Wednesday, 23 January 2013

Briggs' 'Face

Simple, yet good sounding booster/OD, emulating the sounds of Black/Brown/Silver-face amps. This one can be run from  4.5V all the way up to 18V. So You could of course run it with 9V, but consider using a charge pump to double it up to 18V. And yes. I do believe it'll work well with 5K trimmer :)

From Mr. Briggs: I really like this one, it's a booster/overdrive with a nice Fendery feel and voice to it. As you can see there are some values which you can alter on the schematic that allow you to tailor the tone to your exact needs. Firstly the supply voltage - the lower the voltage the less overall gain and clipping you'll get, more voltage = more gain and clipping. The jfet choice also makes a difference to the tone of the circuit; 2N5457s give more gain but are more compressed sounding (better for the Fender "tweed" emulation) than the MPF102s (better for the larger Fender "twin" type emulation) which have more headroom but less gain. You can use almost any jfet type in this circuit 2N5458, J201 (I didn't like these, they were too compressed and gainy - almost fuzzy!), you just need to find your type! If you want to learn a little about jfet's and their properties check out this article over at runoffgroove: http://www.runoffgroove.com/fetzervalve.html. Finally the 3n3 tone cap can be altered to your preference, a higher value will lower the high end gain, a lower value will allow more high end drive.

In my final build I'll be using MPF102 jfets running at 18V with a 4n7 tone cap to keep the high end under control. This setup provides a nice "cranked twin" tone and is really dynamic and reactive to your playing, a decent low end is retained while it still fizzes out at higher gains.



27 comments:

  1. ah, nice! Glad I came around when I did...

    One question on this; the charge pump will be downline of the v+, correct? so power from battery/wall >charge pump > v+ into circuit?

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  2. Wait, what's the bias switch for? I get it if there's a bias knob but switch?

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    Replies
    1. In the schematic it's called "bias". I really have no clue what Briggs meant by that...
      +m

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    2. It just increases gain and adds a bit of bass.

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    3. Well if it does both those things, wouldn't it make sense to just externalize it with a full size pot?

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  3. What would be fantastic is to mod this circuit to fit 3 or 4 defined Fender sounds with a rotary switch.

    Unfortunately, I have neither the experience with the specific sounds of the different Fender amps, nor am I particularly gifted at vero-boarding these circuits :/

    I mean, I could see (and even I can see that this would be far more duplication than necessary) creating 3 or 4 little boards, since they are pretty small, configuring each to a different Fender Amp sound, and feeding the inputs through a rotary switch, and then to the stomp switch, etc, but that's a pretty brutish way to accomplish it.

    Anyway, cool looking circuit. As soon as I get my parts restocked (I think Futurlec sends their stuff by carrier goat), I'll be building this one.

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    Replies
    1. I was just thinking about that too. BeavisAudio did a cray-zee "screamerlab" project where he literally stacked 3 ics, one on top of another, soldered the respective legs to power and ground, then made a pile of switches go to and thru each IC so he could swtich between the three Tube Screamer selects for IC voice. He did this for diodes as well. The project is effing nuts, and I don't think he ever finished it.
      With this I imagine it could be something as simple as a rotary switch with the different fet pairs. If you wanted to do two, maybe just set them up on a switch.

      I'm looking forward to seeing the Azabache get tackled, if it'll fit on a board. I think they have it on the to-do list.

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    2. About the to-do lists - I think so too :)

      I'm guessing that you could get pretty close of different amps by just playing around with cap values. That would give you slightly different frequence responces, and those would be somewhat doable with a rotary switch...
      +m

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    3. Kinda like the idea behind the Azabache, by Run Off Groove. On the other hand, this circuit by Fred Briggs is so easy to build and so low in parts that you could simply build 2 or even more of them, with different JFET combos etc and have them in a 125B/1590BB enclosure with their own footswitches, much easier IMO.

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  4. No luck with this one, total silence so far. Can't find any solder bridges; cuts are ok; layout follows the schematic from what I can tell. These are voltage readings:

    Q1. D. 1.29v
    S. 1.03v
    G. 0v

    Q2. D. 9.05v
    S. 4.42v
    G. 1.29v

    It's probably staring me in the face.any help would be appreciated!

