ff bias

[Gus]

Why do people still adjust the Q2 collector resistor to bias a Si FF like circuit?
Take a clue from one of the the Vox circuits.  Look at the 820 ohm and 1K gain control or the 2K gain control FF like circuits.  Think about what is happening there is a greater emitter voltage at Q2 and adjusting the Q1  DC bias AC feedback resistor 47k, 100k etc (pinch) in another circuit you are adjust the bias of Q1.
Looking at the Vox circuit note any wiper noise if a potentiometer is used as a bias control instead of the 820 ohm is bypassed by the gain control cap (gain control wiper to ground)

Think about what you are doing adjusting the collector resistor.  A circuit using a trim potentiometer should be "DESIGNED" to be at about 50 percent of it revolution at close to the desired setting  AND any noise from the wiper is not bypassed.

Who says 4.5VDC is best?

Who started collector/drain trims?

[artifus]

Who says 4.5VDC is best?

i have to admit ignorance and inexperience in transistor biasing but always took 4.5 as a ballpark figure - it'll get you close enough and wont be too far wrong. it assumes a perfectly 9v power supply, not a well used battery. as prr recently posted in another thread we are breaking a stick in two for leverage. are we considering component tolerance and environmental factors here too? pickups, amp, cable, phase of the moon? do we live in an ideal and thoroughly understood world? isn't 90% good enough?

i'm just musing, here - thanks for posting.

[pinkjimiphoton]

i thought is was more like just half voltage supply to b+ to make the transistor bias?

i'm outta my league here, i think!!

[newfish]

i thought is was more like just half voltage supply to b+ to make the transistor bias?

...as did I, Jimi.  I figured the reason a trimmer was used (instead of a fixed divider) was to allow for the leakage / mojo / idiosyncrasies of each particular device.

You'll notice that a Big Muff has no such trim - and is not picky about what Trannies are used - I know this is like comparing Apples and Oranges, but all fruits are related...

[brett]

Agree with Gus that the biasing "rules" don't make sense, especially if based on the collector of Q2.
Looking back at what people have done since FFs were first made, it seems that a Q2c of 4.5V is designed (i) to make the bias of Q2 "right" and (ii) to give an OK bias of the base of a Ge Q1 in a standard Ge FF circuit. "Right" for Q2 seems to be designed to give the widest non-clipping voltage swing possible on the collector of Q2 (so the clipping from Q2 is reduced and even-sided). 4.5 V emphasises any asymmetric clipping from Q1. But Si FF circuits have much less asymetry than Ge circuits (one reason why Ge FFs have more and sweeter harmonics than Si FFs), so I think that the idea of emphasising the distortion from Q1 is less relevant in an Si FF. Any comments?
This is a very big can of worms....

[deadastronaut]

i like a trim on Q2  for mad trumpety type stuff...

http://www.youtube.com/watch?v=oZY2eFGZC20&list=UUGP0eO8ADt0H9FY5UcHTf8A&index=1&feature=plcp

[pinkjimiphoton]

yah,
but a ff's first stage is an emitter follower isn't it? you could directly ground the emitter for max gain, so no real need to bias the first stage...haven't tried it with a ff, but a couple of other things i've tinkered with, i skip the e resistor all together and it makes for a pretty nice fuzzular.

seems like the bias to the second stage is more to prevent the second tranny from overloading and saturating i thought....by messing with the voltage to it, you can dial in the "sweet spot"...which, to my ears hasn't been 4.5 v yet, usually closer to about 6v...a little louder, and a bit more headroom.

i try to adjust the ff circuit so that you get maybe 3db volume diff with the guitar knob...mainly it controls the saturation of the fuzz sound. in a perfect world, it will sweep from clean (or cleaner) to filth without really getting all that much louder...it's almost more of a fuzz control. if it's "glitchy" in the transistion as you sweep it up on the guitar, the bias isn't right.

you kinda wanna find the spot on the trimmer where it's a smooth transition from 0-10 on the guitar with the ff knobs pegged...
then everything seems to be "tuned in" right so the whole circuit becomes highly interactive.

kinda maybe interesting, i recently built a face for another guitarist. he loved the tone, but said it wasn't quite loud enough...he runs a bunch of dave barber's pedals, but at 18v for more headroom.

so i changed the 500k output pot to 100k....for me, it was LOUD enough. not for him.

so i took it back, and swapped the 330r for 1k, and the 8.2k resistor (which i'd already replaced with a 10k trimmer) for a 50k trimmer...i needed 20k, but hadn't ordered any yet.

now it's crazy loud, it's hard to get the "sweet spot"...i do it by ear, not a meter...so i'm gonna swap that out with a 20k i got from tayda...

