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The Millett Hybrid Maxed
Setup and Bias Settings
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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 MAX on and off, measuring the DB bias with your meter each time until you know where you stand. Then, adjust carefully as described below.
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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 Millett Max 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.
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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.
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MOSFET DB Bias is trickier still, because of the higher currents. Try to work up to about 88mV (40ma) and bias
with half turns from that point until you reach your target.
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| Power Supply Voltage |
| Measurement Points: | V+ to GND |
| Adjustment Device: | RR3 |
| Acceptable Values: | 24VDC - 30VDC |
| Tube Bias |
| Measurement Points: | TA2L to GND, TA2R to GND |
| Adjustment Device: | RA1L, RA1R |
| Acceptable Values: | 1/2 of Power Supply Voltage |
| DB Bias |
| Measurement Points: |
TB1L or TB2L with TA2L, TB1R or TB2R with TA2R |
| Adjustment Device: | RB12L, RB12R |
| Acceptable Values: | 66mV - 110mV (30ma - 50ma) |
| MOSFET Bias |
| Measurement Points: |
TB1L or TB2L with TA2L, TB1R or TB2R with TA2R |
| Adjustment Device: | RB12L, RB12R |
| Acceptable Values: | 176mV - 264mV (80ma - 120ma) |
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