    Thanks, Mark

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    Replies
    1. Does the trimmer do anything?
      +m

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    2. Still no sound

      Fully CCW
      D. 0.83v
      S. 0.68v
      G. 0v

      D. 9.05v
      S. 4.15v
      G. 0.83v

      Fully CW
      D. 8.77v
      S. 1.96v
      G. 0v

      D. 9.05v
      S. 8.77v
      G. 8.77v

      Hope this throws some light on it. Cheers!

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    3. I'm not sure, but i think the trimmer should adjust the Q1 drain for that ~4.5V. If you turn the trimmer very carefully through the sweep - it should work when it hits the right drain voltage. Usually the point where it works is very tiny part of the sweep. Not sure, but that's how i percieve it. As it seems you're getting everything between 0.8V and 8.7V at the Q1 drain.
      +m

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    4. Aww... I think there is a mistage in the layout. It's not the drain but source which is supposed to have that 100n connected. Try it. I'll fix the layout...
      +m

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    5. Nice one, Miro! Jingle Jangle here we come! Working and verified with your fixes above. Many thanks. My wife thinks I'm weird, running to my soldering iron at this time! Oh, well!

      Mark

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    6. Thanks for verifying Mark! And i'm glad i noticed that. Normally one would assume that it's always the drain that has the output, but not with these :)

      How come weird? Just last night it was 2:15 in the morning when i cut two boards :)
      +m

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    7. 2.15am! Excellent, Miro you are a legend!!

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  5. Couple of Questions,

    1) Am i able to omit the bias switch?
    2) Will running this into a condor sim work?

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    Replies
    1. for the 2nd question what i mean is, where do i place this, before or after the condor? or do i need something between this and the condor?

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    2. 1) Sure.
      2) The condor would come after.

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  6. Just got this to work properly, a couple of note/questions:
    - the 1M volume pot caught fire a couple of times, added a 1k resistor between volume 1 and ground and it seems ok (for now), wonder what happened since no one that built this had this issue;
    - the cleans are amazing, but sound a little compressed. used two mpf102. do they need to be matched? also, if I run this at 18v, will it sound less compressed or it will just add more gain to it?

    thanks for the layout Miro =D

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  7. I built this pedal and got silence. I worked out any possible problems to no avail three times. I rebuilt it on a new board with new components. I am sure that I have done everything right, as I have nearly memorized the circuit by now. My only variation is that since I could not find a 4k7 trim pot, I used a 5k. Could this be my problem?

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    Replies
    1. The trim pot won't be the problem, at least assuming you haven't just left it in one position and have turned it to make sure you're getting no sound throughout the trim pot sweep?

      Measure the voltages at the transistor pins to try to give us a clue to where the problem lies.

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  8. I have turned the trim pot each time I tested it after adjustments. I'll get back to you with a voltage reading soon.

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  9. The good:
    Great effect. Even more simple to build than a Peppermill and does the work just as well for those Fender-ish tones. Using a 5457 and a MPF102 sounds quite close to my modded Peppermill, but with quite a bit more gain. Dual MPF102's are my favorites so far and I see not reason why trying this @18v cos' gain is plenty. I'll probably get rid of the tone control, which is backwards on the layout ;)

    The bad: never had so much trouble building a pedal: Found I had a bad footswitch, then bad JFET LOL! And either I have a bad trimmer or that's just the nature of the circuit, but there's a very narrow bias adjustment margin for it to work - anything outside 4.5v-6v and I get no sound.

    Thanks everyone for making this available - Fred Briggs and Miro!

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  10. First attempt at building one of these, and no sound. Checked the gaps between strips with a voltmeter, and knifed 'em for good measure. Here's my transistor readings:

    trim pot fully CCW
    Q1 2N5458
    D 9.14
    S 8.28
    G 8.29

    Q2 2N5458
    D 9.15
    S 9.14
    G 9.14

    trim fully CW
    D 8.31
    7.46
    G 7.48

    D 9.13
    S 9.13
    G 9.13

    Two changes I made, due to lack of parts at hand: the trim pot is 10K, and the 47pf is a 100pf. Would either be enough to make it not work at all?

    Weirder to me is that the values are hardly changing, and look nothing like mah62's, whose worked once that error was fixed.

    Any help or suggestions are appreciated. thanks,

    john

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