Biasing The Kt-88 Power Tubes - Mesa/Boogie BASS STRATEGY Eight:88 Owner's Manual

BIASING THE KT-88 POWER TUBES – by Designer Randall Smith
First and foremost: Bias adjustment is only required when replacing power tubes. Checking bias as routine maintenance or if a
tube problem is suspected is quick and easy but shouldn't often be necessary. And since those big KT-88s in your amplifier are only
running about half their maximum rated dissipation, they should last for years.
Nevertheless, we have provided a simple and effective bias adjust system that you can use yourself with only a little care and a small
screwdriver. This deviates from our standard practice for 6L6s over the last 40 years where the bias is fixed to the "sweet spot" as
each amp is built. Why this difference?
First, a little history: Traditional Fender 6L6 amp schematics show the bias setting to be -52 volts. What ISN'T specified are some
important factors, such as the AC mains voltage which can vary up to 14% and affects the bias setting equally.
But most important is: What is the desired Plate Current that the power tubes should be biased to draw? Because getting that current
set right is the whole point of adjusting the bias. And here's where the going gets tougher. Fender offered no spec for plate current,
probably because measuring current (the "amount" of electricity flowing) is difficult. Properly done, it requires cutting the wire leading
to the output transformer and inserting an amp meter in series with that current path. And plate current in a tube amplifier runs at high
voltage, around 400 to 500 volts for 6L6s and much higher for our KT-88s. Unless you're a technician and REALLY know what you're
doing inside an amplifier... you don't want to mess with that.
Obviously Fender's adjustable system and our individually hard-wired systems both work—though in my many years of servicing
Fenders, the bias settings were often found to be far from "correct". And with the wide availability of quality 6L6 tubes, especially our
own selected MESA tubes, adjustments outside our designer's "sweet spot" are unnecessary and usually undesirable (unless you
like paying a tech to re-set your amp's bias voltage.)
But KT-88s are different and sampling them from various suppliers revealed a far greater variation in plate current for any given bias
voltage compared to 6L6s. And with the 88's greater size and capacity, the current draw varied from 15mA (milli-amperes) to 50 mA
whereas our "sweet spot" was 30 mA. So... we've provided a simple, safe system for bias adjustment that actually measures the cur-
rent flowing through the tubes, not just some standard bias voltage. Use MESA Matched Tubes whenever possible in your amplifier
and you will find they are far closer to the specified range than those available elsewhere. Because of this you will save yourself time
and ensure your amplifier's Tone will stay intact as the BIAS setting is a critical element in its performance.
BIAS ADJUST: OPERATE/SET BIAS
NOTE: The optimum BIAS SETTING was arrived at through EXTENSIVE scrutiny during the development of your amplifier. We
STRONGLY RECOMMEND adhering to the Suggested Optimum Factory Setting for both Tonal and Reliability reasons. MESA has
historically avoided Adjustable Bias Controls on our amplifiers for one important reason; the average musician has neither the training
nor equipment to properly adjust the Bias in their amplifier. We see too many examples of this in our Authorized Repair Stations in
the field and in our Repair Department here at the Factory to believe otherwise. The Internet is full of people espousing the virtues of
a (too) high Bias Setting. Don't Be Fooled!
The Recommended Bias Setting your amplifier was shipped with delivers the absolute best blend of Tone and Reliability. This has
been confirmed time and again throughout the entire course of its development. Please—Do Yourself A Favor—Don't Mess With The
Bias Looking For "Better Tone". It's not there.
NOTE: IMPORTANT! Adjust the Bias only when replacing Power Tubes and Adhere to the Owner Achievable Bias Setting Described!
FAILURE TO ADHERE TO THE RECOMMENDED FACTORY BIAS SETTING WILL VOID YOUR WARRANTY, WEAR YOUR POWER
TUBES FASTER AND COULD CAUSE DAMAGE TO YOUR AMPLIFIER!
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