Home Forums General DIY Pedal Discussion A Fuzzy Sunday Drive with a Fuzz Face Circuit

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  • #18436
    Cybercow
    Moderator

    Finally got around to assembling and testing the four PUFF PCBs I’ve ordered from Barry over the past year. After populating the boards and setting u the test jig, I went thru a rather large selection of Ge transistors for testing.

    What an adventure.

    For clarity’s sake, the charge pump is being used merely to invert the DC supply polarity. So this is just a 9v circuit. And the test unit is an “Ultra Tester” from FuzzDog.

    Tried out the following transistor types . . . .

    MP16B

    MP20B

    AC125

    AC128

    AC188

    2N404

    2N1305

    2N1309

    NTK275

    OC42 (NOS Mullard)

    OC44 (NOS Mullard)

    OC71 (NOS Mullard)

    OC75 (NOS Mullard)

    OC602 (lo gain NOS Mullard)

     

    I was truly surprised at the results. I tried several low gain matched sets, hi gain matched sets, low (at Q1) and hi at (Q2), hi @ Q1 & low @ Q2, and ratio-matched sets for standard FF circuits. All combinations and types (except the MP16B and MP20B Russian transistors) had bias sweet spot at about 15K Ω.

    The Russian MP16B & MP20B transistors worked “well” no matter the bias setting. At the minimal bias setting they were a bit “sputtery”, but as I turned up the bias towards the max setting, they cleaned up nicely giving some great sustain and warm fuzziness. So for the MP16B & MP20B transistor sets, I will make the bias pot a user control for the user. Lots of great tonal changes in the sweep of the bias pot with these trannies.

    All of the Mullards exhibited an interesting behavior with the bias above ≥12K Ω. At ~12K Ω, they show their sweet spot. (Below ~12K Ω they sounded like garbage.) Above 12K Ω they begin to clean up with less “dirt”, delivering more clarity of the natural guitar with greater sustain when the ‘Fuzz” control was turned up.

    Surprisingly, all the ACxxx, NTK275 and others sounded and performed near identically with their bias sweet spot at about 15K Ω. Also, none of the higher gain combos, or with a higher gain tranny in the Q1 – OR – Q2 position exhibited any increase in volume compared to the others. To my ears, the low gain tranny sets were just as loud as the hi gain transistors.

    The only transistor sets that jumped out at me were the Russian MP16B & MP20B sets because of how sounded at any bias setting. Ergo, putting the bias control on the front panel.

    Of course, all this testing and reported results are completely subjective using the schematic in the attached photo – a GuitarPCB “PUFF”. The only change I made to the circuit was to use a 1N5817 in series, instead of the designated 1N4001 in parallel (to ground from the +9v) for polarity protection. (Personal protection preference.)

     

    Just wanted to share my experience exploring this circuit with several different PNP Ge transistors. Thanks for reading. Happy building!

    #18437
    Barry
    Keymaster

    I am going to have more follow up questions later when I have a chance.

    Also I am working on a GuitarPCB testing platform for future release.

    #18440
    Cybercow
    Moderator

    Because these are pretty much standard Fuzz Face circuits, I’ll be applying another mod with a couple of other considerations. The first mod that will go into these four PUFF builds is Guitar Pickup Simulator. It’s just a 42TL019-RC x-former with only the outer legs of the primary side of the x-former in series with the input, followed by a 330pF cap to ground, BEFORE the 1M Ω dropping resistor. This will allow the guitar’s Volume control to clean-up the dirt somewhat with a handy adjustment – even after a buffered pedal. This only works with pedal still near the front of a chain and works well after a wah or other buffered pedal. I’m also going to experiment with trading up the 30K trimmer with a 50K pot so that the user can access the Q2 bias from the face of the enclosure. The 2nd consideration is to have R2 (the 33K resistor) swapped out for an internal 50K trimpot for more explicit bias control of Q1. The third and final consideration is to the remove the B1K “Fuzz” pot replaced with 1K to 2.2K fixed resistor – although this mod is least likely to remain in a finished build. Finally, I will experiment with a 1K value at R3 in place of the 470Ω resistor. Some of my research has suggested increasing the resistor to 1K increases the volume a bit. At any rate, here is the modified schematic . . . .

    Any and all thoughts, consideration and/or other points of view are welcome.

    #18463
    Cybercow
    Moderator

    And now it has the candy-wrapper graphics done . . . .

    #18473
    Big O
    Participant

    Love the idea of a testing platform Barry.

    I have 3 Fuzz Faces.  Two are from BYOC and another is a Hartman pedal with variable input impedance (I believe) to match pickup strength – that is low output to high output pickups.  The two I built are slightly different.

