WRL Globe 65 Transmitter modifications

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

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    The Globe 65 and 680 series of transmitters is an inexpensive means to own an AM transmitter. Other similar inexpensive transmitters are the Hallicrafters HT-40, Knight T-60, Heathkit DX-40 & DX-60 and a few others. The list of transmitters are low level modulation thus produce about 8-10 watts carrier power. A more powerful and inexpensive low level modulation transmitter is the Knight T-150, but the VFO is bad news. It transmits on 75 and 60 meters at the same time!

    The Globe 65 and 680 are high level modulated using heising modulation. Briefly, heising design uses a choke in series with the audio tube plate and works well. However, the Globe transmitters do not produce 100% modulation. Likely in stock form about 60-70% modulation.

    Modifications are listed here. http://www.w8ji.com/Heising modulation.htm
    A few mods are also listed here. http://www.w8ji.com/globe_scout_modifications.htm

    Read carefully as the first author is technically skilled, but his description of the circuit mods are not well written. One glaring error is using a 22K resistor in series with the 1uF capacitor in series with the 20K resistor supplying the 6146 screen. This also does not show on his schematic. Never the less, the author is talented. And, the audio on a modified Globe sounds very good,.

    If you follow the modifications, the Globe 65 and 680 will supply a 25 carrier and modulate it 100% both positive and negative. A Johnson Ranger makes a 40 watt carrier, but cost at least $350 + heavy weight adds an expensive shipping cost. The Globe is only 29 lbs. My Globe from NY to MI cost under $35 to ship. I paid $65 for it and cosmetics were good plus worked. The difference in-between the Johnson Ranger cost and with shipping leaves enough cash left over that you can buy a good receiver! There is not much difference in-between a 25 watt or 40 watt carrier. But, 25 watts is a serious improvement from the 8-10 watt carrier of the low level modulation transmitters mentioned.

    One item the Johnson has that the Globe does not is a VFO. The Globe 6V6 oscillator is a bit power hungry. Thus, you need a VFO that produces at least 10 volts. A good VFO that is around $50 used is a Heathkit VF-1 with that 10 volts output. Only issue is it does not have an internal power supply. A source for an inexpensive transformer to make a power supply is Antek Transformer Company. However, the Globe has a husky power transformer and can supply the VFO power. An issue is the Globe makes lower voltages for some tubes thru dropping resistors, thus does not have a low B+ voltage. Fortunately the VF-1 can operate off of 20 ma of current. One way to get lower voltage is replacing the Globe 50K ohm bleeder resistor with a tapped 50K 25 watt resistor. The tap can be set to supply lower voltage and a 10uF 450 volt capacitor across the tap provides well filtered DC. The Globe wastes power for the bleeder resistor anyways thus 20 more ma is no big deal. And, the Globe transformer is husky- about a 500 watt CCS rated transformer. One issue is the Globe power supply when not transmitting raises in voltage due to the choke input power supply and insufficient bleeder resistance. The difference is not huge, about an extra 100 volts. Thus, the regulator tube will draw about 35 ma (within regulator tube specs) when the Globe is not transmitting. During transmit this current drops to about 20 ma.

    The VF-1 needs two tube changes for better stability. Replace the 0A2 with a 0B2 and the 6AU6 with a 6AH6.

    Two Globe issues is the power supply design. Software simulation shows a large voltage spike on the 5U4 rectifier at 3100 volts. That is far above 5u4 specs. Adding a .01uF or better yet a .02uF on the rectifier tube output removes that spike. The .02uF capacitor draws about 8ma thus I would use a poly type capacitor vs a ceramic unless that ceramic capacitor is transmitter output rated. I would also use a 4000 volt rated capacitor. A better rectifier tube is a 5R4GB rated at 3100 volts PIV. The 5R4 drops more volts, but is insignificant. Globe 115 volt rated line input on 120 volts makes up the difference anyways.

    You should replace the original electrolytic capacitors. 12uF on the choke output produces about 20 volts on AC ripple. That is a little high IMO. I replaced with 32uF for about 8 volts AC ripple output. The original capacitors were so large there was no room for larger value capacitors.

