RAT gm/mu Tester Ap Note 2 - Using the tester to set bias point. Hi All, Use can use the gm/mu to set up the bias point for your proposed design. This can aid resistor selection for those who do not have the full set of curves for the tube, or to simplify this procedure. The RAT tester can independently vary the plate (anode) voltage and plate current. By placing a DC voltmeter between cathode and ground, the bias voltage can be measured for this selection. Let's consider an example. A 6SL7 is chosen for operation with a supply voltage of 350 volts. The load resistor is desired to be 250k ohms. We wish to see what appropriate bias points may be established. Step 1: Calculate the "load line". From a 350 volt supply, we can establish several points: At 300 volts, 0.2 mA flows (50 volts drop across a 250k resistor). Similarly, 250V and 0.4 mA, 200V and 0.6 mA, 150V and 0.8 mA, 100V and 1.0 mA all fall on the load line. Step 2: Measure the characteristics using the RAT Tester along this load line: Vp Ip Vg gm mu 300 0.2 60.7 250 0.4 3.07 0.98 63.4 200 0.6 2.14 1.14 65.1 150 0.8 1.22 1.27 66.6 100 1.0 0.46 1.33 65.3 Step 3: Establish appropriate operating point. Notice that with this load resistor, the plate can swing DOWN to slightly below 100 volts before grid current starts to happen, and the mu starts to degrade above about 300 volts. This seems to indicate that an appropriate quiescent point is 200 volts and 0.6 mA. At this point, there is 2.14 volts on the cathode. Therefore, the cathode resistor should be (per ohms law .. R=E/I) 2.14/0.6 or 3.56k. You could use 3.6k 5% resistor here. You can also now determine stage gain in your circuit. Lets assume the next stage grid resistor is 470k. Here's the data.... For the chosen operating point, gm=1.14 mS, MU=65.1, so rp=57.1k. Plate side resistance=57.1k parallel 250k parallel 470k or 42.3k. Cathode side resistance (bypassed)=0.877k (1/gm) or if the cathode is not bypassed, 4.44k. Therefore the circuit gain bypassed is 48.2 and unbypassed is 9.5. Notice that from the RAT tester, the 6SL7 is a very linear device, as its effective mu doesn't change very much over the chosen operating load line. Notice also, that for the chosen operating point, at least 200V p-p (70V RMS) can be obtained. Regards, 1Steve