Hi, i don't have 1N914 diodes nor 1N4148. With which can i replace this diode? What sort of traits are desirable from a diode? I measured some of these that i have with a multimeter and most of them have 0.7 v. So can i use any diode with 0.7v?
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You can use pretty much any diode, even led's work. The important parameter is the forward voltage. Basically, at a lower forward voltage the diode will clip the signal earlier, and thus there will be more distortion, at a higher forward voltage more of the signal gets through before it clips. If you use a diode with a higher forward voltage, the signal will be much louder and also cleaner, if you use a diode with a smalled forward voltage the signal will be quieter but you will get a much more pronounced distortion effect.
Hope this helps!