Help!...GGG's Slow Gear replica working voltages?

[gigimarga]

Hello,

I'd built a GGG Slow Gear replica almost one week ago and still i can't make it to work
The sound after passing it it's clear and louder, but no swell
I replaced the 10K trimpot (which had no effect at all) with a 22k trimpot which seems to act ok...but i couldn't find no setting of it and the two trimpots where to swell.
ALso, after reading some posts here, i put before it a louder distorsion (rocktron rampage)...but the same problem.
I used a 2SK30A and BC549C.

Can anyone who has it and which is working good to post the voltages?

Thx a lot!

[gigimarga]

 

[gigimarga]

 

[mdh]

How about posting some non-working voltages, per the debugging thread?

[oldrocker]

This was a past thread that I thought might help.
http://www.diystompboxes.com/smfforum/index.php?topic=55388.0

[gigimarga]

I put it after a strong distortion, but i don't hear the swell

I used the schematic from http://www.generalguitargadgets.com/pdf/ggg_sgo_sc.pdf and the layout from http://www.generalguitargadgets.com/pdf/ggg_sgo_lo_parts.pdf.

Anyway, my voltages using a 9.25V battery, are:

IC (741):

Pin 1 = N.C.        Pin 8 = N.C.
Pin 2 = 3.90        Pin 7 = 8.35
Pin 3 = 3.65        Pin 6 = 4.60
Pin 4 = 0.00        Pin 5 = N.C.

T1 (BC549C): E= 3.65        B=2.95        C=9.25

T2 (2SK30A): D= 5.35        G=1.85        S=5.20

T3 (BC549C): E= 7.14        B=4.40        C=9.25

T4 (BC549C): E= 3.15        B=2.65        C=5.70

T5 (BC549C): E= 2.85        B=2.95        C=2.90

T6 (BC549C): E= 2.95        B=3.05        C=2.90

Thx a lot!

[gigimarga]

I replaced all the BC549C with 2N3904, the 2SK30A with 2N5457 and now it's working (very subtle...of course)...anyway, i am very happy that i discovered the most stupid stompbox in the world, and, paradoxically, very hard to build!

[oldrocker]

I used 4401's and 5952's in mine.
http://randallbarnett.homestead.com/SG1_1.mp3

[gigimarga]

mine sounds like crap...

[gigimarga]

 

[gigimarga]

 

[blackboxx]

hi gigimarga, my SG replica does not work at all.....
i controlled the various voltages, unluckily they are so very different from yours..
could you confirm that yours are the right ones?

thanks a lot!

[gigimarga]

my voltages are the right ones...but it sounds like crap...a very subtle swell...anyway, i hope it swells and i'm not schizo 

and, moreover, this subtle swell i obtained when i've put before it an original mxr dynacomp with sensitivity and output maximized...or an original rocktron rampage with the level maximized

what i heard in the oldrocker's sounclip posted above it's fabulous...i am very curious how he obtained it

[mdh]

Do you notice any change in the sound of the effect as you twiddle the trimmer?  I wish I remembered which knob settings I used to make my samples (in this thread), because as I recall, it was kind of finicky with respect to attack and threshold settings.  Unfortunately, mine still isn't boxed up, so I haven't had a chance to take a look at it yet.  The build notes say to set attack fully clockwise and threshold at 12 o'clock for calibration, so that's probably a good place to start.

One thing about this circuit, though, is that it's kind of finicky about JFETs.  I used a 2N5485 with a Vgs_off around -2V in mine.  R.G. has stated elsewhere that the optimal range is about -3 to -4.  Do you have enough FETs around to do some more swapping?  I know the transistor footprints on this board don't work with the SIP sockets that most people around here use, but if you can find a transistor socket that fits the footprint (or even just break 3 machines pins out of a socket and solder them in individually), it would be easier to audition different parts (assuming you haven't done so already).  It may also be worthwhile to look at R.G.'s JFET matching article and build the matcher, because even though you're not matching multiple FETs, the matcher gives you an estimate of Vgs_off, which may be useful information for selecting among the FETs you have.

[gigimarga]

Thx a lot man for your reply!!

I noticed changes in the sound of the effect as i rotate the trim...in the first 25% almost no sound...a sweet spot around 25%...a volume increase at 50-60%...almost no change until 100%...the pots were set intially as in the building notes...after some tries i've began to chane them.

I like very much your soundclips...that's what i want from it!!!

I will try to change some other JFETs...until now i changed only two 2SK30A and two 2N5457...i have no chance to found 2N5485 here...i have a couple of BF244, BF245, 2N3819 and 2N5952...i will try all.

Can you tell me what JFETs are the Vgs_off between -3 and -4?
My Zenner is 5.6V as in the schematic...does this matter very much?

Thx a lot again!

[mdh]

Unfortunately it's hard to say what Vgs_off to expect from a JFET, even given a datasheet. They often list maximum and minimum values, but JFETs are notoriously variable, so the ranges tend to be pretty big.  Any chance of measuring the ones you have in R.G.'s JFET matcher and letting us know the results?  I may be off base here since I don't understand the behavior of the circuit in detail, but I wonder if maybe your JFETs happen to have Vgs_off too close to zero.  Mine measured -1.9V or so in the JFET matcher, and I definitely got the most intensity out of the effect with the trimmer at the extreme of its rotation.  It's also a problem if Vgs_off is too large (in absolute value, that is), and I believe that in that case you may be able to improve performance by using a higher voltage zener.  But of course, you don't want to get too close to the supply voltage, which is why transistors with Vgs_off around -3 to -4 are optimal, if I understand correctly.

If you want to understand these issues more completely, try reading some of the other Slow Gear threads, including my build report thread linked above.

[gigimarga]

OK..thx a lot again for your kindness!