asymmetrical clipping using three diodes.

[davepedals]

Hi,
I want to increase the gain somewhat and change the sound of some of my overdrives to asymmetrical clipping using three diodes. I like using GE 1N270 for smoothness and thinking about a 914 or 4148 SI for third diode. But to retain the smoothness should I connect two GE back to back, or two SI back to back?  I think I also need to mismatch them If I read correctly.
Image below:
Thanks!

[davepedals]

A quick follow up:  I have lots of J201 trannies that measure below what I like, around 200 - 250.  Good candidates for replacing the SI diode, or diodes?
Thanks!

[Mark Hammer]

The solution will be found by measuring the forward voltage of the diodes you have or are considering, and picking on that basis.  You'll find that any category of diode - Schottky, Ge, Si - can show considerable variation in Vf.  In some instances, a pair of Ge diodes might get you exactly what a single Si does, such that it is only asymmetrical in number of diodes but  not in its impact on the clipping.  Conversely, a single Ge + 2Si might have forward voltages of 230mv and 1260mv, respectively, for each half cycle.  And that may not be the sort of asymmetry you were think of.

So if you don't want to be measuring a mess of diodes, looking for some magical combo, here's something I did on a TS808 a decade back that was well-received by the purchaser.  I simply stuck a variable resistance in series with one of the diodes (5k-10k ought to do the trick).  This lets you dial in degrees of asymmetry, without having to futz around with different diode types and combos.

[rezzonics]

SI diodes have bigger forward voltage than GE diodes, 0.6v vs 0.3v.  2SI diodes back to back will be more asymmetric vs 2GE. But in both cases you will have the smoothness on the GE forward conduction cycle (positive or negative). You could have smoothness in both cycles with a GE - GESI combination. You can add some kind of smoothness to SI if you add a series resistor, and still you keep the bigger forward voltage, and hence voltage range.

Sent from my SM-J320FN using Tapatalk

[PRR]

Forward voltage, at what current, makes what sound?

Me, I'd put in a socket and mix-n-match diodes and jumpers until thrilled with the sound.

[reddesert]

A quick follow up:  I have lots of J201 trannies that measure below what I like, around 200 - 250.  Good candidates for replacing the SI diode, or diodes?
Thanks!

JFETs don't measure gain the same way that bipolar transistors do, and JFETs also don't have the same kind of voltage drop from gate-source the way that bipolars effectively have a diode from base-emitter, so taking your statement at face value, I would advise: (1) your J201s might be fine and you should save them for the usual JFET applications like buffers, gain stages, and phasers; (2) no, they wouldn't be a good substitute for clipping diodes.

[davepedals]

Thanks for the replies!  Will take this info into play.  Forgot to mention, the reason for the post was that last night I pulled one of the two 914's out of one of my od pedals, replaced it with two 1n270 back to back orientation, results were great!  Was suprized that gain increased, not a bunch but noticable, and the crunch was more the way I wanted it as well.  Gonna study this diode clipping a lot more now!

As for socketing, I normally do but not with this one.  However, I just etched a new board for an od and soldered wires with alligator clips where caps and diodes go, and one resistor. The girlfriend suggested that. (blew me away!) A nice way to experiment since I still get headaches fooling with breadboarding!  Thanks again!  Crap - one more question, what common transisters ARE good for clipping?  Thought I read somewhere that jfets are good for getting somewhat of a more tube-like crunch?

[Mark Hammer]

Nothing changes about the gain.  Nothing.  What changes is how much signal amplitude is allowed to escape the gain stage.  The higher the forward voltage of the diodes used for clipping, the greater the maximum output level. 

People use that principle in different ways.  For instance, if the overdrive tone you like comes from pushing an amp harder, then using something like red LEDs for clipping in the pedal will allow you to provide a much hotter input signal to the amp.  On the other hand, if the overdrive tone you want needs to come specifically from the pedal, to feed a clean amp, then you'll likely want Ge diodes for clipping.

The old TS-9 vs SD-1 comparison was essentially a comparison between 2 vs 3 diodes in the feedback path.  The SD-1 used a trio, which permitted a louder output, even at identical gain settings (based on the specific component values, not where the knob was pointing).  Some players appreciate what that can do for/to a tube amp poised at the edge of breakup.  In contrast, using only 2 diodes can provide a nicely compressed sound, which has its own charms.