amp stuff

Tube Swapping and Biasing: The Fortin Advantage

For decades, the "back of the amp" was a forbidden zone for the average guitar player. We were told that unless we were trained technicians, touching the internal components of a tube amplifier was a one-way ticket to a 500v hospital visit. While the safety warnings are valid - tube amps hold lethal voltages even when unplugged - modern engineering from brands like Fortin Amplification has finally demystified the process, putting the power of tonal customization back into the hands of the player.

If you own any Fortin Amp (Sigil, Meshuggah, Cali, etc), you are sitting on a rig designed for "user-serviceable" tone. Here is the definitive guide to understanding biasing, how Fortin’s external controls change the game, and how to swap your output tubes to find your signature sound.

What is Biasing, and Why Should You Care?

Think of biasing like setting the idle on a car engine. If the idle is too low, the car stutters and stalls; if it’s too high, you’re burning fuel and wearing out the engine for no reason.

In a tube amplifier, "bias" refers to the negative DC voltage applied to the control grid of your power tubes. This voltage regulates the flow of electrons from the cathode to the plate.

Hot Bias: More current flows. The tone is often described as warmer, more saturated, and "touch-sensitive." However, tubes run hotter and will die significantly faster. Extreme hot bias leads to "red-plating," where the internal structure of the tube literally glows red before failing.
Cold Bias: Less current flows. The tubes last much longer, but you risk "crossover distortion"—a harsh, jagged sound that occurs as the signal passes between the two sides of a push-pull circuit. Cold bias often sounds sterile or "thin."

The Fortin Difference: External Mastery

Traditionally, adjusting the bias required "cracking the chassis." You had to remove the amplifier from its wooden headshell, expose the circuit board, and use a probe to measure high-voltage pins while turning a tiny internal plastic screw.

Fortin Amps changed the workflow for the gigging musician and studio pro. On the back of the Fortin Amps, you will find External Bias Test Points and Adjustment Pots.

Tube Swapping: Choosing Your Sonic Flavor

EL34

(Fortin Cali, most Marshall amps)

The "British Growl." Strong high-mids, aggressive crunch, and classic "swirl."

6L6 output tube for electric guitar amplifiers

6L6GC

(Mesa Boogie / Fender)

The"American Sound." Massive low end, "glassy" highs, and scooped mids. Perfect for modern metal.

KT88 output tube for electric guitar amplifiers

KT88 / 6550

(VOX, ENGL Savage 120)

Extreme headroom, ultra-fast transient response, and bone-shaking low-end clarity.

Step-by-Step: How to Bias Your Fortin Amp

If you’ve just bought a fresh pair of matched power tubes, here is how to install and bias them using the Fortin external system.

1. Safety & Preparation

Ensure the amp is off and has cooled down. You will need:

A matched pair of power tubes.
A digital multimeter (set to mV DC or mA, depending on your meter's labels).
A small flat-head screwdriver.

2. The Swap

Gently remove the old tubes by rocking them slightly in a circular motion while pulling upwards. Align the "key" (the plastic nub in the center of the pins) of your new tubes with the socket and push firmly.

3. Power Up

Connect your amp to a speaker load (never turn on a tube amp without a cab or load box!). Turn the amp on and let it warm up in Standby for at least 2 minutes. This allows the heaters to stabilize the cathode.

4. Taking the Reading

Insert your multimeter's black probe into the "Common" or "Ground" jack on the back of the Fortin. Insert the red probe into the "V1" or "V2" test point.

Flip the amp out of Standby (Volume at zero).
Your meter will display a reading in millivolts (mV). Because Fortin uses 1-ohm resistors for these points, 1mV = 1mA.

5. Doing the Math

To find your target bias, you need to know your Plate Voltage (usually found in the Fortin manual or measured internally). A common rule of thumb is to bias for 70% of the tube's maximum plate dissipation.

The optimum reading for Fortin Amps is as follows (with the output tubes at production.

Cali - 32mV (per tube)
Evil Pumpkin - 36mV (per tube)
Meshuggah - 32mV (per tube)
Sigil - 22mV (per tube)

FOR MORE DETAILS, PLEASE REFER TO THE USER MANUAL OF YOUR AMPLIFIER.

For a standard 50W amp with EL34s (25W tubes) and a plate voltage of roughly 450, your target would be around 38mA.

6. The Adjustment

Use your screwdriver to slowly turn the external bias pot. You will see the numbers on your multimeter climb or fall. Set it to your target, let it sit for 10 minutes to "settle," and then do one final tiny adjustment.

Why "Matched Pairs" are Non-Negotiable

When buying tubes for your Fortin, always look for Matched Pairs (or Quads). Since the bias pot usually controls the voltage for the entire set of tubes, you need tubes that "draw" the same amount of current at the same voltage. If one tube is running at 40mA and the other is at 25mA, your amp will sound unbalanced, hum loudly, and potentially blow a fuse.

Conclusion: The Freedom of Tone

Fortin Amplification has removed the "technician tax" from the tube-swapping experience. By providing external test points, they encourage players to learn the "feel" of their amp. Whether you want to run your Cali "hot" for a vintage saturated lead tone or "cold" for a surgically tight rhythmic attack, the control is literally at your fingertips.