    The Globe 6L6 modulator tube is under stress after the modifications as the 6L6 screen voltage is set for a plate dissipation around 28 watts. Metal 6L6 and 6L6GB or 5881 are rated 19 watts plate dissipation. The vintage 6L6GC are a 30 watt tube, but so expensive due to audiophile and musician demand. A inexpensive 30 watt tube is the Sovtek 6L6WXT. Some seller state 5881 WXT is that is only a 19 watt tube. The real 6L6WXT has a stepped base.

    So, have fun.
     
    Last edited: Mar 29, 2020
    W4KJG likes this.
  2. N8FVJ

    N8FVJ Ham Member QRZ Page

    BTW- a 25 watt carrier Globe 65 with 100 watts peak is perfect for most amplifiers. That would make about 250-300 watts carrier with 1000-1200 watts peak. You can 'run' with the big boys or about the same as a Johnson 500 or Globe King 500!
     
  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page





    W8JI never identifies his Point “C” nor voltages at any nodes which would have helped future modifiers and experimenters. In addition, all L1’s in the Globe Scouts I have seen are 7H. He also swapped R101 and R106 in his discussion.

    This was addressed over here: https://forums.qrz.com/index.php?threads/globe-scout-65b-and-heathkit-vf-1.698102/


    Software simulation should be followed up by actual observation. How many 5U4's have failed in Globe Scouts due to transients? How many 5U4's have failed due to old age, i.e., decreasing filament emission over time?

    What were the values you used in Spice to characterize this diode?



    Also addressed over here with a number of better solutions: https://forums.qrz.com/index.php?threads/globe-scout-65b-and-heathkit-vf-1.698102/



    Pheel
     
  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    Two items. I doubt any 5U4GB failures existed, but for a few dollars I will use the .01uF capacitor to remove the spike. Some do not care for operating the VF-1 from the Globe, but it can be performed without causing damage to the Globe or VF-1. My article is how to get on AM with a reasonable signal and low budget. I would rather own a Johnson 500, but............
     
  5. N8FVJ

    N8FVJ Ham Member QRZ Page

    Speech amp mods. Thanks to AC00B
    upload_2020-3-30_1-24-51.png
     
  6. N8FVJ

    N8FVJ Ham Member QRZ Page

    BTW- if you want a Globe 65 or 680, they do not show up much on ebay. Place a want ad on the Ham radio sites. I had two show up in a day for sale. The 65 and 680 are identical except the audio preamp and band coverage. The 65 has a 6SJ7 driving a 6C5 for the preamp. I also heard some use a 6J5 vs a 6C5. The 680 uses a 6U8 preamp tube. The model 65 is 160 to 10 meters. The 680 is 80 to 6 meters.
    Note- for 50 ohm antennas the output load capacitor needs 220pF in parallel. 160 meters needs 680pF on the non-doublet output connector. Use the doublet antenna connector on 160 & 80 meters. Use other antenna connector for 40 thru 10 & 6 meters.
     
    Last edited: Mar 30, 2020
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    The was a Globe Scout 66 made same year of the 680. The 66 covered 160 to 10 meters. The 66 model uses the 6U8 speech preamp.
    The last model of same heisting modulator design is the Globe Scout Deluxe. Many different tubes vs the 65, 66 and 680 and makes about 35 watts AM carrier output. Globe Scout Deluxe below[​IMG]
     
    Last edited: Mar 30, 2020
  8. N8FVJ

    N8FVJ Ham Member QRZ Page

    I mention using 400 volts on the 6L6 screen with an adjustable resistor. Original screen resistor is 22K. A fixed resistor of 16K @ 10 watt will provide about 400 volts on the 6L6 screen.
     
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    The GScout 66 speech amplifier upgrade is similar as I have done both, just a different tube. Fun to work on and very simple rigs but I think I prefer the GScout 65 with the octal tubes.


    Pheel

     

    Attached Files:

  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I wouldn't.

    Examine the tube specs and curves again.

    This tube, while being a fairly tough tube, and with the voltages imposed on it, is already screaming for some relief, with Vsg = 320V@6.3 mA screen current and Ip = 50 mA@430V plate current and voltage.

    The voltage divider in the previously posted schematic places 320 volts on the screen at about 6 mA with a voltage stabilizing capacitor. A voltage divider always provides a more stabilized screen voltage than a simple dropping resistor.

    I suggest the following schematic if you want variable power:


    Pheel

     

    Attached Files:

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