Hipster questions

[trickpony]

Just built the hipster with BC550 trannys and have a few questions.

These three items are from the build docs:

1)R1 is optional, and may not be needed.
    Ok, so what is the downside of putting this in, does it change the tone at all? I can't tell the difference.


2)C2 can be increased if oscillation occurs at maximum fuzz. Incremental increases in value will result
in incremental decreases in treble. Suggested alternate values are 22pF, 47pF and 100pF.
   So, when I turn past about 95% I start getting oscillation. I started with 10pf and still have the same oscillation with 100pf.....I haven't really noticed a change in treble except at the low fuzz side of the pot. Should I just keep increasing up to 1n?


3) Adjust the BIAS trimpot until you read approximately 4.5v on the collector of Q2. Note that 3.5v – 5.5v
is the nominal range so feel free to experiment with your bias setting. 
     This is very sensitive to adjust, I finally managed to get it to 4.6....What would changes in this typically do? Could I solve my oscillation problem by playing with this?



I am also going to try some 2n3xxx and see how they sound.

Thanks,
Ben

[midwayfair]

First, required reading for this circuit if you haven't already seen it:
http://www.geofex.com/article_folders/fuzzface/fffram.htm

Bookmark that site, you'll be heading there a lot in this hobby!

1) R1 is a pulldown resistor, very common in pedals to prevent DC from building up on the switch and causing switch pops. It does not affect any meaningful frequencies on the guitar or the harmonics associated with notes played on the guitar. It can slightly affect input impedance, though not in a way that's an issue with a Fuzz Face.

2) BC550 are actually fairly high gain for a Fuzz Face. It's quite possible that increasing C2 won't fully solve the issue, though I must say I've used higher gain transistors and not had them oscillate with a 100pF cap there.

Play with the bias (see below) -- you might find that the problem is fixed by running the voltage up. It depends on which transistor is oscillating, but usually it's Q2.

There's something you can do if you're willing to go offboard:
1) Place a ~100R (that's 100 OHMS) resistor in series with the gain pot. To do this, you have to desolder lugs 2 and 3 of the gain pot, solder the resistor into the spot for lug 2, and then solder the other side of the resistor to lugs 2 and 3 of the pot itself.

This creates a maximum amount of gain. If you want to get really spicy, you can measure the pot at exactly the point the oscillation occurs and use the closest value resistor.

3) The "ideal" setting for the trimpot, assuming all other voltages in the pedal are also "ideal," is exactly 1/2 the supply voltage. It is indeed very sensitive to adjustment.

Supplying more voltage means that the transistor has more voltage to play with, and thus more headroom. You may find that increasing the voltage tends to fatten the sound some, too.

Supplying less voltage can cause gating, spitting, and so forth. It can also cause aliasing -- and some measure of oscillation. But some transistors sound cool this way.

[BraindeadAudio]

Made one that sounds absolutely deadly with BC550 in 1st transistor and a hand picked 2n3904 with HFE of 375 in transistor 2. For fun I took a switch and wired 3 different PF values, 10 in the center, 100 on the right, and 220 on the left for changes in sound. I sort of like the tone I get with the crazy oscilation for blasted out HC tone.

[trickpony]

Wow, thanks for the replies. I tried out various configurations of the 2n3904 and bc550 using my 9 yr old daughter as a judge, and we both liked the 2 bc550s. Sounds punchier to me. Ended up with the 100n even though still have a little weirdness there...but am trying to embrace it as BrainDead said (Right Said Fred?)

I did manage to kill the oscillation with 220N but it did deaden the sound for me. Putting a switch for the different values would be cool, but I cant fit it in the tiny box.

Bringing the voltage up did also solve the oscillation but made the sound too ummm...big I guess.

Didnt try out the resistor in the gain pot...that will be next.

Thanks again for the help.
Ben