another fuzzface silicon transistor question. Various BC10x

[brianh]

I've tried a bunch of random NPN silicon transistors from Smallbear in my faceface build.  So far I think the BC108 workalike sounds best to me, though I don't hear that much difference between them all.  I haven't tried the BC108B, BC108C, BC109, BC109C (all work-alikes) or real BC109 NOS. 

Would any of them give a bit more thick fuzz tones than the BC108, even with a little more "spittyness?"  It's cheap enough to experiment but it's getting pricey!  Just looking for some guidance.

thanks
brian

[BAARON]

The BC10x with an A or no letter after it is the lowest gain of the family.  BC10xB are higher gain, and BC10xC are highest gain yet.  I don't think that a higher gain transistor is going to make the sound thicker Or more spitty, because a BC108 is already pretty high gain compared to an original fuzz face and going higher is just going to get farther and farther from that sound.

Have you tried adjusting the collector resistors on Q2 to change the bias voltage at the collector?  That'll affect the sound of the fuzz quite a bit too, ranging from sputtery to smooth, depending on whether you bias it at half the supply voltage (smoothest) or farther from there (nastier).

If you're looking for that really big, fat, smooth, muscly tone associated with Hendrix and Johnson, I would also suggest running your fuzz through the overdrive channel on your amp rather than the clean.  (Jimi apparently ran his Plexis at full-tilt, with all knobs turned to 10 most of the time when he played live, and got most of his tonal changes with his guitar's controls.  I wasn't there to see it first hand, though, so I can't verify it...    Meanwhile, Eric Johnson uses his silicon FF (both knobs set at 10) through a Plexi with the bright and normal channels jumpered together, with the controls set thusly: vol1 9, vol2 10, treble 0, mids 3, bass 5, presence 0.  In other words, dirty and dark.)

[brianh]

Very helpful, thanks.  I've been running it thru a Hiwatt DR103 clone dimed, but not yet tried it with my JTM45 build.  I have the bias pot installed and keep it dimed to the "right."

[BAARON]

You might want to check the bias voltage with a multimeter and see what it is with the bias pot cranked over to the right, as you say.  I tested a FF on my breadboard with a 10k bias pot once, and discovered (in the end) that I actually needed 12k in order to bias it properly... oops.

Do try it through your JTM45!  Pairing the right distortion/fuzz/overdrive with the right amp can sometimes bring out the charm in a pedal that would otherwise seem awful, and knowing how to set the controls right (on the pedal And the amp) can make a world of difference toward getting the classic tones we expect out of a piece of gear!

[zombiwoof]

Yeah, just turning the bias pot all the way to the right is not the way to go, you need to check the bias voltage.  If you can't do that, set the knobs on the FF where you usually do, and hook it up upside down with the bottom plate off, so you can adjust the bias pot, and play through it till you find a spot that sounds good, and cleans up a bit when you turn your guitar volume down.  In other words, bias by ear.  If you don't get a good sound this way, something is wrong, you either have way out of spec trannies or not enough resistance on the bias circuit.  You could try adding a small resistor in line with the bias pot (which most people do, anyway, to get a good range of bias).  If you have a 10k bias pot subbed for the 8.2k resistor, try adding a 2.4k or slightly higher resistor in line with it, to give you enough bias range to dial it in.  I take it you're running it into an already distorted amp, but if you run it into a cleaner channel, putting an OD after the FF helps to mimic the "FF into a cranked amp" scenario.  Anyway, getting the bias sorted out is the key IMO.

Al

[doitle]

The BC10x with an A or no letter after it is the lowest gain of the family.  BC10xB are higher gain, and BC10xC are highest gain yet.  I don't think that a higher gain transistor is going to make the sound thicker Or more spitty, because a BC108 is already pretty high gain compared to an original fuzz face and going higher is just going to get farther and farther from that sound.

Have you tried adjusting the collector resistors on Q2 to change the bias voltage at the collector?  That'll affect the sound of the fuzz quite a bit too, ranging from sputtery to smooth, depending on whether you bias it at half the supply voltage (smoothest) or farther from there (nastier).

If you're looking for that really big, fat, smooth, muscly tone associated with Hendrix and Johnson, I would also suggest running your fuzz through the overdrive channel on your amp rather than the clean.  (Jimi apparently ran his Plexis at full-tilt, with all knobs turned to 10 most of the time when he played live, and got most of his tonal changes with his guitar's controls.  I wasn't there to see it first hand, though, so I can't verify it...    Meanwhile, Eric Johnson uses his silicon FF (both knobs set at 10) through a Plexi with the bright and normal channels jumpered together, with the controls set thusly: vol1 9, vol2 10, treble 0, mids 3, bass 5, presence 0.  In other words, dirty and dark.)

Maybe try the BC516 if he really wants higher hFE...

