CATHODE RESISTOR METHOD
This is the easy way to bias your amp and its also the safest (for you). There are two other
ways to do it also but this is what I recommend. Before you do this bias metod you have to
install a 1 ohm 1% (2-3 watt) resistors from the cathode to ground on each power tube.
Before You Power Up Your New Amp/Board Be Sure To:
With all tubes in their sockets, connect your multi meter across the 1 ohm power tube cathode
resistor. Polarity does not matter so pay no attention to the positive or negative sign.
Set your meter for DC millivolts and connect it across this 1 ohm resistor. Turn on the amp,
and as you watch the power tube current, be ready to turn the standby switch off if the
current (millivolts) rises too high. Turn on the standby switch keeping a close eye on the meter.
Remember that with the cathode resistor method (millivolts converts directly to milliamps) So
if the meter reads 20-40 milivolts then you can bias the amp to where ever you are going
to bias it (using the adjustable bias trim pot).
If the current rises up to above 40 millivolts you may not have enough negative bias voltage and
the bias range resistor (RB) may have to be a smaller value. OR If the current rises
up to only 10-25 millivolts, then you have to much negative bias voltage and the bias range resistor (RB) will have to be a larger value.
Here is a tip and seen in the pic, if at first shot
the bias is not correct and it problably wont be, I unsolder and remove RB then connect the two alligator clips (as seen) to a new value RB ressistor and power up and re-check you may have
to do this a few times in increments to get it right. Note the Heat Sink used When working with any kind of silicon chips or even a diode you can't over heat them or they will be destroyed. When you unsolder the one side of resistor RB close to the diode put a heat sink on the diode just incase.
The figures above are based on a plate voltage of aprox. 250-300 volts. measured at pin 7 of
the power tube. The formula you need to know for a precise figure is:
.7(watts)/actual pin 7 voltage=your bias setting.
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