first time fuzz build .. debugging help??

[saint44]

Thnx for responses... I changed npn's to 2n5088.  Pins are verified.  The issue now is voltage at Q1 not right .  The one pic of topside of board has the trannies the right way nd i changed ground (grey wire) Q2E ground now.  Plus I guessed it would have a little more bass ... the one cap value I changed from 1200pF to 1000pF.

https://db.tt/EwLvAf9l

[LucifersTrip]

A couple notes. Q1 is not setup like a standard amplification stage. I believe it's there for impedance matching.

Q1B didn't wind up one voltage drop (.5 - .7) above Q1E, at least on all my builds, it's less. I asked about this years ago on this forum and the answer I got was that it's a poorly designed emitter follower (EF) stage. Someone suggested the culprit was the 1M.

Secondly, the sound of this thing is smooth, thin & trebly with nice sustain...perfect for spaghetti westerns (it's no surprise the Jen is Italian).

And lastly, I believe the orig transistors had gains in the 200 range. If you use much higher gains, the tone will be even thinner, harsher and more trebly, which was never my choice.

[saint44]

thnx for clarifyin Lucifer, you are describing exactly what i hear.  The sustain is great.  i just figured it might have  a meatier, more bass along with the grit, the low ends seems cutoff.  and buttery was my goal  , does it make sense what i did for the Q2E ground?  Do you think putting 1n4002 diodes in place of the 914 will smooth the output?  also, what else would you suggest for a transistor in the low gain range (200), i looked locally for 239C but not avail.?

[R.G.]

A couple notes. Q1 is not setup like a standard amplification stage. I believe it's there for impedance matching.

Correct. It's an emitter follower, intended to prevent loading the incoming signal, and to drive the following stage harder.

Q1B didn't wind up one voltage drop (.5 - .7) above Q1E, at least on all my builds, it's less. I asked about this years ago on this forum and the answer I got was that it's a poorly designed emitter follower (EF) stage. Someone suggested the culprit was the 1M.

The problem was that the 1M makes measurement of the base-emitter voltage difficult for the meter, because the meter loads down the 1M. As a bit of EE 102 (not 101) you can usually get a decent measurement by measuring a lower-impedance point - like with the meter leads directly across the base and emitter. This is a much lower impedance measurement, and will almost certainly come up with 0.5-0.7V. All that's to say - the B-E junction has the 0.5-0.7V difference, it's just hard to measure.

Secondly, the sound of this thing is smooth, thin & trebly with nice sustain...perfect for spaghetti westerns (it's no surprise the Jen is Italian).

And lastly, I believe the orig transistors had gains in the 200 range. If you use much higher gains, the tone will be even thinner, harsher and more trebly, which was never my choice.

If you look closely, this is a Fuzz Face-like circuit (Q2 and Q3) with a buffer in front of it (Q1). The sound is quite different if you remove the 0.01uF from the emitter of Q1 and input the guitar right into the open lead of the 0.01uF into the base of Q2. It gets bassier yet if you make this cap not 0.01, but 0.1 or even 1uf.

And yes, high gain silicons do make for harsher clipping.

The odd biasing on the first emitter follower would be much improved by putting a second 1M from the base of Q1 to ground.

[saint44]

thnx again,

I really appreciate all the knowledge and experience y'all have and passed on.  Much appreciated.



Bri

[LucifersTrip]

RG:
>The problem was that the 1M makes measurement of the base-emitter voltage difficult for the meter, because the meter loads down the >1M. As a bit of EE 102 (not 101) you can usually get a decent measurement by measuring a lower-impedance point - like with the meter >leads directly across the base and emitter.

Thanx for this piece of info. I did a lot of experimenting with that stage back then, including adding a B to ground resistor. Nothing made anything significantly better or improved the overall sound.  Next time I have one open, I'll check across B-E.

saint44:
>Do you think putting 1n4002 diodes in place of the 914 will smooth the output? 

I'm not sure what you mean by smoother. Just  experiment with different diodes and input caps until it's where you like it. The biggest
difference will be with ge diodes.

