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What's wrong with this picture DX-40

Discussion in '"Boat Anchor" & Classic Equipment' started by K1OIK, Jun 28, 2019.

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  1. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    Meter reads .85ma maximum it can be loaded to, plate voltage 600 V=52 watts input
    Power out 45 watts.
    It should load to 125ma.
    So it the current meter off?
    What is the typical efficiency of a DX-40? I guess 70%?
  2. W1BR

    W1BR Ham Member QRZ Page

    I am surprised you can resolve .85 mA on that meter. Regardless, the power input would around 1/2 watt if you are correct.
  3. N2EY

    N2EY Premium Subscriber QRZ Page

    Not anywhere near enough information provided to answer the questions. We're not mind readers! You don't even tell us if that .85 mA is grid or plate current.

    Let's see...we need to know:

    Band? Mode? Crystal or VFO? Grid and plate currents as indicated on the meter? Measured plate and screen voltages? Test setup (dummy load? Wattmeter? Indicated SWR?)

    If the meter accuracy is questionable, why not test it? All that's needed is a battery, a resistor of proper value, a known-good milliammeter, and some clip leads.

    Even this guy can't answer your questions without more info:

    Last edited: Jun 28, 2019
    KS4W and W1BR like this.
  4. WN1MB

    WN1MB Ham Member QRZ Page

    Place current meter in series with meter of known accuracy. Pass current through meters. Note any variance. Answer own question.
    K7TRF, AG5CK, KS4W and 2 others like this.
  5. WR2E

    WR2E Ham Member QRZ Page

    I would probably think eff more like 60%

    And if that is so, then loaded to 125 ma @ 600V says 75W input.

    At 60% eff that's 45 W out.

    So yeah, it's likely that the meter is off. Check that first as others have said.
    K1OIK and KS4W like this.
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Several things:

    First of all, 125 mA, at 600-volts, with the DX-40, does NOT really indicate 75-watts plate input power. The meter is actually reading cathode current which includes the screen current which is around 12 mA. "Assuming" 12.5 mA for the screen current, the real plate current is 112.5 mA. With a plate voltage of 600 VDC, the actual plate input power would be 67.5-watts.

    Heath did this for several reasons including the fact that having the meter in the cathode circuit is a LOT safer than having the meter in the plate circuit. Also, since the Novice Class was limited to no more than 75-watts plate input power, and the fact that most Novice Class operators, in fact, most higher class operators as well, indicating that 125 mA was the maximum reading that insured the power limitations were being met, assured that the Novice Class operator stayed within the limitations.

    The meter, itself, is probably OK. What is causing the problem is the fact that the meter shunt has changed values. This is very common. The shunt, for the "plate" (actually cathode) current reading is a 0.67-ohm resistor that is, if I remember correctly, wired directly across the "grid - plate" meter switch.

    Then, there is the accuracy of the wattmeter. Most amateur radio grade wattmeters have a tolerance of at least +/- 10% of the full scale reading. If the full scale reading is 100-watts, then an indication of 45-watts could be anywhere between 35-watts and 55-watts. If the full scale reading is 250-watts, then an indication of 45-watts could be anywhere between 20-watts and 65-watts.

    Glen, K9STH
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  7. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    85Ma at 600 volts is 52 watts. I shouldn't have put the period in front in the first post but I was rushing to get out the door to bike and play volleyball, but in retrospect it should not have been hard to see my error when I indicated 52 watts.
  8. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    80 meters, CW, Crystal, grid current 3 ma (between 1ma-4ma it doesn't make much difference), load was a light bulb but changed to a resistor at 50 ohms, wattmeter is accurate, SWR 1.1:1, screen voltage will be tested. The meter is not a current meter but is across a shunt which I will try to test.
    My main question from a 6146 what efficiency should be expected? I am guess the current reading cannot be accurate as I am getting 45 watts out with 52 watts in.
  9. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    That analysis makes total sense. I have used this watt meter with two Flex's, an Icom 7300 and some vintage rigs. It agreed with the internal wattmeter's on the modern rigs tracking nearly perfectly. I am guessing the shunt is off but I also want to check the screen voltage.
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    If you want any kind of life out of the 6146, don't run more than around 2.5 mA for the grid current. It is the grid, in the 6146 family, that is the weak point. Running more grid current than necessary will substantially reduce the life of the tube.

    RCA originally specified 3.75 mA for maximum grid current. Then, they reduced it to 3.5 mA, next 3.25 mA, and, finally, to 3.0 mA grid current per tube. Running between 2.0 mA and 2.5 mA grid current per tube is a good target. I have seen a lot of the 6146 family that put out full power at 1.75 mA grid current.

    Glen, K9STH
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