Tube Screamer Diodes (again, sorry!)

[saxtim]

Thanks for all the suggestions from my previous thread for possible diode combinations etc.  I am however still completely unsure about how the forward voltage of a diode effects distortion.

The reason I ask is I built the tube screamer circuit w/germanium diodes, a single pair, one in each direction and I did not get any distortion even with the drive pot set to max.  With 1n914s in I got a bit of distortion.  I then tried subbing LED's in (still only using 1 in each direction) and I got 0 distortion again even with max drive.  

I measured the Vf of all these diodes - the Ge ones where .3ish, the 1914s .7ish and the Leds 1.6ish.  

Come someone explain the relationship between forward voltage and distortion?  I was expecting with different forward voltages there'd be not just a change in the tone of distortion, but actually how much the signal clips.

thanks
tim

[Dai H.]

I did not get any distortion even with the drive pot set to max.  

something is wrong with your build. A correctly working tube screamer will give a BIG boost in volume and ample distortion w/the drive knob. Try checking everything over for errors.

[RobB]

When I first built mine I had no distortion at all and low output.  The tone control worked but no clipping.  It turned out I had the wrong value for C3.  0.0047uf instead of 0.047uf.  Missed by a factor of 10.  
Perhaps you should check all your component values, especially around the feed back loop.

[WGTP]

The LED's will be the loudest with the least distortion, althought there should still be plenty.

The Ge's will have the least output, with the most distortion.

The SI's will be in between.

The more juice the diodes allow to go thru the feedback loop, the less volume you have.  The Ge's start at .3v., the LED's at 1.6v.

All should distort, even without diodes the op amp should distort.  Keep checking.

[saxtim]

It turned out I had the wrong value for C3. 0.0047uf instead of 0.047uf.

Good call - I'd done the same!

Well now I'm getting a lot more distortion with the 1n914s, and some (but still substantially less than the 1n914's) with the Ge diodes.  

This is backwards right - more distortion with silicon, less with the Ge's.  I've checked my circuit voltages and all the IC pins and transistors check out fine.  

tim

[RobB]

Did you purchase your cap's at Jaycar?

[saxtim]

Did you purchase your cap's at Jaycar?

yes. All parts were from Jaycar with the exception of the opamp and the germanium diodes which were both from small bear

tim

[RobB]

They must have a batch of mislabelled cap's.  
As for the less distortion thing with germanium goes, it may be a case of how you have your amp set.  With the silicon diodes you may be hearing a combination of ts distortion and amp distortion.  Because you will get a smaller signal from the ts with germaniums installed it may not be enough to overdrive your amp and so you could be hearing only the ts distorting without amp distortion.  I suggest a/bing with your amp set as clean as it will go.  
Other than that, if you want to satisfy your curiosity about clipping thresholds, feed a steady signal into the ts and view the output on a scope.  You could use a synth for the steady tone and I've read about some freeware scope software for pcs at freeware sites.

[Mark Hammer]

"Come someone explain the relationship between forward voltage and distortion? I was expecting with different forward voltages there'd be not just a change in the tone of distortion, but actually how much the signal clips. "

Your expectations were accurate.  The problem seems to be in the build itself.  At some point, you'll have it completely debugged and the relationship will be more apparent.

Think of forward voltage, voltage drop, etc., as being like a level/voltage-dependent resistance, the way that you can think of caps as being frequency-dependent resistors.  In a clipping situation, the diode conducts voltages up to its own maximum and then it sort of chokes.  That "choking" is what sets the ceiling on signal conduction/level and produces the clipping.  If you have a pair of diodes in the feedback loop or tied to ground at the output, then it doesn't matter how much gain you apply to the signal, the maximum signal output will be determined by the "choking point" of the diodes.  By your own measurements, you can see that there are different choking/clipping points for different categories of diodes.

Where different distortion pedals often have their own unique characteristics is in terms of the amount of gain applied to the signal prior to where the diodes are in the circuit.  If the clipping threshold is low (e.g., a pair of Ge or Schottky diodes), then very little gain is needed to bring the signal level up to a point where the signal bangs its head against the ceiling set by the diodes.  If the same amount of gain is applied, but the number or type diodes used sets a much higher threshold (e.g., a series pair of diodes with forward voltages of 298mv and 323mv would set a threshold of 621mv for that half cycle) then there will be fewer points/occasions where the signal hits its head against that threshold, making the output clipped a smaller proportion of the time, hence less consistently fuzzy  Crank up the gain, though, and the proportion of time that is spent potentially above threshold (hence clipped) increases back to where it was before.

The general rule of thumb is that switching from Ge to Si to LED will allow greater and greater amounts of signal to pass.  If the amount of amplification prior to the diodes is not proportionally increased, you will hear more and more volume but less and less distortion, simply because the signal is not being "choked off" as much.

As you note, Ge diodes have a smaller forward voltage, and they set a much lower ceiling and maximum output level because of that.  Since gain can be applied before OR after clipping, though, do not confuse output level with distortion amount.  In some instances, you'll see pedals that have a clipping stage followed by a gain-recovery stage.  The clipping stage sets a limit on maximum output level, so an additional stage amplifies that clipped signal more.  Making the output loud increases the likelihood that you will overdrive the amp as well, adding another flavour of distortion on top of what was generated in the pedal itself.

Those circuits that use asymmetrical clipping arrangements, whether by means of dissimilar diode types or numbers (e.g., 2+1 diodes, Ge*2 plus 1 LED, etc.), have an interesting effect on tone.  Since the ceiling for one half cycle may be set much lower/higher than for the other half cycle, any signal amplified in a symmetrical manner (i.e., same boost for each half cycle) will clip on one half cycle at a lower level than where it clips for the other.  If the input signal is *precisely* the right amplitude, you can get clipping for one half cycle while the other remains clean, hence only odd harmonics.  In practice, though, this rarely happens since guitar signal levels are pretty UNconstant.  Apply enough gain and pick hard enough, and both half-cycles clip very hard such that half-cycle differences are really only in terms of average amplitude rather than frequency content.

Up to the point where that heavier clipping sets in though, you can hear a little more dynamics in the sound since the signal is freer to "wander" over a wider range without experiencing clipping.