I don’t feel there are any practical negative impacts of doing so. Several well-regarded classic amplifiers leave small sense resistors in the cathode circuit (e.g., Marantz 9, Heathkit W-7M, Luxman LX-33, Radford STA-25R, and Harmon-Kardon Citation II). Bypassing with a capacitor is not practical due to the low impedance of the sense resistor. If this is a concern, you could shunt the sense resistors with a switch when the meter is not being used. For example, the predicted gain loss for a 10-Ω sense resistor and a triode-connected KT-88 with a 5-kΩ plate-to-plate load, mu of 8 and rp of 650 Ω is 0.2 dB. In practice, the gain loss is minimal for the values recommended. The sense resistor in the cathode introduces a small amount of negative feedback, theoretically reducing both gain and distortion. Cathode-bias amplifiers generally do not have bias adjustments, or if they do, the configuration varies widely, making it hard to recommend a specific bias-meter design.įigure 2b: Independent bias configuration. The meter is designed for fixed-bias amplifiers, not those with cathode bias. In the last section of this article, I describe how to modify the meter to work with independent bias controls. I prefer the arrangement in Figure 2a, and this article describes the corresponding meter configuration. Push-pull output stages can be configured with bias and balance controls (see Figure 2a), or independent bias controls for each tube (see Figure 2b). To the best of my knowledge, this arrangement for measuring bias is unique. The reference voltage is set based on IO, the target bias current per tube, according to VR = IO × RS. Bias is measured by averaging the voltages A and B at point C and comparing the result to a DCA reference voltage, VR. This type of balance meter configuration is fairly common. Balance is measured between points A and B. A small value sense resistor RS is connected in series with each output tube (see Figure 1).
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