HELLO BRETT -- PIGGYBACK QUESTION

[brian wenz]

Hello Hello --
       Hi Brett!  I didn't want to hijack Joe's Fuzz Face thread with this one so I decided to start a new  thread.
 I put together a mutant FF / Tonebender this  weekend  with a piggybacked pair of 2N3906's for Q1 and a germ for Q2.  The resistor that I used between the emmiters  of Q1 is a 100k.....seems a little high.  How would the overall sound be affected by lowering this res.??    [Go down to 20K or-so?]
thanks!
Brian.

[RedHouse]

I'm not Brett but I can tell ya I've used 220k and 330k to tame some hfe>450 transistors for a FF build piggyback style, I think they sounded great and use it as my default modern FF.

[brian wenz]

Hello Red House--
          22oK ??    Wow.....didn't know it was o.k. to go that high. ...wonder what that does to the gain structure of the circuit??   Mine sounds pretty good with the 100K.....hhhhmmmm..need more info!
Thanks!
Brian.

[will]

Hi Brian,

My understanding of this is the higher the value of the emitter to emitter resistor the more you remove the influence of the 2nd transistor. The higher the value more HFE gain you will get, the less it will act like a germanium transistor. Perhaps you could put in a pot and see where the sweet spot will be for your circuit and the Hfe of the transistors you are using. This resistor greatly affects the overall hfe. You can also experiment with differing transistors.  Good luck.

Regards,
Will

[petemoore]

I used the dual opamp IC socket to piggyback, the emitters resistor between pins 4 and 5, plugging in resistors is easy.
 Also used a 10k pot, plugging it in there, I noticed the most difference between 0k-20k IIRC, ending up with a 2k2 or 4k7 series with 10k pot.

[brian wenz]

Hello Will!--
      How are you??   Thanks for the response.
 I originally had a 100K pot  wired between the emitters and played around with different settings for a day and found out that the bias of Q2 would go up and down accrdingly.   The  voltage on the collector of Q2 would raise up  as I would increase the res. value on my 100K pot [ 20K would get me around 4v on Q2 collector and  and  40K would get me around 5v...etc...] so I just decided to leave it at 100K so I wouldn't have to lower the res. on Q2's collector [it's down to 3.3K].   I wonder if this isn't  due to the fact I'm using a germ tranny for Q2 instead of silicon???
Thanks to you, too, Pete!
Brian.

[brian wenz]

Hello Hello--
    bumpity-bump
b.

[RedHouse]

I'll need to go double check my proto-board, I have a test fixture I made up and it has trimpots for everything so I can dial-in the values that I feel are best sounding.

I agree it seems high so I'll check it tonight when I get home and re-post tomorrow.

I got the same thing as brian did, whenever a tweak was imposed, I had to re-bias.
(but is easy on my test fixture)

[brett]

Hi all.  I'm obviously not spending enough time here in the forum!  Now that my PhD is finished I'll definately be catching up and building some more stuff.  I've got the bits for a Plexi clone - that will be a hoot.

Anyway, what I found with most transistors was that 10k reduced the hFE of the pair by a factor of 2 or 3.  That made pairs of 2N3904 and 2N3906 trannies really smooth and more like Ge trannies.  If you want a BC108 sound, a 47k or even 100k transistor might be cool.

But here's something really odd.  When I used no resistor or small values (like 330 ohms), the hFE was ridiculously low when measured on the meter (e.g. 10), but the sound was quite good in a fuzzface.  Weird!?

Of course, since Philip Bryant "discovered" 2N2369A transistors for Q1, with hFE around 70, the piggyback thing has become less important.  I use BD139s for Q2, which have hFE around 120 to 150.

However, for persistent experimentalists among us, the piggyback thing might still turn up some cool results.  Where could you use a pair of transistors with hFE of 5 to 100 ?  A booster maybe?

cheers

[RedHouse]

Checked my perf-board jig last night ...it was 22k and 33k I had actually used, not 220k and 330k.

Sorry if my mis-info above caused any problems for anybody.

For me the higher resistances worked well for my 2N5087, I have a 50k pot where the emitter bridging resistor goes so I can dial it in.

After I built my first two FuzzFace boards, then had to re-solder in numerous parts to tweak it to be "just right" I decided that sucked, so I built a Fuzzface circuit on perf-board that has all trimpots and sockets for transistors and has reversable power switch (so I can test the PosGnd-vs-NegGnd when needed) and has 3 little 5-way rotary switches for the input, output, and fuzz caps and test terminals to hook up my DMM for bias adjustment and it only took about an hour to build and has saved me countless hours in debugging and mod'ing time. It's really nice just to be able to pop-in a set of transistors and find the correct values of the support components, then solder those into a PCB once and for all.

Not only does it allow me to dial-in my components and test/experiment for the sound I like, When I'm asked to build one for someone else, it lets me sit them down with their guitar and amp and dial-in the sound they like in a FF.

Plus it's nice to be able to use transistors meant for low noise audio applications, while the 2N2369A has the right hfe it is a high-speed switching transistor and switching transistors don't have real great linear regions, which is where we want to dwell for audio applications.
(IMO)

[brian wenz]

Hello Hello--
    Brad or anybody else--
         Did you notice  if  piggybacked trannys introduced any more "hiss" or noise into your FF circuit??  
Brian.

