The second change provides proper grid bias. The 6DQ5 requires approximately —70 volts on its grid, where the 6146 likes —45 volts. Since the 6DQ5 draws little grid current (more on this later), grid-leak bias is impractical. Battery bias is recommended. Lift the ground end of the existing grid resistor from ground and connect it as shown in Fig. 2. At any convenient location, install two 30-volt hearing-aid batteries connected in series to provide 60 volts. Ground the positive terminal, and connect the lead from the grid resistor to the negative terminal. This applies —60 volts fixed bias to the grid; current through the grid leak will supply the additional 10 volts needed.
005 mt fig 2
The third change provides correct screen voltage. The 6DQ5 requires 150 volts on the screen, at 20 ma screen current. This is 8 ma more than the 6146 screen takes, although the voltage is the same, so the screen-dropping resistor will have to be changed. To determine the new value, subtract 150 from the plate-supply voltage, and divide the remainder by 0.02, Wattage rating of the new resistor, including a safety factor of two, is equal to 0.0008 times the resistance.
With many transmitters, this completes the modification. However, the output capacitance of the 6DQ5 is appreciably greater than that of the 6146 or 807, and as a result you may have to remove a few turns from the 10- and 15-meter tank coils to be able to tune through resonance. In addition, slight pruning of the coils may increase efficiency by getting a higher tank-circuit Q.
If your rig uses fixed tuning in the final grid circuit, re-peak the tuning slugs after making the modification. This is standard procedure whenever the final tube is changed, but the adjustment may be greater this time since input capacitance, too, is greater than that of the tubes replaced.
Don't expect grid current of the 6DQ5 to be so great as it was with the previous tube; the 6DQ5 is capable of delivering 175 ma of plate current without ever driving the grid positive, which means that no grid current at all need flow to be able to load the tube to its maximum ratings. The fixed bias added in conversion step two makes grid current unnecessary for tube protection.
The best way to determine the amount of drive needed with your rig is to hook up an output-power indicator of some sort (lamp bulb, wattmeter, etc.) and fire up. Start with maximum drive, and decrease the drive until output just begins to fall off. Now, increase the drive a hair. Check to see that power output increases with modulation. If not, increase the drive a bit more until upward modulation is obtained. This is the correct operating point for your rig.
So far, we've talked only about modification of existing equipment. Naturally, the 6DQ5 can also be used in new-design homebrew equipment. The no-grid-current feature eliminates the need for bulky drivers, since the oscillator itself may furnish enough voltage to drive the tube to its limit. Thus, high power becomes practical for mobile use—or for apartment dwellers,
Was this article helpful?