of us are buying the SMD J201 from chromosphere's shop I think. I haven't seen any reports of issues with those, and they've worked very well for me.

I think smallbear is always safe if you're in the US. They have through-hole J201 for $0.55 each. I've always gotten good parts from them and I'm sure Steve would take care of you in the unlikely event of an issue

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[Beaker]

You know you are are up against it when companies like Banzai start selling dodgy JFETs.

Unfortunately the pool is drying up, and the number of suppliers of decent J201 and 2N5457 is getting smaller and smaller all the time.

As already mentioned, it's time to stop wasting time and money tracking down decent JFETs at a decent price, and start using SMD versions instead.

I know you are in Europe Lobbetto, and you can get SMD JFETs from Bitsbox here in the UK.

[Lobbetto]

You mean something like this, right?
https://www.youtube.com/watch?v=IjcsL6tHdl8
On musikding there is this version, hoping that they are working.
http://www.musikding.de/MMBFJ201_1

or even where you have suggested on bitsbox
http://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=140_151_154&products_id=2783


I could also avoid using an adapter.
I could pay off the excess rheophore on SMD resistors to connect in the veroboard socket.

[Beaker]

Those from Das Musikding and Bitsbox will be fine Lobbetto.

They are TINY, it is possible to use them without an adapter board, but it is much easier to solder them to the adapter board, them mount the boards onto the vero.

Even with the added price of the adapter board, it still works out cheaper than using through-hole J201s.

[Beaker]

Here is a pic I took of one - it's in the SMD Jfets soldering tips? thread.

[marcus262]

Hi,

Sorry to hear that you too had a problem with j201 from banzai.
I wrote them a letter explaining the problem and to check their supplier for future customers, they didn't respond yet.

Before I learned about smaller smd j201, I bought the regular j201 from this seller, he says he is a guitar pedal builder, so I took a chance,

http://www.ebay.co.uk/itm/201541198885

[Lobbetto]

I'm sorry, but now I have become suspicious of the J201 TO-92.

I buy from musikding with adapters, as even include the connector with the pins.
Practically:
1 MMBFJ201
1 PCB ADAPTER
1 STRIP 3 PIN

just € 0.63

less than a J201 TO-92.


Diefet sounds very different from 7thvn and blackforest?
I was looking for a Engl distortion.

[Beaker]

I think you are right to be suspicious - I don't think you can  absolutely guarantee to be buying good J201s anywhere.  And yes the SMD versions are much cheaper, even including the PCB.

Do yourself a favour, and buy some of this:

http://www.ebay.co.uk/itm/Gel-Flux-Solder-Paste-BGA-SMD-SMT-Assembly-and-Repairs-1-4-ml-/111740168992?hash=item1a043bab20:g:NOAAAOSwDNdVyPUf

A tiny smear on the pads of the PCB will glue the JFET in place while you solder.

Another good tip for you: The SMD JFETs have interchangeable source and drain pins - as long as you get the gate pin in the right hole, you cannot fit them backwards. This makes mounting them on the veroboard easier.