[wavley]

 I like a BC141 in Q1 with an AC127 in Q2.  Of course they are in sockets and really depends on the day because right now I'm using two Ge devices.  The BC141 is dark and fuzzy

[brianh]

Anyone have a quick bias how-to?  Couldn't find it doing a search.  I'm a pedal newb, so please forgive me.

Here's the guts.  Since the pic I removed C4 and added C6 + D3.

[petemoore]

  Test the battery voltage, take notes...
  Clip the Black DMM lead to a ground [chassis should work].
  Touch the red DMM probe to the transistor leads.
  Q1 emitter should read exactly 0.0v and = Gnd.
  Both of those transistors have 'pricetags' on their hats marking where the emitters are.
  Move to Q1Base [should be = to the transistors diode drop voltage above emitter].
  Then Q1Collector [about 1.15volts...or something like that.
  Q2 emitter...above yet near ground
  Q2Base [being connected to/should read exactly the same as Q1C...
  Q2Collector...when this ones somewhere near 1/2v [4.5vdc is 1/2 of 9vdc], the transistor will have room +/- to swing the signal +/- starting from a bias point near ''from the middle of the supply voltage'', without hitting one or the other power rails harder [Gnd or V+].
  Use 1/2vdc or 'V-bias' in this case as a way to set the FF up initially [take voltage measurements to determine if it can be biased as is], tune that bias carefully by ear...let notes ring out with the guitar volume turned down some, try using the 'gain knob on your guitar' [with a well tuned FF, the guitar volume controls gain/fuzz] and knobs on the amp and FF volume...all interact.
  I like 4k7 added to 10k linear pot for setting bias as FF Q2 collector resistor...if I need less than 4k7 or greater than 14k7 [the R range of a 4k7 + 10kpot circuit], I start hunting for the other transistor that'll bias with R's that look like sorta stock FF values...that or I have to start debugging. 

[brianh]

Beautiful, thanks!  I will try that.

Anyone try the "hendrix" mods from Geofex?

Mods and Magic
OK, what about things other than the transistors? Roger Mayer is reputed to have tweaked Jimi's gear heavily, so maybe there's some tone to be wrung from the stones there.
There is a set of changes collectively referred to as the "Hendrix" mods or the "Roger Mayer" mods. These are

Replace the 470 ohm output resistor with 1K
Replace the 8.2K resistor at the collector of the second transistor with 18K
Replace the 1K control in the emitter of the second transistor with 2K.

What can be expected from these changes?

[petemoore]

Replace the 470 ohm output resistor with 1K
  It boosts output, and makes my volume control be set lower, the series resistance of the volume pot [when set a bit lower] takes a touch of highs off.
Replace the 8.2K resistor at the collector of the second transistor with 18K
  IIRC that's the schematic using MPSA18's...
  Between 4k7 and 14k7 [above, I described 4k7 + 10pot] is a good range to try various transistors, allows fine-tuning of the bias pot.
Replace the 1K control in the emitter of the second transistor with 2K.
  I would leave the 1k control there, and put about 800 ohms between it and ground to 'bump' the gain R range up [that'd give you between 800ohm and 1800 ohms [800 - 1k8] range.
  The gain of the transistors, the gain pot value, the guitar volume setting...all this stuff, rebias and try everything after a transistor swap, try to stop swapping when you've discovered you can clean up at the guitar volume, start working wildly with every knob you can see that your signal passes through [on the amp, fuzz, guitar].
  Sorta low-ish gain transistors should fuzz-hard through clean up @ guitarvol., the gain knob can then be used to work with them.
  ...or see 'Many faces of Fuzz'' and examine the various ways available there to control gain/frequency around the gain knob.
 

[brianh]

Thank you again, you've been incredibly helpful.  How would I go about tacking on a 4.7k to the trim?  Can this be done under the board?

To add the 800 ohm from the 1k pot to ground, which "pot lug" am I connecting it to (F2, F3?)?  I assume I can tack this from the board to a ground point?

Thanks again for indulging these very newb questions.

brian

[BAARON]

The resistor needs to go between lug one and ground: currently, the wiring diagram attaches lug one to ground by running a wire from the fuzz pot to lug 1 on the volume pot.  You can just replace that wire with the 800R resistor.

R5 (1k) is already in series with the trim pot, so just replace it with a 4k7.

[brianh]

Thanks, Baaron!

[doitle]

I'm disappointed no one offered any input on my suggestion...

[brianh]

Just realized I don't have a 470 ohm resistor in my clone - should I replace the 330ohm (R4) with 1k?

thanks

[BAARON]

You can replace R4 with a 1k, yes.  It'll give you a little more output volume if you do.

R5 should still be replaced with something between 3k-6k, I'd say.

[brianh]

Thanks again, everyone.  Did the mods tonight aside from the 330ohm --> 1k, I'll get to it.

Biased it to 4.5v which I found was right in the area of what my ears liked.