SFT V2 Clone - Troubleshooting

[elbeeuk]

Hi all,
I'm hoping someone can help point me in the right direction with debugging this build.
I've built it all and the circuit lets the signal through when engaged but the gain is weaker than demos. The layout I used is over at http://tagboardeffects.blogspot.co.uk/2016/06/catalinbread-sft-ii.html
The main problem I'm having though is that I get a weird squeal when stoner is engaged. Here's a quick vid to explain better in terms of pots involved:
https://photos.app.goo.gl/1Bwv7Qc03aNyy5g03

Can anyone think where to start with debugging? I've built 4 other pedals without issues, this is the first that's giving me bother.

DPDT switch is wired lkike this:
https://photos.app.goo.gl/d839CmHh40PKJbrE2

Solder on the veroboard looks like this:
https://photos.app.goo.gl/Vqg7srdPfQ6kB6a03
https://photos.app.goo.gl/YRxqSaZpcJJ4YAnO2
https://photos.app.goo.gl/O91edceJB54DqYpu1

Give me a shout if you need other info that might help.

Cheers
Elbee

[rankot]

I am also having problems with this pedal, when I switch to "stoner" mode, it works fine, but when switched to "stones", gain pot is barely usable - must be maxed to avoid farting. No ideas why... Any idea what to trace or look after is appreciated.

I have used this schematic:


All JFET transistors are MPF4393.

[rankot]

Hey, any hints on this? I still have no solution...

[PRR]

> any hints on this?

I don't see any "modes" on your schematic.

I see places which may lead to switches, but no clue what makes stoned/stoner/stones modes.

Just dart-throwing, I would look for a large (to rail) change of a DC condition, possibly from a bad cap in the switching path.

[rankot]

Thanks PRR! There's only one switch, but I forgot to put it on schematic, it is 2PDT, connected to points 02/03 and 04/05 this way:

[duck_arse]

I don't wish to seem rude, but I don't see any DC gate reference on Q1. also, Q5 might benefit from a failsafe gate bias resistor instead of relying on the convoluted path thru the tone stack.

[rankot]

You are not rude But I also have Ampegulator (copy of SVT v1) and it has almost the same input circuit, without DC reference on Q1 gate. And it's working.
http://pedalparts.co.uk/docs/Ampegulator.pdf

I have also tried to build this first part in LTspice and it works - but not as I expected. So I looked a little bit more at schematic differences and found the error: pull down resistor after C1 is missing! I simply copied this schematic from Internet as they said it is verified, but they have omitted this! When I put 1M into LTspice, it starts working as expected. Now I will try to solder the same on my PCB and report here if it have fixed the thing!

Thanks!

[rankot]

Well, I was wrong with this pulldown resistor, if I add one after C1, it will bias gate of Q1 too high and the sound will be too much distorted.

However, I found out that drain voltage of Q3 was too low (2.3V), so I tried different JFETs and finally found one J202 with drain voltage of 4.5V. It really made sound better - at least in Stones mode, but when I switch to Stoner, I have problems with oscillations, microphonics and some kind of tremolo-like noise. Really have no idea what's going on here...

Edit: some 10 minutes after writing first part of this message, I tried to join switch pins 2 and 5 (as someone proposed on another site), but it didn't bring anything except no sound at all. However, when I removed that connection, pedal started to work as expected out of the blue - no oscillations, no noise. I'm really puzzled, but it works

[Dr. Satan]

I built this too and it works, but not properly.  I used MPF4393 transistors because I still have quite a few from when I bought 100 of them years ago (It was literally cheaper than buying 20 to get 100), but I'm about 100% certain that R1 is on the wrong side of C1.  It will work without it, but if you run a hot signal, like from a boost that puts the AC voltage around 2VAC, it will throw the device out of operation until the voltage settles back down and the FET starts conducting again. Slapped a 470K on the other side of C1 and it works fine, even with a heavily boosted signal. It will still throw the FET out of the proper range of operation, but it snaps right back so you get this gated sort of sound when you stop playing, but it's only for a fraction of a second and only on higher gain settings- I actually like it personally. Catalinbread used the MPF4393 JFETs for a reason.  They are the most consistent across the board JFETs you can buy.  I had initially bought my stash with the intent of testing and sorting and after testing all 100 of them, there wasn't enough variation to even sort them into batches, unlike the horribly inconsistent J201 or 2n5457 that seem to be so popular. Sadly the MPF4393 is obsolete in through hole package, but still made in SMT, which shouldn't be too hard to implement some sort of adapter for.   I would suggest trying some J113 JFETs as they have similar specs to those that are popular, but are significantly cheaper.  They may not work well for applications where you want low noise, but on a distortion or OD, most people aren't looking for clean tones so the noise is a non-issue.

[rankot]

I built this too and it works, but not properly.

What happens on your build?

[Dr. Satan]

If the input voltage exceeded 2VAC it would knock Q1 out of whack and the volume would drop and the distortion would be very sputtery and fuzz like. Like a dying battery type of sound, but not in a good way.  Let it sit for a minute and things would settle down and start acting right again, which is typical behavior for a mis-biased JFET, in my experience at least. 

Also, if you double check the schematic on the Ampegulator, you'll see that R1 on that schematic is the DC reference to ground for the gate of Q1.

[rankot]

They are the most consistent across the board JFETs you can buy.  I had initially bought my stash with the intent of testing and sorting and after testing all 100 of them, there wasn't enough variation to even sort them into batches, unlike the horribly inconsistent J201 or 2n5457 that seem to be so popular.

What are your Vgs and Idss values for them? I've bought few batches and I'm not sure they are all legit, also variations are big.



Everything over 30mA or over 3V is out of spec.

[rankot]

...and here is the photo of my PN4393 JFETs:

[duck_arse]

do those natsemis have colour-bandings on the back, rankot?

[rankot]

Only those with "F" have red and yellow bands.

I really have no idea what those brand marks represent. 

[duck_arse]

...and here is the photo of my PN4393 JFETs:

first one looks like Silliconix, but I've only ever seen inked examples, not etched.
second is the famous Fairchild F, a mark of quality.
third is the later NatSemi mark, fourth is an earlier National Semiconductor logo. you can say the fourth is older than xx years by the logo, but you'd need to research [or wait for PRR].

national use those colour bandings as batch identifiers as far as I can tell, it makes an easy "first sort" when you buy a bag of them.

[rankot]

This Siliconix is actually some kind of print with silver or white ink, although it looks like etch on this photo.

What do you think, which ones shall I use for this pedal?

[Fancy Lime]

Hi Ranko,

the farting at less-then-max gain that you describe is typical for mis-biased circuits and in the schematic you posted, Q1 is not biased. I the Ampegulator schematic, R1 is the biasing resistor for Q1. The missing biasing resistor is that which you call the "pulldown resistor after C1". With that in, it should work.

However, you write:

Well, I was wrong with this pulldown resistor, if I add one after C1, it will bias gate of Q1 too high and the sound will be too much distorted.

I don't see how the gate of Q1 could be too high with the resistor to ground. It should be 0V. What might be too high is the source of Q1. You could try replacing the source resistor of Q1 (R5 in your schematic) With a 10k pot wired as a variable resistor and see if that lets you dial it in right. JFET biasing is always fun!

Cheers,
Andy

[Dr. Satan]

Hey, sorry I don't remember the values of my tests, just that there wasn't enough difference to need any sorting. What I have, IIRC, are genuine Motorola devices that I got from Mouser. This was years ago, of course, back when they were cheap and plentiful. They are nearly impossible to photograph too. I took about 20 pictures under various lighting and this was the best I could get with my phone.