WRL Globe 65 Transmitter modifications

Discussion in 'Amplitude Modulation' started by N8FVJ, Mar 29, 2020.

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    If the tube rectifier is in good condition and with 121 volts from the mains the screen voltage is 320 Volts.

    The problem with these entry-level transmitters is that the time the transmitter is in the AM mode the Speech Amplifier
    and Modulator Tubes have B+ on them.

    What is needed is a relay circuit to switch the voltage to the speech amp and modulator only when needed and that is on Keydown.

    Pheel


     

    Attached Files:

  2. N8FVJ

    N8FVJ Ham Member QRZ Page

    Yes, kind of a pain to switch the CW and Transmit switch at same time when transmitting AM to keep the 6L6 and speech amp off when receiving.
     
  3. N8FVJ

    N8FVJ Ham Member QRZ Page

    Some further investigation shows the stock 6L6 plate dissipation is 24 watts. A 5881 6L6GB is a 24 watt plate dissipation tube thus will work and appears the plate over-voltage is not an issue. I am more comfortable with the Sovtek 6L6WXT 500 volt plate rating. The Svetlana 6L6GC has the same voltage rating as the Sovtek.
    The 6146 shows a little red on plate during a minute key up. The 6146B does not. It seems either tube works, but the 6146 is the original design for the Globe Scout 65.
     
    Last edited: Apr 6, 2020
  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    One more item. The Globe Scout arced at 5U4GB base during warm up. The 3100 volt spike was to be eliminated as viewed on the software simulation with the .02uF 4KV capacitor added on the choke input, but arced anyways. It did not ruin the 5U4GB, but the 5R4GB is highly recommended and B+ is only 10 volts lower. Also unloads the power transformer by 5 watts.
     
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    You mentioned that before so why repeat it?

    Why would a Final plate show any color during a non-conduction period?

    6146 tube gains can vary widely no matter the flavor as I can attest to after having used and tested many of them.

    You could do yourself a favor by studying some tube theory.


    Pheel
     

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  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Why would it arc unless you had some conductive crud across the pins?

    You lose 7 Watts of filament power@6.3V by using a 5AR4.


    You never did answer my question on what tube characteristics you used in the .SUBCKT listing for the Spice simulation.

    Did you ever hear of something called a, "Self-fulfilling prophesy?"

    Go ahead and do what you want in spite of the many suggestions from experienced members here.


    Pheel
     

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    Last edited: Apr 6, 2020
    W7UUU and W1BR like this.
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page


    You lose 6.5 Watts (approx. = to 7 Watts rounded up) of filament power@5.0V by using a 5AR4.



    Pheel
     
  8. N8FVJ

    N8FVJ Ham Member QRZ Page

    The 5U4 arced inside the glass, not outside. I performed I believe every mod you suggested except using discrete components in the speech amp that is not needed at the moment while bringing the Scout up on line. So, self-fulfilling prophesy is not a nice or true comment. I think I will stop posting on the Globe Scout now.
     
    Last edited: Apr 7, 2020
  9. N8FVJ

    N8FVJ Ham Member QRZ Page

    I am finished with the Globe 65. I used 300 ohm dropping resistor on the 6146 plate vs W8JI recommended 450 ohm and tried shorting across the resistor. Output power was the same at 20 watts on 75 meters with or without the resistor. The resistor with the parallel 10uF capacitor sounded a little better on AM. I used the 20K in series with the 1uF capacitor on the screen and removed it. Power output was the same, but the 20K resistor and 10uF capacitor sounded a little better on AM. The mic drive with a D-104 and series capacitor with 5 meg ohm on 6SJ7 grid has a lot of gain thus setting to 2 on the gain control sounded best.
    The Globe Scout after mods does not make anything near 100% modulation as stated in W8JI's article. My peak reading analog meter swings to 30 watts peak and this is with a new 6146 and 6L6 modulator tube. As a comparison my Gonset G-76 makes a 75 watt carrier and swings to 300 watts on the same watt meter. Unfortunately I would have thought 50 watts input in Class C would produce a 30 watts carrier and per the few articles the heisting choke would produce 100% modulation. There are no wiring errors, soft tubes or other issues in the Scout. It is interesting I get 2 watts more power with a used 5U4GB vs a new 5R4GB. I owned two Sylvania 5U4GB and one arced at the glass inside at the base during warm up. Other did not. The 20 volts higher output from the 5U4GB makes a difference. SS rectifier would make more power, but the 6L6GB is at the limit of 500 volts with the tube. The Globe Scout 65 does not produce the power I want for my Amp Supply LK-500zb amp. I guess I will be looking for a Johnson Ranger. The Globe Scout was fun to rebuild and modify.
     
  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Use a scope and monitor the voltage across a 50 ohm dummy load. The peak to peak voltage is then divided by 2.83 to get Vrms. OR, take the peak-to-peak voltage and multiply it by 0.354 to get Vrms.

    You actual power is then P = Vrms X Vrms/50 ohms.

    10 to 20 Watts into ANY linear amp is about nominal.

    SS rectifier would make more power, but the 6L6GB is at the limit of 500 volts with the tube. Yea, and that's why there is a 180 to 220 ohm resistor in series with the PS choke in the schematic we gave you earlier to reduce the resultant B+, and that is why Don KYV suggested an 807 as the modulator and in which I followed up with an 807 modulator schematic.

    The resistor with the parallel 10uF capacitor sounded a little better on AM. Sounded better Than what, the 20k with a 1 uF capacitor in the SCREEN circuit? Did you try the 33k with a 1 uF cap? What was your Screen Voltage at those values?


    Pheel

     
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