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Johnson Ranger audio driver mod

Discussion in 'Amplitude Modulation' started by N8FVJ, Apr 14, 2020.

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Did you modify the speech amp and sweep just the speech amp to see if your rolloff is occurring there with the same speech amp mic setting?

    If you have too much roll-off or a lower modulation percentage at the higher end then you might want to lower the coupling capacitor value to say 4.7 uF, unless you are satisfied with what you have.

    I usually shoot for around 6kHz as my rolloff (-3 dB) point.

    Pheel
     
    Last edited: Apr 23, 2020
  2. WZ5Q

    WZ5Q Ham Member QRZ Page

    Nice Bench!! :D
     
  3. W1BR

    W1BR Premium Subscriber QRZ Page

    Pheel

    I did not see much improvement with the plate load resistor; I was using a 22 mfd coupling cap. Three or four capacitor changes did the most improvements. I am satisfied with the high end, but the low
    end could use some work. I am almost tempted to replace the existing interstage transformer with a higher quality piece of iron that has better frequency response. I have a ton of room
    on the side apron. If I have time I may measure the turns ratio and see what Hammond has available. It would not make much sense to waste money on a Hammond just to use it for
    for reactor choke.
     
  4. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Phillip,

    I follow what you are doing with the tube charts. But you continue to overlook or ignore my previous and simplest question. If the Ranger worked in the original circuit with +300 volts and stock grid bias, why won't it still work with +300 volts on the plate after the modification, using stock bias voltage?

    300 volts is 300 volts, regardless of the source. Furthermore, the tube paremeters are based on initial static conditions, with no signal presest. How does changing from the transformer to a plate choke, or returning the 12.5 k resistor to +550v, increase the plate dissipation or change anything else at the tube if the plate voltage is the same is it was before?

    If the tube is running class-A and the grid is not being overdriven, the signal will have no effect on DC plate current as measured with a milliammeter. The plate current and plate voltage will remain static. The tube might actually dissipate slightly less with the presence of a signal, since some audio power is inevitably dissipated in the resistor, or as transformer/choke losses, but the DC input is invariable (average plate current doesn't vary with the signal in properly operating class-A), so that dissipated power has to come from somewhere, and the only possibility is to subtract it from the plate dissipation.

    This modification is designed to address one problem and one problem only: the poor performance of the driver transformer with DC flowing through the primary. It was not intended to address other design flaws that may have been present with the stock circuit. Once the driver transformer is operating optimumly, then other issues like optimum grid bias and plate current/plate dissipation, including cathode resistor, can be addressed and corrected.

    If the resistor is used and the resulting plate current is incorrect, the value of the resistor must be tweaked until the correct plate current is achieved. In such a case, the plate voltage will once again measure +300v as it did orignially.
     
    W8KHK likes this.
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I answered your questions in three previous posts with detailed analysis in the last post giving a detailed procedure and analysis to show the results.

    At this point all I can suggest is to print out the charts and work them as outlined in my last post to you and do the math.


    Pheel
     
    Last edited: Apr 23, 2020
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Did you make any mods to the speech amplifier and sweep it?

    To improve the low end one could increase the cathode capacitor at the 12AU7A cathode to 47 uF and place a 22 uf cap across the last stage speech amplifier resistor.

    I have had good luck capacitor coupling the Hammond 124D driver transformer without a driver reactor; see reply #83.

    DO you have a schematic of your present speech amp and driver?


    Phil

    Pheel
     
    Last edited: Apr 23, 2020
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    I'll sweep the audio stages to the interstage primary, and then again to a 1614 grid.

    I suspect the Ranger transformer is 9:1 Z? I'm thinking of ordering a 124B 3:1 interstage, 41 bucks at Hawk electronics.
     
  8. W1BR

    W1BR Premium Subscriber QRZ Page

    I'm seeing some LF roll off in the first 12AX7 section that I can not explain. It is not for lack of
    cathode bypassing. I need to rest my eyes as I may be missing something obvious.
     
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Do you have the original component values or have you modified the speech amp?

    What mic are you using? If it is a D-104 R17 needs to be at least 2.7Meg, C52 needs to a .01, C56 needs to be removed, C57 needs to be a 0.1. See my earlier response to the OP.

    I suspect the Ranger transformer is 9:1 Z? I'm thinking of ordering a 124B 3:1 interstage, 41 bucks at Hawk electronics.

    http://www.hammondmfg.com/pdf/124D.pdf


    I doubt it. I think the 124D would be more appropriate.


    Pheel
     
    Last edited: Apr 24, 2020
  10. W1BR

    W1BR Premium Subscriber QRZ Page

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