The AM Forum

My problem is the opposite from Dan's high output issue. My DX-60B runs 720 volts plate current, but power output only 38-40 watts. Dosen't this seem low?

73

Daryl  K8FP

720 volts is more or less normal for a 6146.   What is your plate current?  We need to know this before we can continue.

This transmitter loads nicely with no trouble maintaining 150 mils plate current with 2.5 mils at the grid. All into a good dummy load with no SWR and numerous wattmeters tried.


Thanks

Daryl  K8FP

Hmmm.... That  should produce about 108 W input, and should theoretically produce  about 70W output (assuming about 65% efficiency).  Hmmmm....    Now 150 mils is a  bit high for a 6146.  You ought to be running it at about 125 ma max.  Still....    Are you sure the 6146 is good?   Grid drive sounds right.  Hmmmm....

Both of my 6146 final transmitters have the output power of about 40W.   They are a Ranger II and an AF-67.

Yup, could be. Depends on the plate voltage.  I think that those rigs run lower plate voltage than the Heath rigs.

How are you measuring the plate current? I had a similar problem and thought the tube was bad. Two brand new ones did the same thing. Then I checked the meter resistors for the plate current.They were waaaaayy out of tolerance and what was showing on the meter as 150 mA was really only loading up at about 80-90 mA.

Daryl

I agree that you might want to check the meter shunt resistor (the one used when the meter is in the plate current position) to be sure that the plate current reading is correct.

Separately:

When loading up the transmitter: make sure that you start with the loading capacitor set* at too high a capacitance (which should result in a lower value of average plate current)... and then gradually adjust the loading capacitor in the direction that produces less loading capacitance... until you get the most output power (as measured on your power meter) when you "dip" the plate current by adjusting the tuning capacitor**.

*In most transmitters, the loading capacitor is at maximum capacitance when the loading control knob is set to the "0" position on the loading scale (maximum CCW). If the loading capacitor has no mechanical barrier to keep it from turning the full 360 degrees... then it would be easy to accidentally set the loading capacitor knob incorrectly... with the capacitance at minimum when the knob is pointing to "0".

**The impedance of the tank circuit, as seen by the tube, goes up as the impedance of the loading capacitor goes down. The impedance of the loading capacitor goes down as the capacitance is increased. Therefore, by starting with too much loading capacitance, you start with the tank circuit having too high an impedance ... which results in less plate current... which is where you want to start.


If you go past this point (i.e., using even less loading capacitance), then the impedance seen by the tube at the output rf frequency will be too low. This will result in somewhat more average plate current when you "dip" the plate current... but less rf output power. (I.e., more electrical input power and less rf output power => less efficiency => more plate dissipation).

Make sure that the knob on the DX-60 is attached so that the loading capacitor is fully closed (maximum capacitance) when the knob is pointing toward "0" (fully CCW on the scale).

If the transmitter is properly loaded, and if the average grid current is correct (which indicates that the peak grid drive voltage is about right)... then the tube should be running at about 65-75% efficiency (rf output power / (plate voltage x average plate current)... depending upon the details of whether the tube is running closer to Class B (which is what a DX-60B is designed to do) or well into Class C (which is what a Ranger is designed to do).

 Stu

Daryl

Also carefully check the DC value of the grid bias voltage when the average grid current is 2.5 mA. The place to make this measurement is on the bottom side of the 1.1mH choke... where it connects to C12, R11, R12, and the grid side of the meter switch.

The voltage shown on the schematic is -38 volts... which is about right for biasing the tube near Class B operation... actually very slightly into Class AB (~65% efficiency if it is loaded properly). Based on the schematic... -38 volts is what I calculate for 2.5 ma of grid current... if all of the component values are as shown on the schematic

On the DX-60B, the grid bias is affected by:  the output of the negative bias voltage supply , the value of the average grid current, and the values of R30, R12, R11, and R10 .... so there are a whole bunch of things that could cause the grid bias to be less negative than it needs to be. I would be particularly concerned about a reduction in the value of R10 or an increase in the value of R30.

If the grid voltage is not negative enough, then the tube will not be operating near "Class B"... and the efficiency will be significantly lower than it would be with the grid bias set properly.

Stu