switchable sym./asym. clipping for Distortion + clone

[bettycat]

Hi guys, I´m actually building a Distortion Plus clone on the GGG board, using the wiring scheme on www.generalguitargadgets.com/diagrams/dist_plus_lo.gif
Like in the original box I will use NOS 1N270 germanium diodes instead of the shoen 1N4148. The board is designed for asymetrical clipping, right ? I guess so because the two diodes point against each other. I would like to have a switch to dial in symetrical clipping as an option but I failed while trying it. Any ideas how to wire such a switch or anyone knows a diagramm where such an option is shown ?

[Brett Clark]

The setup shown if for symmetrical (same both ways) clipping. For asymmetrical (more clipping on one side), add a switch that changes  one of the diodes in some way. For example, you can:
1. make one of the diodes a 1N270 and the other a 1N4148. OR
2. have one 1N270 one way, two the other way - you can just make the switch short across one of these two to make it symmetrical

[bettycat]

The setup shown if for symmetrical (same both ways) clipping. For asymmetrical (more clipping on one side), add a switch that changes  one of the diodes in some way. For example, you can:
1. make one of the diodes a 1N270 and the other a 1N4148. OR
2. have one 1N270 one way, two the other way - you can just make the switch short across one of these two to make it symmetrical

Hi Brett,
thanks so far for your input ... do you have any circuit drawings for how to do this ? Anyone else maybe ?

[petemoore]

-->l--->l--
 connect an spst switch's leads to the 5th and 6th dash lines to go from one to two clipping diodes in series, the switch simply shorts across the 'left' diode, leaving the right one in circuit.
 If you have one or two diodes facing ---l<--- [the opposite way, in a back to back config] hitting the switch will increase/defeat assymetric clipping.
 two one way and one the other should be assymetric
 Back to back diodes of the same type and count [1?, 2? each way] should be as symmetric as their FTV.
 You can read the foreward voltage threshold [at what voltage the diodes begin conducting] of diode configurations with a DMM.

[Mark Hammer]

Try to remember that if the signal has a "choice" between going through a piece of wire, or going through a device that restricts it in some way (diode, capacitor, resistor), it will *always* "choose" to go through the wire.  So, if you wire up a switch in parallel with a diode (i.e., a shunt), closing the switch so that there is a straight wire connection running parallel to the diode, will make it as if there is no diode there at all.

Extending this, if one had 2 end-to-end diodes in parallel with a single reverse-biased diode (a 2+1 diode complement), and you used a switch to short/shunt one of the diodes in that end-to-end pair, it would be as if you had a 1+1 diode pair.

Note - if you haven't already done so - that adding diodes in series will increase the signal ceiling such that there will be greater volume (assuming no other subsequent constraints on signal level), and more gain or harder picking will be needed to produce the same amount of clipping.

Here is a simple way to wire up a switch that changes clip symmetry and compensates for volume at the same time.  I've shown just the Dist+ output stage for simplicity. It uses a DPST switch but obviously you can use a DPDT and ignore two of the lugs.  The way it works is by simultaneously shunting one of the diodes and an added series resistor labelled R2 here.  Since I don't know the precise diode being used I can't specify the resistor value needed to compensate, but chances are that a 22k might be a good place to start.  If it's too much volume drop, try 18k or even 15k; 33k if not enough.

What this mod does is "pad down" the output with an extra resitor when the louder 2+1 diode arrangement is used.  Then, when you switch from 2+1 to 1+1 diodes by shunting the added diode, you also shunt the added resistor and "un-pad" the output.  This will not compensate for the different degree of clipping, but for gigging purposes, it should be able to provide a tonal change that does NOT obligate fiddling with the volume as well.

[primalphunk]

Maybe you should consider using a warp knob/pot(as the AMZ site suggests for diode clipping distortions) to tweak from symmetrical to asymmetrical.  Mark posted some great info about this mod and how to do it with a TS some time ago.

peace,
James

[petemoore]

Sometimes I like also having a series cap in with the diodes, just the right size of course lets the bass not get clipped as much.

[Sam]

Sometimes I like also having a series cap in with the diodes, just the right size of course lets the bass not get clipped as much.

Hmm. I'd been experimenting with that kind of clipping diode setup but haven't had any real success improving tone. I figured this would possibly improve the dynamics and responce in a bass distortion but wasn't able to get satisfying results.

What range of values have you felt useful with this setup?

I thought about having two sets of clippers one for the bass/mid and one for the treble using caps to make some kind of pseudo-crossover.