Mesa/Boogie BASS PRODIGY Four 88 Owner's Manual page 27

Owners - it is not necessary to remove the Bottom Cover on your PRODIGY to set the BIAS when replacing Power Tubes. Located
on top of the chassis is a 2-position mini-toggle switch, along side a precision Trim Pot and an LED. The BIAS ADJUST circuit al-
lows for precise setting of the Bias for the Technician using a Current Meter - or - a "near perfect" setting of the Bias for the not-so-
technically-minded Musician/Owner with a very small "flathead" screwdriver. Owners and non-technicians can adjust their Bias when
re-tubing by setting the BIAS ADJUST Switch to SET BIAS and following the Bias Adjustment Instructions below.
Like some other amplifiers, this system measures current at the cathodes which, with no signal running through the amp, is the same
as plate current (plus the minute screen current). But unlike other systems, the BIAS ADJUST switch entirely removes the adjustment
circuit and its cathode resistors from the audio chain when adjustment is complete and it is set back to OPERATE.
OPERATE: This position is for playing your amplifier. The BIAS ADJUST switch should be set here at all times unless you are adjust-
ing the BIAS after replacing a worn set of power tubes.
Technicians – you know what to do. Set BIAS ADJUST SWITCH to SET BIAS Position - Check and Adjust Cathode Current: Set to
120 – 125 mA.
This Bias Adjust Mode also allows for a near-perfect Owner Achievable Bias Setting with nothing more than a small jewelers flat-head
screwdriver. Again – adhere to the directions below and adjust your Bias ONLY after replacing worn power tubes with a new set.
NOTE: When the Bias switch is in the Set Bias position, the Mute LED will be on and the amp will be muted. This is to prevent the
amp from passing signal while the Bias circuit is engaged.
Owners – the following simple procedure is also printed on the chassis top (Tube Deck) and consists of the following steps:
SET BIAS (For Non-Technicians)
1. Set the BIAS SWITCH (located on the upper Rear Tube Deck) to OPERATE. Make sure the amp is set to FULL POWER—set-
ting the Bias while in HALF POWER will give an incorrect measurement.
2. Follow the Cold-Start Procedure under POWER in the Front Panel Section of this manual and Switch the PRODIGY to STAND-
BY ON after three minutes. Then allow the amp to warm up for another 5 minutes (POWER and STANDBY ON).
3. Set the BIAS SWITCH to SET BIAS. The MUTE circuit is activated by this setting and you may proceed with the procedure with
no load connected to the SPEAKER OUTPUTS. BUT DON'T FORGET TO CONNECT A LOAD when you are finished with the
BIAS setting procedure!
4. Using a small flat screwdriver, twist the trimmer pot counter-clockwise until the LED just goes dark... then:
5. Twist the trimmer EXACTLY ONE HALF turn clockwise toward greater brightness – Stop there. You're Done!
6. Set the BIAS ADJUST switch back to OPERATE and Play! Your BIAS is set for optimum performance and Tone. CHECK YOUR
SPEAKER OUTPUTS! Don't forget to connect a load (cabinet) BEFORE playing!
NOTE: IMPORTANT! It is important when re-tubing your PRODIGY, to make sure that all the power tubes are matched to each other.
This is critical to achieving a proper Bias Setting and also for obtaining the best performance from your amplifier. If your local dealer
doesn't have a complete set of identically matched KT-88s (Preferably MESA Tubes), you may order perfectly matched sets direct
from us at www.mesaboogie.com or, by calling 707-778-6565 and asking for a Product Specialist - who will be happy to assist you
with your purchase of these matched sets of KT-88 Power Tubes.
NOTE! Original PRODIGY OWNERS! MESA offers a HALF-PRICE DISCOUNT on your FIRST SET of Replacement Power Tubes.
This can be obtained by ORDERING DIRECT FROM THE FACTORY ONLY and the order must be accompanied by your ORIGINAL
SALES RECEIPT as proof of ownership.
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