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Probably about the most simple distortion circuit out there, the Electra, has a few variations floating around the internet. I am not quite sure why the different resistor values exist, and maybe someone can give me why they may differ. Maybe for differing levels of distortion? Below is the simple drawing of the circuit.
Most of the circuit schematics have R2 at 680R, although there is one with a C10K Pot at R2 for a dirt control (actually replaces a 470R resistor in this version).
It is at R3 where there is the most variation. With R2 of 680R, there are circuits with R3 values of 3.3K, 4.7K and 47K. The rest of these circuits appear identical.
The Trotsky Drive version has a 3.3K resistor paired with a 50K pot (linear I believe, not log as shown). This is supposed to be a gain pot. So this would provide a range of 3.3K to 53.3K of resistance at R3, which is close to all 3 values above (3.3K, 4.7K and 47K).
The circuit with a C10K Pot at R2 for a dirt control has an R3 value of 68K.
I guess I will breadboard it and see what the different values do to the circuit. In the meantime, maybe someone could give me some more insight on this. I will probably put this on a GBOF board or veroboard it with other GuitarPCB boards employed in the overall circuitry.
Did my own investigating via breadboard. For R3, the higher the resistance the greater the distortion. So using a 50K Pot and a 3.3K resistor is the best way to go IMO to have controllable amount of distortion. I tried 680R and 470R (the value on one of the schematics) at R2 and there didn’t appear to be much of a difference. Leaving R3 at 68K and using a linear 10K trimmer at R2 does also control the degree of dirt well, although it would be better to have a Log Pot here. Instead of finding a C10K Pot to use here, maybe I could use an A10K Pot for a dirt control.
At C1, there does not appear to be much of a difference using a 0.1uF (100n) capacitor vs. a 0.047uF (47n) capacitor. I may make this a switchable option on my build. Changing Q1 from a 2N3094 of ~150HFE to a 2N5088 of ~630HFE does slightly increase the overall amount of distortion, but not by very much.
I was going to use 2 Rototones for differing Symmetric and Asymmetric clipping that would be switchable between the Symmetric and Asymmetric variations as below. However I found out I like blending a diode labeled 1N34 with the other diodes I have, so I may keep that one in fixed position and the other diodes on the Rototone. I still have to test the 1N34 to see if it is really a Germanium one or a fake (purchased from Mammoth a few years ago). I also have some true Ge diodes from Small Bear I am going to try in the circuit as well.
I would consider switching the coupling capacitor from 100n to perhaps 10n, 15n or 22n on a switch to tighten up the tone which would be more noticeable using a rig with a larger speaker cab and of course volume would factor in as well.
Bass artifacts that are not part of the natural guitar tone drive me nuts so this is a mod I often employ to help cut back on that.
Barry, you are right. I will experiment with the different coupling capacitor values. This is maybe why the Trotsky Drive has a 47nF coupling cap (passing AC and filtering out DC). That is unless you are talking about the coupling cap on the other side the circuit, which I believe some may call that a decoupling capacitor (filtering out the DC and passing the AC signal).
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