off track a little, but i discovered with the 50k pot and a couple of 2222's with a gain around 220 or so, i could dial it in to be an octave fuzz face. it's rude, it's noisy as f*ck, but it sounds almost like the superfuzz i've been tinkering with (which i thought i had screwed up, but now realize is misbiased).

that one trimmer can really do a lot in that position.

that said, i've built a couple silicon variants without the trimmer, and it doesn't seem to matter... i think the whole reason to use it is to be able to set the final volume of the circuit...and the tonal effects are just a bonus?

you can bias a fuzzface anywhere from fuzzy as heck but below unity (around 3.5 v) to an almost "clean" compressor at about 6v. above that , it begins to become more of an overdrive with an octave effect. granted , it depends mightily on which transistors.... and i'm a newb, so i'm probably wrong...

but i think the main purpose of that more than anything else is to try and achieve unity gain. to adjust the first transistor's characteristics, use the knobs on the guitar itself.

like i said...over my head here, i better swim for shore!!

[pinkjimiphoton]

nice demo, rob!!

[R O Tiree]

The clipping in a Big Muff comes from the nose-to-tail diodes in stages 2 and 3. That's why it's not very picky about the transistors you use.

Unless I'm totally off-base, here (sorry - the Devil made that pun) increasing the resistor (within limits) will lower the DC bias point at Q2C, so it would clip earlier on the down-swing and have a little more headroom on the up-swing, at the expense of less output (which can be restored by slightly increasing the value of the other resistor) and vice-versa. Perhaps it's all debatable, given the combined gain of 2 Si transistors strapped together like this - you get a kind of skewed square-wave but you can change the "duty-cycle" (for want of a better term). Pinkjimiphoton's post above would seem to bear this out.

As to whether to insert a trimmer or not... well, to my mind, if all Fuzz Faces sounded exactly the same, then it would get very boring. According to RG's "Technology of the Fuzz Face" article (right towards the end), it might have been Mike Fuller who kicked this off, suggesting a 50k variable resistor at the input (to reduce loading of the source) and a 1k variable resistor in between the 820R and 6k8 to boost/restore the output. As RG puts it, the input VR is a "less" control and the 1k VR is a "more" control. It might be that people have forgotten or ignored the input VR. However it happened, inserting a trimpot in place of the 6k8 to Q2 isn't really the same as that Fuller mod. You need to look at both these resistors leading to Q2C to get the character you want as well as the output level (again, see pinkjimiphoton's post).

All that said, if people want to mess around with it to see what happens, why worry about it? If you know what you're doing, and why, then you can satisfy a customer's needs. If someone wants mad trumpety stuff, then why not? Especially if he wants it to be selectively mad and trumpety? If people are doing it for a clear reason, let them be. If they're just messing around to see what happens, let them be.

[John Lyons]

I believe that Gus is having issue with adjusting at the collector of Q2, not adjusting bias in general.

[Electric Warrior]

Who says 4.5VDC is best?

Q2 c depends on the transistors leakage from what I know. Transistors with low leakage bias lower than 4.5V, ones with high leakage ones higher.

[mac]

Q2 collector voltage depends mostly on Q1 gain and leakage, not on its own leakage.
Finger-heat Q2 only and you'll see a small deviation, after a few minutes do the same with Q1 and Vc2 will go to the roof.
Or swap transistors at Q2 using the same at Q1 and you'll note that even if put low and a high leakage germs, Vc2 will not vary that much.
That's why SiGe FFs are stable.

http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=41446&g2_GALLERYSID=14da711ddc3ee133b5dc7f9252606b54

mac

[brett]

For clarification:
the bias of Q2 affects the bias of Q1 because the circuit is a voltage feedack circuit (AC and DC voltages are fed back to Q1 via the 100k resistor).
with bad Ge devices, leakage current adds to the bias current, but with good Ge and Si devices, all of the bias voltage and current comes from the voltage feedback
an Si FF has a different need for bias voltage and current than a Ge FF. The base-emitter voltage drop is ~0.4 V greater (and there's an additional voltage drop if there's a resistor on the emitter). Also, the cut-off current for Ge devices are much higher. In a good Ge FF, Q1 is turned off A LOT during playing, and for low signals is barely on. For an Si fuzzface to get the same amount of cut-off, the input signal has to higher (this is also affected by the value of the feedback resistor and gain-product of Q1 and Q2).

RG Keen is the friend of everyone messing the FuzzFace, and his "Technology of the FuzzFace" contains 90% of what we need. Other than that, I've found Gus' threads really good - particular his understated reminders that the input to Q1 is a summing/cancelling point for multiple voltages affected by guitar, cable and pedal resistance, *capacitance* and *inductance*. 
cheers