Distort-O-Matic XI from Stompboxology

[hkc94501]

I came across this design and am thinking about building one.
But I have tried to do an LTSpice simulation and there seems to be something wrong.
Either there's something wrong with my schematic input (likely) or something wrong with the design.
My simulation shows the device as a serious attenuator not a high gain distortion effect.
I think that the problem is in the bias of the initial amplifier stage but I don't know how to design or bias jfet amplifiers so no progress.
It really looks like a problem in the bias circuit because there is no setting of the bias pot R18 that gives a dc operating point at the collector of Q2.
It's way off, between 4 and 8 volts.
I don't seem to be able to attach a file to this post but the LTspice file is https://drive.google.com/open?id=1HIIRPwpBx0GPLZKX4gGjJq_DjSNbN81S
If anyone has any ideas about this I would love to hear them.
Best regards to all,
Hank Cohen

[Mark Hammer]

Well, first off, I'm sure you realize you're never going to be able to get clarification from the designer himself.

Second, a great many of Boscorelli's designs are drawn in a way that makes them very hard to follow.  I don't know if it was intentional, or simply one of those things that a guy woodshedding circuits simply doesn't pay much attention to.  But the bottom line is that there are decent odds that the circuit you entered into LTSpice might diverge a little from the actual circuit itself; not as a result of any ineptitude on your part, but because Boscorelli didn't make it easy.

And welcome.  We don't get many Cohens around here.  Levites, yes, Cohens not so much. 

[PRR]

Welcome.

What plan are you building *from*?

Many of us can't open LTSpice files. It would be good to show a screenshot or schematic export of what you built in the sim; many of us can spot slip-ups that way.

I found one plan (there seem to be many). The Q1 Q2 stage "should" bias-up fine unless you have a high Vto JFET.

Ah: 2N5457 may have Vgs as high as 2.5V at these currents. That could require Q2 R16 to pull-up to 8V to get active.

[Mark Hammer]

Well THAT's a lot easier to follow! 

[ElectricDruid]

That wonderful comment "missing 10K near here" reminded me so much of this from xkcd:

[PRR]

OK. Poking at it, I think C15 R17 should be *series*, not parallel.

Parallel, the gain is HUGE, like 100. Clip the snot off guitar. Lose the dynamics needed for the Q4 Q5 stage to do its thing. Also the JFET bias is somewhat fussy.

Series, the gain is almost 3, suitable for guitar under a 9V battery. And the bias becomes much less fussy. Even with an antique part I easily found a working bias.

[hkc94501]

Wow.  THanks for all the leads.
What plan?  Well I found a link http://moosapotamus.net/stompboxology.html at www.valvewizard.co.uk there are several scans of stompboxology issues there.

I was pretty sure that there was something wrong with the bias circuit for the first amp stage and these posts confirm that.
I will try some of the suggestions here when I have a little time to play with Spice (working the day job now

Has anyone actually built one of these things?  Was it worth the effort?

Thanks,
Hank

[PRR]

OK, the magazine shows C15 R17 parallel and says 40dB(!) of gain here.

(And it does show the 10k I said was missing.)

http://moosapotamus.net/files/stompboxology-going-discrete.pdf

Post your actual voltages with R18 one way and the other.

[antonis]

OK, the magazine shows C15 R17 parallel and says 40dB(!) of gain here.

Which reminds me the mind trouble to verify it from NFB formulae calculations.. 
(never managed to get even close..)

BTW, here follows another picture with simulated inductor..

[duck_arse]

well, I have no idea what that list of numbers in yr googoo drive thing is or means - however, if Paul has taught us [us who have never even used a sim program] anything about sims and simming, it's that you need to enter 1M when you need one meg, and not 1 milli.

check your entered values for R6, R8, R9 very carefully. then check any other Meg value resistors .....

[PRR]

> enter 1M when you need one meg, and not 1 milli.

You may have a point:

SYMATTR InstName R1
SYMATTR Value 1m

FWIW: my antique sim won't know "m" from "M" (from days when some terminals only had one case) and I have to spell it out "Meg".

[hkc94501]

Oops!   That's embarrassing.  Definitely makes a difference. 
Now I can at least bias the first amp stage but there doesn't seem to be any interesting distortion happening yet.
More experimenting to go.