Amplifier overdrive and distortion is usually achieved by signal clipping--pushing the gain until the signal peaks are chopped off. "Real" tube overdrive isn't possible without major changes (adding additional preamp stages, etc.), but here's another option:
Preamp clipping can be simulated by installing a diode-clipping circuit. In fact, some big-name amp manufacturers (Marshall and Fender, for instance) have used diode clipping for a more ballsy sound (especially at low volumes.) In fact, it's made a bit of a resurgence in boutique amps...
The danger of converting a tube sound into a harsh "solid state" sound exists. Tubes clip with a softer "shoulder" and have a more pleasant, warm sound....of course, it's a matter of taste, after all.
But a carefully designed diode-clipper circuit can mimic those tube characteristics. "Peak clipping" more closely approximates tubes than "crossover clipping," which is common to transistor design. Plus we can utilize asymmetrical clipping to emphasize the "correct" harmonics. And since our new sound is contained within a tube amp, it will help to soften the effect.
Of course, diode-clipping can be used (and is) for solid state amps, too.
Video clip shows the three switch-selected settings: 1) clean; 2) medium clipping; 3) max clipping:
Warning: This is a "mod," and does require a bit of rewiring. All the usually cautions about discharging power supply filter capacitors apply here. To restate them:
Don't touch the internal components of a guitar amp without first draining the filter capacitors. Please. You could really harm yourself. Here's a link to my older amp project, and the section Discharge those Caps!
Step 1: Let's Examine Diode Clipping...
In theory, diodes conduct current in only one direction. But they don't operate "perfectly." All diodes have a "forward voltage" -- they won't conduct until that voltage is reached.
If a diode is connected between the signal path and the ground, the signal won't be channeled (shunted) to ground until it exceeds the forward voltage. And even then, the whole signal isn't dissipated, only the portion of the signal above that forward voltage. So it's peaks are simply "chopped off."
But it also results in more attenuation, as some of the signal is lost. That isn't all bad! Clipping the loudest parts is also a form of "limiting" -- you loose some dynamics, but accentuate the lower-volume sounds.
Plus, many amps don't begin to distort until turned up too loud for many venues. Like a stomp-box, diode-clipping lends a killer sound at a lower volume. Your family, roommates and neighbors will thank you.
Step 2: Types of Diode Clipping
The first illustration shows:
1) An unclipped signal
2) Asymmetrical clipping, one side of the signal (not limited to a single diode.)
3) Symmetrical clipping, where both the + and - signal is clipped equally.
4) Asymmetrical clipping, both sides.
Variation #4, asymmetrical clipping, offers the most natural and "tube-like" sound. Plus, it's the most flexible--any number of diodes in series can be used to shape the effect.
Picture #2 shows just a few variations of asymmetrical clipping. Notice that LEDs are used as diodes in examples B and C! LEDs have a much higher forward voltage than typical diodes, so the clipping can be softer and more subtle.
Step 3: The Design
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.)
Step 4: Inserting the Circuit
--Don't insert at the input circuit (before the preamp.) Guitar signals are quite weak, and even if the circuit functions partially with a really hot pickup, it likely won't work at all with other guitars.
--Don't insert in the driver portion of the power amp (output transformer primary.)
--Don't insert in the output transformer secondary / speaker loop.
The last two examples will greatly attenuate (lessen) the volume of the amp, probably "fry" the diodes, and possibly ruin the power tubes or the output transformer.
So, where should it go?
--Within, or just after the preamp circuit. The red line illustrates the signal path between the preamp and the output stages. This amp has a single preamp stage, so this is the only acceptable place (in this case.)
The points B and C are all possible places to connect the clipping circuit. I chose C, since this amp is never too loud, and turning down the volume will change the character slightly. Connection point B are probably better if you wish to retain the full clipping effect when the volume control is turned down. Point A is unsuitable, due to the DC plate voltage.
Amps with a dual-tube preamp are ideal--any signal attenuation lost in the clipping can be recovered by increasing the gain of the second preamp stage. (Of course, that pre-supposes that you have the expertise to do so...)
There will be some attenuation with diode clipping--some signal will be lost. This isn't really tragic, however. Most players would prefer to get "that sound" at lower volumes.
Step 5: Other Options...
And there are many diode types we haven't covered: zener, silicon, other LED colors, etc. Even MOSFET transistors have a voltage drop.
Flipping the asymmetry--switching the directions of all the diodes, will also have an effect. This can emphasize odd or even harmonics. Choose whichever sounds best to you.
There are a great many variations of diode clipping circuits. Here are a few links to get you started... :
-- DIY Fever: Diode Clipping
-- MOSFETs as diodes
-- AMZ - Diode clipping staturation controls
-- AMZ - "Warp" controls--balancing diode clipping
-- AMZ - Mosfets and Zeners as clipping diodes
Step 6: But How Does It Sound?
It definitely gives a harder-edge to the natural distortion of my amp. But it's a small amp, and the attenuation is fairly significant in the "full-on" setting.
At least one thing must be noted:
A simple mod like this isn't going to give you an over-the-top metal-chorus-delay-flange-mega-distortion effect. You're better off using a stompbox or fancy rack-mount fx unit. But if you like the sound of your amp clean, you probably will like it even better with a little "dirt."
To be honest, a good tube amp with some flexibility really brings out the character of different guitars better than the external FX. And you can always use the stompboxs with the mod anyway...