This project is a 'hybrid', combining integrated-circuit and tube technology in one circuit, It was developed by the author as a case study that's elaborately described in his book The State of Hollow State Audio -in the Second Decade of the 27st Century, published by Elektor. The 12EL6 tubes used here are low-voltage "space charge' types discussed at length in the book. The IC seen in the circuit diagram in Figure 1 is a type OP27 op amp. The type LTC6090 IC originally used in the design proved hard to get in experimenter quantities. Since the input resistance of the op-amp stage must be about 1 MΩ, an op amp is needed that has very low bias current, offset current, and offset voltage. Also, the op amp must have very low noise. The OP27 meets these requirements and is readily available in small quantities. The LTC6090 boasted rail-to-rail input and output voltage ranges, whereas for a 2 kΩ or higher load the OP27 has a typical span of 1.5 Ⅴ (maximum 3.5 V) between maximum output voltage and the supply rail. This requires a change in our thinking about how to use the op amp to set the control-grid bias of V1. Other low-bias-current, rail-to-rail op amps are manufactured, but I did not find any that are easily available in small quantities. Thus the OP27 was selected and using it we need a quiescent DC output voltage at least 3.5 Ⅴ more positive than the negative supply (which is zero volts dc in this single-ended-supply configuration). We also need to allow about a volt for the output voltage swing to avoid clipping in the op amp stage. Then we need for the V1 control grid bias to be adjustable from about zero to -0.2 V. Of course, with a single supply, we cannot produce a negative voltage, so we'll have to make the cathode somewhat positive. In the original circuit, we used two diodes in series between cathode and ground to place the cathode about 1,2 Ⅴ above ground. Thus a 1-V control-grid voltage gives us an equivalent bias of -0.2 V. But we can't drive the output of the OP27 quiescently that low. Since the forward voltage drop of a semiconductor diode is not well-defined (0.6 Ⅴ is the approximate value), I found experimentally that by using only one diode between cathode and ground of V1, and making the adjustment range of the non-inverting input to the op amp span from 0.387 Ⅴ to 5.6 V, I could obtain the desired range of control-grid bias. Knowing from the earlier work that the input resistance of the cathode follower would load the V1 stage, I examined the output waveform from V1.1 found that it required a larger signal than a typical humbucking pickup would produce in order to create the distortion signature I wanted, so I increased the op-amp gain by changing feedback resistor R4 to 10 MΩ. Then I verified that the op amp could not be driven into clipping until the triode was already in hard clipping, so that our circuit would provide a 'tube 'sound.
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