A new matched pair of Sovtek EL84/6BQ5 output tubes and NOS 6EU7 dual triode and 6CA4 rectifier! On the DG-SE1, the output stage is cathode biased. That means the grids of the output tubes sit at zero volts, and the cathodes have positive voltage. Dave Gillespie, who designed the mods that make this amp a DG-SE1, found that different versions of the EL84/6BQ5 required different values of the bias resistors to achieve the desired bias current, about 49 mA. Also, tubes change as they age. Replacing the tubes in the output stage therefore requires checking and, likely, adjusting the bias current. Checking the bias current means measuring the voltage drop across the "sense" resistor (100 ohms in this case, 120 ohms in Dave's original design). For a 100-ohm sense resistor, the bias current is the voltage divided by 100, so 4.87 volts means 0.0487 amps or 48.7 mA. Setting the bias current means turning the little screw that changes the value of a variable resistor (potentiometer) that is in series with the sense resistor. I've installed these potentiometers so that the little screws are accessible through the top of the chassis, and turning the screw clockwise turns up the voltage (and the bias current) by decreasing the resistance.
In this video, I demonstrate this process.
25 авг 2024