    The first is a stock NPN circuit with NOS AC128 Germanium transistors and an external bias control, which is very handy.  When I first built the pedal, it didn’t work at all.  I was about to give up on pedal building as this was my first attempt and I spent many hours trying to debug my build job.  Finally after socketing supplied low HFE S9013 transistors, the pedal roared to life.  I found out that one of the transistors guaranteed to be in working order, did not.  The other was fine and worked well paired with one of the S9013’s.  I contacted BYOC and they sent me a new matched pair and the pedal worked fine.  Pedal building almost became a dead end hobby for me, but sorting out the problems and finding I could perform diagnostics and “fix” a problem got me hooked.

    The second FF is a hybrid Ge-Si NPN using the leftover Ge transistor from the bad set and a low gain S9013.  I just ordered a PCB from BYOC and built it with components I had on hand.  I honestly can’t tell the difference between the Ge-Ge pedal and the Ge-Si pedal.  They sound exactly the same.

    #18484
    Cybercow
    Moderator

    Barry, are those “Miller Caps” across the collector and base of the transistors? They are in none of the standard FF circuit schematics I’ve ever found.

    #18507
    Chuck
    Participant

    One of the Beavis mods was putting the 470Ω resistor on one side of a DPDT and a 1K resistor on the other for a switchable boost. Making the bias an external pot can be real handy for resistors that are a little fiddly and need frequent tweaking for warmer or cooler temperatures, different pickup outputs, etc.  The 220p caps on the PUFF are optional for noisy transistors.  The Build Doc refers to them as hiss filters and says they can be left off.

    Chuck

    #18508
    Barry
    Keymaster

    One could incorporate a 2 Knob Job here on several fronts and make it all happen with your feet.

    #18509
    Barry
    Keymaster

    On another note you can have more stability by creating a “Hybrid Set”

    The Foxrox CC comes to mind:

    As well as the recently released Dunlop Hybrid featured in “The Pedal Movie” and hand-wired by Jeorge Tripps.

    This uses a Russian GT308B Germanium in Q1 and a 2N5087 PNP in Q2.

    Nothing hard to get there but if you want one you will need about $2,000.00

     

    All of that said there is absolutely nothing wrong with Pure Silicon PNP Sets.

    #18512
    Cybercow
    Moderator

    OK, so I’ve been up and at a hybrid germanium\silicon PNP transistor pairing test adventure with the stock PUFF build.

    Here’s what I got . . . .

    1. Q1: 2N404 @ 46; Q2: 2N404 @ 55 – Sounds good with the bias ~14K. With bias above 14K it cleans up only a little bit. With bias below 14K it gets farty and dies at lower values than ~12K.

    2. Q1: 2N404 @ 46; Q2: (Si) 2SA970 @ 435 – Sounds really good with the bias ≥13K. With bias above 13K it cleans up a good bit retaining some excellent bite and sustain. With bias below 13K it gets sputtery and dies at lower values than ~11K.

    3. Q1: (Si) BC560 @ 500; Q2: 2N404 @ 55 – Sounds OK with the bias ~14K. With bias above 14K it cleans up only a little bit. With bias below 14K it gets farty and dies at lower values than ~13K. (I cared the least for this combo.)

    4. Q1: 2N404 @ 55; Q2: (Si) BC560 @ 500 – Sounds OK with the bias ≥14K. With bias above 14K it cleans up only a little bit. With bias below ~14K it gets sputtery and dies at lower values than ~13K. (I feel the same about his arrangement as I do #3 – meh.)

    5. Q1: (Si) BC560 @ 500; Q2: (Si) 2SA970 @ 435 – Sounds terrific with the bias ≥12K. With bias above 12K it cleans up a good bit retaining some excellent bite, a bit brighter (but darker sounding at lower bias settings), sustain and some surprising tonal change. With bias below ~12K it gets sputtery and dies at lower values than ~11K. I recommend this transistor paring with a 12K resistor between TR1 and the collector of Q2 – and replace TR1 with a 10K user accessible pot. I tried some other, lower gain silicon PNP transistors and they all seem to work & sound pretty much the same as germanium in the same gain ranges – maybe a bit brighter with some more crunch while the sustain was comparable to the Ge counter parts.

    I was disappointed that the Ge/Si combo pairing tests didn’t pan out as I’d hoped. But the the pair of hi-gain Si trannies was a pleasant surprise. As always, your results may vary. But the endpoint is to encourage experimentation.

    And here’s another marked up schematic to reflect the changes suggested in #5 . . . .

    #18516
    Cybercow
    Moderator

    And of course, there’s a dedicated candy wrapper for the all silicon version . . . .

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