Grid bias voltage should set a tube in the center of its operating curve, a point between tube cutoff (no current flow) and tube saturation (full current flow). This is true with both Fixed bias and cathode bias. Fixed bias connects the cathode directly to ground, then applies a bias voltage to the input grid. Cathode bias does not apply a bias voltage to the input grid, but instead a resistance is connected between the cathode and ground.
For the DIY amplifier builder the choice with less chance of bias error is cathode bias. The following demonstrates why using a test circuit.
When operating a tube with fixed bias the usual thing to do is use the grid bias voltage specified in a datasheet. Problem is the required grid bias for a tube varies with B+ supply voltage. A tube datasheet may only have one or two bias voltages specified. For example, a 6SN7 specifies a grid bias voltage of —8VDC for a B+ supply voltage of 250VDC. At 90VDC supply voltage grid bias is specified as 0VDC.
If you build a voltage amplifier or output stage with fixed bias, there is a chance you will not be able to use the grid bias voltage specified in a datasheet. To demonstrate this a test circuit is set up to determine the grid bias voltage required for a 6SN7 at two different plate voltages.
Figure 1 is the 6SN7 test setup circuit using a breadboard. A 25 mA meter is in the cathode circuit and measures current flowing through the tube.
The plate load resistor R3 for a 6SN7 could be any value between 20K ohms and 50K ohms. I had a 30K 10 watt resistor on hand so I used that.
Grid bias is adjusted using R2. One side of R2 taps positive voltage from the B+ through a 1M ohm resistor. The other side of R2 goes to a negative supply. My breadboard has an adjustable 50VDC negative supply. A volt-ohm multimeter is used to measure grid bias voltage at TP1.
The volt-ohm multimeter is also used to measure the 6SN7 plate voltage at TP2. The breadboard DC volt meter is used to monitor the B+ supply voltage. The breadboard volt meter has a 100VDC scale, but a switch expands the scale to 1,000VDC full scale.
An audio generator and oscilloscope is used after cutoff and saturation points of the tube are determined. There is no signal applied to the grid while determining these two operating points.
Figure 2 is the test setup to determining cutoff and saturation points of the 6SN7. This is done at two different B+ supply voltage values, +195VDC and +350VDC.
With +350VDC B+ supply: Cutoff occurred with a grid bias of —24.6VDC, cathode current was 0mA. Saturation occurred with a grid bias voltage of +.5VDC, cathode current was 9.1mA.
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The correct cathode current for center of the 6SN7 operating curve is halfway between 0 and 9.1mA, in this case 4.55mA. With R2 set for —9.0VDC of bias voltage set the cathode current to 4.55mA. The 6SN7 plate voltage measured 220VDC.
Using +195VDC B+ supply: Cutoff occurred with a grid bias of —14.0VDC, cathode current was 0mA. Saturation occurred with a grid bias voltage of —.15VDC, cathode current was 4.95mA.
The correct cathode current for center of the 6SN7 operating curve is halfway between 0 and 4.95mA, in this case 2.48mA. With R2 adjusted for —4.8VDC of bias voltage the cathode current is 2.48mA. The 6SN7 plate voltage measured 119VDC.
At +350VDC B+ supply, if —8.0VDC grid bias was used as specified in the datasheet, then the —9.0VDC bias voltage would have been close to correct. But, with a B+ supply voltage of +195VDC using either —8.0VDC or 0VDC bias voltage as specified in the datasheet would have set the 6SN7 operating point way off center of linear operation.
In a cathode biased configuration voltage at the cathode is the grid bias voltage. When cathode bias is used voltage developed at the cathode self adjusts to the plate voltage. As plate voltage increases, then current through the tube increases. As the tube draws more current voltage drop across the cathode resistor increases resulting in voltage increase at the cathode. As plate voltage decreases the tube draws less current, voltage drop across the cathode resistor decreases reducing voltage at the cathode.
Using cathode bias in an amplifier design simplifies bias requirements. You can find cathode resistor values for cathode bias operation in some tube datasheets. For voltage amplifier stages they generally range from 1K ohms to 4.7K ohms, higher in some applications. For power output tubes they can range from 150 ohms to 1000 ohms.
Figure 3 is a 500HZ signal test used to check signal symmetry. The circuit was tested at 350VDC and 195VDC B+ supply voltages, bias set in the center of the 6SN7 operating curve. Input signal level was 1.5V and output signal was 21V.
For those who might question how one can possible read an analog meter down to tenths of a volt or mA. Having lived through the age of analog meters one can interpret the scales pretty close. In reality 100% accuracy is not required when working with tube circuits.
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