[Fret Wire]

After I built my first two FuzzFace boards, then had to re-solder in numerous parts to tweak it to be "just right" I decided that sucked, so I built a Fuzzface circuit on perf-board that has all trimpots and sockets for transistors and has reversable power switch (so I can test the PosGnd-vs-NegGnd when needed) and has 3 little 5-way rotary switches for the input, output, and fuzz caps and test terminals to hook up my DMM for bias adjustment and it only took about an hour to build and has saved me countless hours in debugging and mod'ing time. It's really nice just to be able to pop-in a set of transistors and find the correct values of the support components, then solder those into a PCB once and for all.

Not only does it allow me to dial-in my components and test/experiment for the sound I like, When I'm asked to build one for someone else, it lets me sit them down with their guitar and amp and dial-in the sound they like in a FF.

I did the same thing. I used sockets for the transistors and caps, and pots inplace of the resistors. I used 1/8" mini-jacks to connect the pots to the board. The pots are wired to the male end of the jacks, and the female end goes to the board. After I bias the transistors to my liking, I un-hook the jacks and measure the pot resistance. If I want to switch a 1k fuzz for a 2k, I just unsnap the 1k pot, and plug a 2k pot in. Same for volume pot. Works great, and like you said, I can dial in a fuzz to someone else's guitar or amp. You'd laugh if you saw it, I should mount it properly.

[brian wenz]

Hello Hello--
      I had a rig similar to that..........gave me "option anxiety" !
 Actually I got to the point where I really needed to  put the circuit into a metal box  and play it with a band in order to see if it was a keeper.
Cool thing to have, though!
Brian.

[WGTP]

I have not tried this, but it seems that a dual 10K pot with one for each Q1 and Q2 wired in reverse, would be a way to inversely vary the hFE betwee the 2 transistors that might be interesting.  Sounds like you would also need an external bias control to, but that would allow for a lot of tonal variation.   Combined with Joe Davidson's Antiquite Fuzz and some of the other mods around hear, lots of possiblities.  

[RedHouse]

Brian,

Yep that's what I did, but I used a tin, like candy or fancy shortbread biscuits comes in, so I could open it instantly just by pulling the lid off. We joke about it and call it the "Fuzz Candy"

Fretwire,

Nope I certainly wouldn't laugh (throw stones, glass house, kettle black etc) as I've gone to some extremes to get "adjustable" circuits for testbed applications.

You should see the amp I built years ago as a testbed, it started out as a Fender and was called "the Boat Anchor" it had adjustable everything and had both Fender and Marshall preamp sections (fully adjustable) and fully adjustable tone controlls (even switching them pre-post CF) it had 2 output transformers (only switchable in STBY mode) and was mounted on a metal stand which held the chassis in just the right position for soldering inside the chassis, then I added a hinged metal "lid" so I could close it and ditch the hummm when testing live with a band... worked great but boy it was ugly, and boy did I get some chidding.

After embracing the "tesbed" methodology though, I find it's more gratifying than simply slapping-up a PCB and populating it according to someone's recipe. Specially after one learns about BIAS and thigs like that, as a byproduct I can (nearly) allways get my stuff sounding great (IMO) when I start with a testbed then move to the PCB when all the proper components are known, even if they diverge from the original schem-o.

[brian wenz]

Hello Hello-

    Errrmmm..........so, did anybody have any hiss or noise problems with piggybacked trannys???
B.

[brian wenz]

Hello Hello--
   Oh well........mention "hiss" and everybody runs away...!
Brian.

[RedHouse]

I didn't notice any added hiss.
(sorry I missed replying to this thread in a timely manner)

[petemoore]

 Todays Fuzz Face" ...   

[R.G.]

When I first proposed the piggyback setup to lower hfe, I had this naive idea that gain would be reduced by half.

I had forgotten that the internal base-emitter resistance would make the active device have a higher Vbe even if it was identical to the piggyback device.  Brett's work on using an emitter to emitter resistor lets you dial in a gain; and as noted, as that resistor approaches infiniity, the piggyback transistor approaches no effect at all.

There is, and must be, an increased noise because you're attaching an active device at the input of the gain stage. Whether this is objectionable, or even audible depends on the device you attach and how much gain follows it. If the piggyback device is identical to the main device, the first approximation to the noise will be that it's doubled (i.e., +6db), which is a just noticeable increase in noise loudness. If the devices are low noise and have the right impedances driving them, this may not be noticeable at all. And it's no worse than the noise of a differential amplifier, which also has a second device connected to the input.

so I built a Fuzzface circuit on perf-board that has all trimpots and sockets for transistors

One guy in our EE circuits design course lab did that. He was the only one whose lab design actually met the performance criteria. He spent maybe 15 hours tuning it in and it was perfect.

The prof failed him, noting that the point of the exercise was not to meet the performance requirements but to be able to design something ahead of time that would meet requirements. Proffessor Allison thought that endless tweaking was a sign that he did not understand the design process.

Fortunately Jack Allison is not watching us here. 

I hope.  8-)