Johnson Ranger audio driver mod

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

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    I should not post here due causing trouble with my posts. But, one more post. I seen many modifications to the popular Johnson Ranger transmitter. One of the best IMO is W8JI. He states the transmitter is well designed and perhaps Johnsons best design. He recommends the 500pF cap at the 12AX7 pin 7 to pin 1 be replaced with a .01uF for lower frequency response. That is about it.

    One item I noticed and have not found any reference to is the 12AU7 modulator driver tube. Looking at the plate curves it is poor (non-linear) and especially so at 310 volts plate. Here is the plate curves of the 12AU7 at 310 volts plate. As the tube output swings to less than 5 ma, it is not linear. The steps in-between the bias points should be close to equal. The tube looks a lot better at 150 volts plate.
    [​IMG]

    Now, lets look at the 6CG7 plate curves at 310 volts plate. It is much more linear.
    [​IMG]
    I recommend replacing the 12AU7 with a 6CG7. The 12AU7 and 6CG7 is almost identical in operation, ie bias points, plate resistance, etc. To use in the Johnson Ranger simply cut the jumper on pins 4 & 5. Then, remove the two filament wires from pin 9 and move to either pin 4 or 5 that is not grounded to the chassis. Either pin could be grounded, on my Johnson pin 5 was grounded,

    The 6CG7 used .6 amps filament and the 12AU7 uses .3 amps. The extra filament draw will not cause harm to the transformer. If you have concern, you can replace the 6AX5 with a solid-state rectifier freeing up more current than the 6CG7 draws. I do not mind the 6AX5 solid-state mods as only an extra 20 volts is on the low B+. I do not like the 5R4 solid-state replacement as about 50 extra volts is supplied. Other have used solid-state replacing the 5R4 and have had no issues that I read.

    Will the Ranger sound better? It may with a strong signal, but not likely noticeable during weak signals due to band noise. Changing the 12Au7 to a 6CG7 is not for everyone, but I thought it is interesting.
     
    Last edited: Apr 14, 2020
  2. WR2E

    WR2E Ham Member QRZ Page

    What class is the AU7 operating?

    How much bias?

    It probably never gets that close to 5 ma.
     
  3. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The bottleneck in Ranger audio is the driver transformer. Used a plate resistor to feed the parallel sections of the 12AU7. It was 40 years ago, so I don't recall whether I used a 10K or a 30K plate resistor, but it was rated at 5 watts. The plate was coupled to the driver transformer through something like a 0.5 or 1 mfd non-electrolytic 600 volt capacitor, and the bottom side of the transformer primary grounded. This takes the DC plate current off the primary; this current flowing through the primary saturates the core.

    The plate resistor drops the plate voltage to the 12AU7, so to bring it back up to normal, the resistor is returned to the HV supply (the same one that furnishes plate voltage to the 6146 and to the modulator tubes). The resistance should be enough to drop the plate voltage back to what it was with the transformer directly connected. Try a 30k resistor; if the plate voltage is substantially too low, try the 10K instead.

    That modification along with capacitor change mentioned above should help tremendously. If you are using a crystal mic like the unamplified D-104, make sure the input resistance to the 1st audio stage is at least 5 megohms, preferably 10 megs.

    I further modified my driver transformer by taking it out of the circuit, disassembling it, and re-stacking the laminations so that adjacent E and I laminations are reversed direction, thus re-stacking the core like a power transformer, eliminating the core gap, which isn't needed with no DC passing through the primary. That made an additional tremendous difference.

    Since I wasn't using mine as a class-B driver for a bigger rig, I disconnected the negative feedback and removed the capacitor across the secondary of the modulation transformer, which opens up the high end. Don't recall if there is a capacitor across the driver transformer, but if there is, remove it as well. The reason for disconnecting the negative feedback is that the modulation transformer has enough phase shift to change the feedback from negative to positive at some frequency above 3000~, causing the modulator to go into self-oscillation with about a 4000~ tone.

    If the 6CG7 is nearly identical to the 12AU7, what is the point of swapping them out?
     
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  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    Operating points are identical, plate curves are not. My Johnson does not use plate resistors on the 12AU7. B+ goes to the driver transformer input and that transformer output goes to the 12AU7 plates. Some voltage drop exists across the driver transformer, but not much.

    Perhaps you changed to a long tail phase inverter. It would need plate resistors and drop the 12AU7 voltage into a much better area on the plate curves. I seen this mod on the internet.
     
    Last edited: Apr 15, 2020
  5. W1BR

    W1BR Premium Subscriber QRZ Page


    Shunt coupling the transformer, clever idea.
     
    KM1H likes this.
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    A couple of problems here:

    1) The voltage chart in the Johnson manual shows 11V on the cathode of the 12AU7A driver. 11V/820 ohms = ~ 13.5 mA. So each tube with no signal is carrying 6.8 mA.
    2) On the 12AU7 "AVERAGE PLATE CHARACTERISTICS" chart draw another curve in between the -10 and -12 Ec curves for -11V. This is called "Interpolation."

    From 280 volts at the bottom draw a vertical line up to and through your new -11V curve. The intersection point plate current is about 9.5 mA as read on the left side. The intersection point of the new -11V curve shows the triode is operating in a very linear region of the curve.

    They wanted the total plate current in T3's primary to vary between 13.5 mA and 19 mA in order to induce sufficient voltage in T3's secondary to drive the modulator tubes.

    Most of the Class A1 data in the tube specs are for resistance-coupled amplifiers. For inductive loads, the rules change somewhat.


    Pheel
     
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  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    My recommendation for the speech amp if you are going to be using a high impedance microphone and you don't want that restricted SSB sound:

    Pheel
     

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  8. W1BR

    W1BR Premium Subscriber QRZ Page

  9. WA3VJB

    WA3VJB Ham Member QRZ Page

    Huh. That does seem a bit prescient.

    There was a guy on the main QRZ.com forum who would ask a wide range of questions, none of them related, but starting a discussion, and then he would disappear from the thread, seldom closing the loop with advice and perspective he had been offered by well-meaning respondents.

    I had noticed the pattern and called him out on it, using the forum's search function to tally HUNDREDS of his questions that all followed the same pattern. Never got a direct reply, and my theory is that he was just some kind of lonely person who enjoyed people talking about HIS question. Or else he is writing a book about all the mysteries of HAM radio and how others have solved them.
     
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  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

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