the beauty of jfet biasing...

[vdm]

now this may be old news to some of you... no probably most of you, but for anyone out there who has ever made a jfet based (or even buffered/boosted) pedal, please read on...

a while back sometime last year, i made a ROG 3leggeddog, and at first i was really happy with (being my first pedal) i socketed some caps and even the biasing resistor. i used the cap sockets to get rid of the really harsh high notes it was giving me, by increasing the coupling caps, but i never changed the jfet bias resistor.

just today i thought i'd crack it out after not using for a while cuz i no longer liked it, and thought why not mess with the bias, because on the off chance it wasnt working this could make it better...

as i started changing resistances, i had a 6.8k in there to start with and i parralelled it with a 10k, for about 4k total resistance, as i plugged it in, i think the gain about tripled in the pedal!!!!

it was incredible a 2.8k change in bias and the pedal was giving the thick amp-like distortion that was described as CMOS distortion.

the gain control needs to be about half cuz i cant much change (other than it getting more wooly and less useable) in the second half of rotation, and the first small portion of turn is no signal or transistor radio-esque sound, and it also needs a 9V power socket cuz all my batteries are about half dead, and even then they started oscillating if i turned the volume up a lil (they didnt used to until i'd used the pedal for a while killing the batteries)

so for anyone who's never played with the bias of the circuit your working on, or could never get something to sound good CHANGE THE BIAS!!! im sure it'll make a difference, and for those who dont believe, try it and you will be surprised!

sorry bout the ramblings... i just needed to talk about this....

trent

p.s. my next project will have to be the ROG double-d after hearing it's cousin in action... thanks guys

[Brian Marshall]

sometimes 100 ohms for jfet bias will make a huge difference.

[Alpha579]

yeh, jfets can be really sensitive to very small amounts of bias change...
Also, changing the bias in jfet OD circuits can change the type of harmonics created quite dramaticly, because (if you already know, stop reading,):
Say you have a single stage jfet OD run with 9v...
If the Jfet has 4.5v on the drain, you will get more symetrical clipping (because each half of the wave-form has to swing 4.5v + or - to clip to the rails).
But if you change the drain resistor to get, say, 3v on the jfet, the waveform can only swing -3v and +6v.
Now if your still reading, the above isnt entirally true. For reasons i dont know, Jfets naturally clip one side more than the other, so u will actually clip assymetrically to a certian point, but decreasing (or increasing) the drain voltagewill cause more assymetrical OD, thus more even harmonics.

If any of the above is wrong, please correct me  :oops:

[Paul Perry (Frostwave)]

Well, now you know why I try never to design anything around fets! not to say that there are some fabulous boxes using them, but if you are trying to make multiple units that sound the same, it is a ballbreaker
Mind you, they are great for buffers..

[Phorhas]

Hi vdm...

can you post that JFET's pin voltages?

[petemoore]

Troublesome, sllightly, but not too troublesome.
 Expecially when they're on. They have that way of sounding just Jfetastic!!! [George Jfet Jetson recommends them too].
 Theres just something I like about the harmonics and smoothe Jfet gain.
 Not to say they are the end all, but alot of new designs that are picking up popularity fast use them.
 I just re-wangled my Minibooster, which Is really getting it's feet on the pedalboard these days, and I'm working up a May Queen...
 The Minibooster is quite the little lovable unit, gets along just swimmingly with the input tube and most all other inputs...very nice smoothe 'breakover can easily be found using it.
 The May Queen is like kind of outrageous...sound sorta like a boosted FF with small input cap and a wah Q on it...talk about the squankin talkin slicey lead penetrator !!! Whew...I'm having to tone it down a notch or two..putting a small B/C cap on Q1, and experimenting with other smallish caps to ground in different spots..getting some cool sounds from this one.
 I would like a gain control on it [thing distorts nice and heavy], thinking of just switching in a parallel resistor on the emitter of Q2...
 Pretty darn interesting playing leads throught the May Queen...

[WGTP]

I guess that is the cool thing about mu-amps is that they self bias.

[vdm]

hey phorhas, sorry i dont have a multimeter (one of the reasons i never properly biased the jfet), but when i go back to school (2 weeks) i can test it there...

send me a PM if you want me to find the voltages...

trent