Step 3: The design
As mentioned, using a combination of diodes, including LEDs, is a great way to begin. I've included a schematic, a wiring diagram and a photo (showing just how simple this circuit is.)
After much swapping in-and-out, I choose a combination of one germanium diode, one silicon diode and one LED.
-- a 1N4148 (D1) and a 1N60 (D2) for a combined forward voltage of ~1.05V
-- a red LED (D3) with a forward voltage of ~1.7V
The diode pairings are connected to SW1, an on-off-on double-pole switch. The center setting is "off," or no diode clipping at all. The other two settings are:
-- the diodes connected directly to the signal path.
-- the diodes connected through a pair of resistors (R1: 47K, R2: 100K)
The resistance softens the clipping, which shapes the effect. When the resistors are connected, more of the natural tube signal leaks through. There are many ways to accomplish this, and this is merely a simple example (See the "Other options" step for more info.)
To help you choose your components, here are the forward voltages for some common diodes:
~790mV -- 1N4148 (Silicon)
~265mV -- 1N60 (Germanium)
~1700mV -- LED (red)
~205mV -- Schottky 1N5819
~740mV -- 1n4001 (Silicon)
Germanium diodes tend to have softer transitions which give a non-linear, more "tubey" sound. But silicon diodes can give a sharper "square wave" metallic distortion, if that's your thing.
Before you build it:
It's important to understand that the peak signal voltages will differ greatly, amp-to-amp. No one combination of diodes will give the same effect in different amps.
And no one combination of diodes sounds good to everyone, either. Experiment! Try 2, 3, 4 or more diodes in series. Keep the sides unbalanced, or use switches to tap the signal in different ways.
(Note: on some amps, the LED(s) will actually light up--they don't in my amp, the peak voltages aren't high enough.)