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The Millett Hybrid MiniMAX

Setup and Bias Settings

WARNING! Bias your DB's first!
Voltage set and tube bias are not critical and may remain out of spec without damage. However, you can smoke your DB's in a few minutes with the wrong setting. Bump your MiniMAX on and off, measuring the DB bias with your meter each time until you know where you stand. Then, adjust carefully as described below.
Power Supply Voltage is a snap to set - simply turn the trimmer screw for the voltage desired Note that 29VDC is about maximum for most 24VAC walwarts under load. The typical MiniMax is set for 27VDC.

Tube Bias is also a snap to set, but slightly more difficult until the tubes are broken in. You should always set bias voltage on the tubes at one-half of the supply voltage. So, for 27VDC supply voltage, you should bias the tubes at 13.5V each. For new tubes, bias may continue to change for several hours. Broken-in tubes will probably reach constant bias voltage in one half-hour or less.

BJT DB Bias is a little trickier - caution is advised. It is possible to burn up the transistors before you ever finish biasing the tubes if the trimmers are wide open. Before applying power to the amp for the first time, screw the trimmers down to their minimums. This is usually 20-25 turns clockwise on the RB12L/R trimmers. Either count 20 turns or listen for a click, if your trimmer clicks. Apply power to the amp, quickly spot-check one channel for the DB bias voltage, then turn the amp off. Repeat for the other channel. Both DB channels should average below 50mV or thereabouts, but this depends on the JFET variance. I've seen some with minimum voltage of 25mV and some with a minimum voltage of over 40mV. If the bias voltage is much higher than 50mV, turn the amp off until you figure out whether the trimmers are operating correctly. Once you confirm that the bias voltage is safe, then leave the amp on and proceed with setting up the power supply voltage and tube bias settings, first.

Click here for a blow-by-blow detail on biasing the DB's.
Power Supply Voltage
Measurement Points:V+ to GND
Adjustment Device: RR3
Acceptable Values: 24VDC - 30VDC, 27VDC optimum

Tube Bias
Measurement Points:TA2L to GND, TA2R to GND
Adjustment Device: RA1L, RA1R
Acceptable Values: 1/2 of Power Supply Voltage, 13.5 - 14.0VDC optimum

DB Bias
Measurement Points: TB1L or TB2L with TA2L, TB1R or TB2R with TA2R
Adjustment Device: RB12L, RB12R
Acceptable Values: 66mV - 110mV (30ma - 50ma), 110mV (50ma) optimum

file last changed:Saturday, May 23, 2009 7:00:00 AM
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