Another boring FET bias thread (azabache)

[kdmr]

Hey all, I think I've exhausted all the available reading on the forum and am still, unfortunately, having trouble with the ROG Azabache, notably Q1 (2n5457) which I cannot for the life of me get biased correctly. The effect sounds cool but when I set the Q1 drain trimmer for 6v, the source shows 2.7v. I've auditioned a few different FETs in Q1 and all show the same voltages when the drain is set for 6v so I feel like there is something fishy going on besides shoddy semiconductors. I've included links to the schematic and the layout I've used for the build.

Sorry for flogging a dead horse with this one.

http://3.bp.blogspot.com/-CL-cQFAg-PE/UWZWOEVf6mI/AAAAAAAACJg/D4plUkOf7uQ/s1600/ROG-Azabache.png
http://runoffgroove.com/azabache.png

[thermionix]

I know absolutely nothing about FET biasing, but if the effect sounds cool, then (not trying to be a smartass here) what's the issue?  Is 2.7v on the source bad for some reason?

[kdmr]

Fair, but correct would be better. Bias on q1 should be closer to .7v at source. I just know it's wrong because when I tap signal in at Q2, the result is really great sounding crunch...How it is now is more like just a coloration with a bit of overdrive, just not nearly as much as I suspect the circuit is capable of. The problem is somewhere between Q1 and the gain pot, just having a hard time figuring out where to start.

[thermionix]

Okay I see, sounds cool but not correct.  I'm still learning BJTs, FETs are beyond me at this point.  I've only built two pedals with them, Rat clones, and I didn't try to bias them.  They sounded good so I didn't question it.

So too much voltage across the source resistor means too much current through the source resistor, right?  Assuming that you double checked that it is indeed 470 ohms.  Doesn't seem like there's much that could influence that, except a leaky cap maybe or something up with the red LEDS, since you're adjusting the drain to a specified voltage, and you've already tried swapping FETs.

Well I'm lost.  I'll go now.

[Frank_NH]

Did someone say FETS?? 

I took a look at the Q1 stage in the Azebache with my handy-dandy JFET spreadsheet, and here's what I calculate:

Q1 = 2N5457
Idss = 3.27 mA (typical value)
Vgs(off) = -1.58V (typical value)

Known values:
Vdd = 9V
Rs = 470 ohms
Vd = 6V

Calculated values (DC bias):
Rd = 2366 ohms
Vs = 0.596 V
Id = 1.268 mA
Gain (no source cap bypass) = 2.76
Gain (w/ source cap bypass) = 6.10

Note that the gain is the voltage ratio, and is pretty small.  The source cap bumps the upper frequency gain up a bit when the bright switch is engaged (bypassing the 1M resistor).

As for the gain loss before Q2, check everything out!  All resistors, caps, diodes...everything.  If you're getting 2.7V on the source leg of Q1, that would imply a DC bias current of 2.7/470 = 5.7 mA, which is greater than a typical 2N5457 Idss!  Hmmm...maybe the source resistor is not connected to ground?  Or the FET is junk?  BTW, you can replace this FET with others such as J201, MPF102, 2N5458, etc. and it should still work if the drain voltage is biased to ~5V or 6V.

Good luck debugging.

[kdmr]

That's the science I was looking for! I've tried MPF102 in its place and its the same deal. I just gave it a once over and everything LOOKS good. Got a batch of 5457s coming in the mail so if nothing else I'll have more audition pieces.

[Frank_NH]

If you just looked at the circuit, that may not be enough.  I've had my share of cold solder joints   and you always need to check everything with the continuity setting on your multimeter.  Look for breaks, cold solder joints, and shorts - I've seen tiny little microscopic bridges cause a circuit to go from non-functioning to perfect. 

You should also make yourself an audio probe if you haven't already.  You could then find where the signal appears to go bad and at least isolate the problem.

Good luck!

[kdmr]

thats whats so frustrating about this problem: I've done all the checks!
I did find a erroneously packaged capacitor in Q1s input path...i was hoping my distributor had given me some bad values elsewhere that might explain it but, no dice. ive filed in between all the strips (i'm using vero), reflowed everything, triple-checked placements and jumpers. I'd chalk it up to a bad tranny but the others i've auditioned show the same problem, i've messed around with a few different source-to-ground resistor values to no avail. I'm positive that when I find the culprit, its gonna be something soooo stupid...it always is whenever i've had really specific problems like this.

[Frank_NH]

Post pictures of your vero build here - maybe someone will spot the mistake.  If you make an audio probe, you can trace the signal from the input to the point where it drops in strength.  Again, 2.7V for the source pin voltage on Q1 looks way out of line, so check everything there (and make sure your JFET's not in backwards - don't ask me how I know this ).

[duck_arse]

I was just going to post: can we see some pictures of your build, but now I'm not going to.

can you measure the resistance setting of the drain trimmer, please? power off first, obvs.

[kdmr]

FETs definitely oriented correctly...Led lights up when in backward. I'll measure the trimmer after work, but since all the other FETs are biased correctly not sure it's a power issue. I feel like it's in the source circuit, but there is so little stuff down there that it's boggling my mind! I've double-checked the tonestack and the components associated with Q1 and I've already audio probed the signal path...No gain at Q1 drain and gets further attenuated through the tonestack. Q2-4 have good gain at each stage.

[duck_arse]

also measure the resistance of that source resistor. please.

[kdmr]

Totally! I replaced the source components last night and measured...470 on the resistor to ground so I know that's legit. I'll measure the trimmer after work and do some calculations. Damn time I learn FETs inside and out anyways...Can't believe I haven't yet but this is the first large FET circuit I've done in the years I've been at this hobby!

[kdmr]

trim pot shows 3.45k

[Frank_NH]

The 3.45K drain resistance is in the ballpark. 

Now, I just thought of something - take a J201 JFET from one of the later stages and use it in Q1's position.  Since the rest of the circuit appears to work, this would tell us once and for all if the JFET was to blame.  You'll have to bias it at Vd = 7.0 V or higher because of the 5K trim pot (you could replace that with a 10K and not affect anything if you go back to a 2N5457), but you'll definitely get some gain.  Let us know what you find.

[kdmr]

This exact fix just occurred to me. I'll try to test it tonight, though I did test a MPF102 in Q1 to no avail (drain biased at 6v with 2.5ish on source i think). Pretty sure I've got a few 5458s socketed in various circuits around the shop that I could dig out too.

[kdmr]

another thought...when i was doing a lot of experimenting with tube amps, i remember the cathode bypass capacitor mod to increase gain. I know a lot of these amp-in-a-box circuits started as kind of FET-by-numbers designs. Not sure how FETs stack up operationally to tubes, but I'm curious about the 220nf cap in the bright circuit. My current stock of 220nf caps was a super cheap ceramic lot from china. they are labelled "224" but they are so tiny i've always been skeptical about them. never had problems so far but do any of the gurus know if changing the value of this cap will affect bias of Q1? I guess I should just try it.

[kdmr]

From the schematic it looks like all the other 220nf caps in the circuit are used for coupling. man i really starting to feel like i'm gaslighting myself here...

[robthequiet]

changing the value of this cap will affect bias of Q1

I would prioritize the FET swap, then worry about the cap. The cap operates on AC, the biasing happens in DC, so the bias is not a function of the cap under normal running conditions. 220 nF is probably a key value for the pedal sound, so try incremental changes if you want to experiment.

[kdmr]

yeah i suppose any dc blocking is going to happen regardless of value