>also, what else would you suggest for a transistor in the low gain range (200), i looked locally for 239C but not avail.?

my cheap modern go to's in the (200-250 range) are usually the cliche 2N2222, 2N3904, which Radio Shack has.

but I usually go for the 150-200 range for the vintage stuff and use 2N5550 or 2SC1815

http://www.taydaelectronics.com/2n5550-npn-transistor-160v-600ma.html
http://www.taydaelectronics.com/2sc1815-c1815-transistor-npn-50v-0-15a.html

[saint44]

sweet,

so at the end of the day, the Q2E ground was the issue, which is because i did not verify the schematic with the layout (PCB)?  or did i just bypass the problem?

so much more knowledge ... will be starting a new project soon then

and thnx for heads up for Trannies

[PRR]

> the 1M makes measurement of the base-emitter voltage difficult for the meter, because the meter loads down the 1M.

Meters suck. Cheap needle meters suck hard. VTVMs and most DMMs suck like a 10Meg resistor.

If you poke a 10Meg resistor around 1K and 10K resistors, the suckage is very small. That's why we like VTVMs/DMMs with 10Meg inputs.

Picture this plan with 10Meg resistors stuck here or there. What changes?


http://i.imgur.com/nC2HddN.gif  -new tab

In A we are not poking. These are the voltages that exist with no meter loading.

In B we poke the 10K emitter resistor. The voltage drops. But not much. By transistor action this is really a 5K point so a 10Meg makes it 0.995:1 of the un-loaded voltage.

In C we poke the 1Meg resistor. By simple 1Meg/10Meg divider action we expect around 10% change, and we get a little more.

NOTE that the emitter also falls. But if you only have one meter, you can't see that.

In D we found another meter and read both at the same time. Both read low (mostly cuz of heavy loading of the 1Meg) but we do compute a 0.6V difference (R.G. is right).

In E: IF you have an UN-grounded meter (not a VTVM; a battery-powered FETVOM or DMM), you can connect as R.G. says "with the meter leads directly across the base and emitter". (Actually if the pedal is on battery and has no connection to external grounds, you can put a wall-power VTVM this way.) Now we find the 0.6V difference we expect. (It may be 0.599V, but this isn't rocket science.)

> The odd biasing on the first emitter follower would be much improved by putting a second 1M from the base of Q1 to ground.

IF the hfe is sure to be 100-200, this plan biases-up with 4.2V-5.6V at the emitter. These are fine values. Un-loaded we'd aim near 4.5V. Loaded, there's a struggle between the 10K emitter resistor and the ~~~10K going into the next stage (can be low when Q2 is socked), so we'd aim somewhat higher.

If Q1 hfe is way higher, like 600, then it all sits up near 7V and there's no up-side headroom. The input cap should charge-up on positive peaks and do some "compression", but I suspect with these values it will be violent (harsh) and fussy about what drives it (hi-Z source won't charge-up the input cap).

AGREE that designs should be hfe-tolerant. And especially with a high-hfe part (emitter voltage over 6V), 1Meg base to ground is the simple fix.

[saint44]

Beauti... changed the first emitter follower cap? from .01 to 0.15 and changed trannies to 2n5550 .... gota bit of bass back nd better fuzz...just what I ordered ... as for a betteR VM.... suggestions?

[saint44]

Thnx PRR

[LucifersTrip]

thanx for this explanation...though, I'm slightly confused.

In B, I place DMM (Metex 3800) black lead ground & red lead  E and read 4.18V?
In C, I place DMM black lead ground & red lead  B and read 3.98V?

So, it will appear (falsely) that B voltage is not only not a diode drop above E, but actually below?

My actual readings were:
EBC
Q1 > 5.96, 6.12, 9.04

Edit:
pulled out my notes for the build with above voltages. I used a PN2222 with hfe 220

For saint44:
On those notes, I marked that increasing the BC cap from the .0012uF (1200pF) on Q3 to either .002uF or .003uF reduced harshness (obviously) and
smoothened it out...and that .005uF was too much.

[saint44]

hey Lucifer, i did change out the 1200pF cap to 2700pF cap ... more like "butter" .. hahahaha ... but as i was searching today i found this ... http://www.diystompboxes.com/smfforum/index.php?action=printpage;topic=92881.0 ... and this is exactly what is happening for me when i crank the gain up ...   not a problem though ... i prolly wont crank up to max anyways, but did you eventually solve it?

[LucifersTrip]

more like "butter" .. hahahaha ...

butta

what is happening for me when i crank the gain up ...   not a problem though ... i prolly wont crank up to max anyways, but did you eventually solve it?

just take a hint from the mkii. simply sub a 500 ohm trim for the 100 ohm and set it to the smallest value that there's no oscillation
http://forum.musikding.de/cpg/albums/userpics/16602/MKIIOC75schem.png