Johnson Ranger audio driver mod

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

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  1. K4KYV

    K4KYV Premium Subscriber QRZ Page

    That was a selling point Continental Electronics used back in the 50s. Like Western Electric, Continental never produced a high level plate modulated transmitter. They had a 250 watt rig that used a pair of 4-250s with a combination of control grid and screen grid modulation, plus negative feedback based on audio derived from an RF sample taken at the output terminal and fed back to the speech amplifier.

    They made it a specific point in the manual to that rig that the total efficiency, comparing power drawn from the a.c. mains to the RF to the antenna, was about the same with low-level (grid modulation or leen-yar) or high level plate modulation, once you calculate in losses in the high level modulator tube plates, modulation transformer, power consumed by the modulator tube filaments, audio driver stage, etc.

    Some of their higher power transmitters achieved improved efficiency by using the Doherty system, including the Continental transmitter at Border Blaster XERF.

    Nevertheless, nothing beats the high level plate modulated class-C final for ease and stability in tuning and frequency agility.
     
  2. W2VW

    W2VW XML Subscriber QRZ Page

    The Doherty system is used in new Rohde & Schwarz solid state UHF television transmitters in order to eclipse the mighty MSDC IOT transmitter for total efficiency.

    As far as transmitter configuration, there are plenty of ways to skin a cat and some of them are very good.

    AM on the amateur bands is all about content. Same as broadcast.
     
    AC0OB likes this.
  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Here is the Continental 317B 50kW AM transmitter.

    Continental manufactured a number of standalone SGM transmitter’s in the late 1950’s with transmitter powers up to 10 kW. Here is a functional diagram of the Continental 317B broadcast transmitter which uses a 5kW screen modulated section to drive a Doherty linear amplifier.

    The 5 kW screen grid driver section is one of simplicity. An 807 first audio amplifier feeds audio to a second audio amplifier, a 4-65A. The output of this second audio amplifier drives a modulator consisting of three 4-65’s in parallel. The 4-65 cathodes then modulate the screen grids of two 4CX5000A RF amplifiers in parallel, producing 5 kW to feed the Doherty linear amplifier, which then increases power to 55 kW.

    The modulator varies the voltage on the screens of the 4CX5000A’s between 325 volts and 575 volts with a current of approximately 50 mA! Due to the power amplification efficiency of the 4CX5000A’s, the average amount of modulating power needed was only 100 Watts.

    Continental 317bschem.JPG

    Notice the Carrier and Peak tube sections and the many adjustments needed to arrive at the correct phases.


    Pheel
     
    Last edited: Apr 30, 2020
  4. K4KYV

    K4KYV Premium Subscriber QRZ Page

    A similar system you don't hear much about is the Terman-Woodward system. It is almost identical to the Doherty, except that the final tubes are control grid modulated rather than class-B linear amplifiers.

    Linear amplification itself is pretty close to the same thing as control grid modulation. With the linear amp, the DC grid voltage is held constant while the RF drive is modulated. With control grid modulation, the RF drive level is held constant while the DC grid voltage is modulated.
     
  5. W2VW

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  6. AC0OB

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    The way you trigger any crowbar but I never saw a crowbar in these transmitters. They used a lot of very sensitive Overcurrent and Overvoltage relays which were very fast.

    Their high power shortwave transmitters don't use screen modulation, rather they use a pulse-step modulation system which is controlled by IGBT's.

    Pheel
     
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    I am going to end up with a 4-hole 813 amp and a dual 813 amp when the dust settles (if I live long enough to finish what I start.)

    I was planning on using the dual 813 amp with the Ranger for maybe around 500 or 600 watts PEP. But the more I think about it, I am wondering if it is worth the effort?
    I could save a lot of heat in the Ranger box by just using it as a CW exciter, and converting the 813 design to screen modulation. Does that make sense?

    Pete
     
  8. VE3AJM

    VE3AJM Ham Member QRZ Page

    If the Ranger driver transformer is such a bottleneck, why not just drop in a pair of 6550s for the modulator. Run them in AB1, use R/C coupling, a phase inverter and voila...no need for any driver transformer. One 6550 is essentially 2 6L6s in one bottle.
    The fewer transformers needed in audio, the better.
    An upgrade to the Ranger modulation transformer which could physically fit into the transmitter is intreging if it could be engineered.
     
    Last edited: May 10, 2020
    W1BR likes this.
  9. W2VW

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    re invent dawheel.png
     
    KD2ACO likes this.
  10. W1BR

    W1BR Premium Subscriber QRZ Page

    Using Don's mods, the driver transformer works okay, the modulation transformer is the bottleneck from what I can see.... too bad a modulation reactor won't fit inside the box. But, the Ranger audio doesn't look all that bad once several value changes are made. It is more of an academic quest than a practical endeavor at this point.
     

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