Sigh, JFETs...

[Silver Blues]

Just venting... had built up a pedal with a circuit of my own design, excited to see how it'd sound, fired it up and nothing but squeal. Volume, tone and gain work (modulate the volume and pitch of the squeal). Discovered less than 1v on each drain... okay. Replaced resistors after some experimentation so 3.7-4.5v on each drain. Still nothing but squeal. Analyzed it with my scope, signal dies after the second stage JFET. No idea why.

Ah well. I was going to redo the circuit op-amp based but then it'd be too similar to another one of my designs.

[IvIark]

I've notice SMD JFETs showing up at better prices on eBay recently like these:

http://www.ebay.co.uk/itm/250976000164

http://www.ebay.co.uk/itm/291064228761

I read on the following thread that the SMD J201s have very tight tolerances so it's easy to get a matched set, so I've ordered some of these along with 30 of the single adapters also in the thread:

http://www.diystompboxes.com/smfforum/index.php?topic=105937

$4.50 for 30 adapters and it took about 2 weeks to get here.  I haven't tried them yet but when I do run out of the known good through hole JFETs it's nice to know that the SMD ones may be a better alternative anyway.

[Silver Blues]

Oh that's neat actually. Thanks.

I might try the circuit with some 2SC1815s I have laying around, because if I remove and reinsert the FET the clipping diodes flash so something must be working in there.

[IvIark]

Sorry I thought you meant the through hole JFETs you were using may have been dubious like many people are suffering with now.  

Assuming the circuit theory is good, if nothing is coming out it must be faulty or misbiased.  What voltage are you getting at the source?

My MMBFJ201's arrived today and I'd forgotten how bloody tiny these things are.  They were in a cut off portion of reel which was bagged, and I saw a few bits at the bottom of the bag which I thought were just bits of dirt or fluff, then I realised a couple had fallen out of the reel.

I may have to buy these and the PCBs in bulk now and solder them all while my eyes and hands are still fairly useable, because in a couple of years I may not even see them!

[Silver Blues]

That is what I meant :P

Before I re-biased it I was getting:

Q1
D - 0.740v
S - 0.658v
G - 0.683v

Q2
D - 0.857v
S - 0.766v
G - 0.753v

Q3

D - 1.374v
S - 1.208v
G - 0.8v

And I biased Q1 to 3.7 or so volts at the drain, and Q2 to like 4.2v, and still nothing. I thought it weird that the resistance necessary to get the drains to that voltage is just a hair over 1K. The circuit is essentially a JFET Muff with one of the clipping stages omitted and a few little embellishments, it's nothing radical and I thought it might be neat.

Hah, yeah I know what you mean. I'm lucky that I'm young and have an uncanny affinity for seeing extremely tiny things, else I would have a much harder time with electronics.