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DX-100/Apache Audio/Modulator Improvements

Discussion in 'Amplitude Modulation' started by AC0OB, Jul 19, 2019.

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  1. AC0OB

    AC0OB Subscriber QRZ Page

    If you can find a non-polarized capacitor of the appropriate physical size, rating, and budget, that fits into the space, by all means use it.

    If you can find a film capacitor of the appropriate physical size, rating, and budget, that fits into the space, by all means use it.

    In our case none of the above criteria was met and we used what the gentleman had available to accomplish the task.

    Again, this design was not done to create Audiophile audio, but to improve the modulated audio within the appropriate physical size, rating, and budget and it does that very well.

    Pheel
     
    Last edited: Jul 21, 2019
  2. W8KHK

    W8KHK Ham Member QRZ Page

    Phil, thank you for the additional suggestions. I have looked at Stu's write-up on his simulation, and it indicates he added a diode and resistor network in parallel to each capacitor for safety. And when he removed the protective components the simulation indicated an increased charging current in the reverse direction on each capacitor. That is enlightening! I have seen cases where back-to-back capacitors have failed, but never looked into if further, thinking it may just be a component age-related failure.

    Stu also suggested the parallel loading circuit, with the resistor and blocking capacitor, since it was directly applicable in that situation. That is an old, standard method that I learned to apply to an audio pre-amplifier when driving a line matching transformer before I was a teen. It is mentioned in much of the literature, for example, it my 4th edition Radiotron Designer's Handbook on page 519, as follows:

    "Parallel feed (Fig. 12.24) avoids the direct plate current passing through the transformer primary and thereby increases its effective inductance, and decreases transformer distortion."

    This coupling method is also presented in almost every edition of the Radio Handbook.

    I have yet to find a document indicating any benefit when using a non-polarized capacitor for blocking DC in an audio amplifier. I understand different types of capacitor have attributes that perform better in certain applications, but that is separate from the polarization issue. I am always interested in learning something new, especially when it comes to circuit design and component performance.

    73, Rick
     
    Last edited: Jul 21, 2019
  3. WZ5Q

    WZ5Q Ham Member QRZ Page

    The Parafeed input configuration is my preferred method of keeping DC off a single winding primary of audio iron. I also like to use choke loading with a plate reactor in leu of resistor loading on the preceding tube stage. One out of the many reasons is because of the energy storage capability the reactor gives which allows it to "swing" over the power supply voltage. Unfortunately the plate reactor requires a lot of inductance at its rated current (usually around 100H depending on the load and frequencies of interest) and must be wound with techniques to keep winding capacitance at a minimum. Jack at Electra-Print winds some fabulous plate reactors. Another major caveat with this configuration is that the polypropylene coupling capacitor value must be chosen with diligence to prevent resonances.

    I use this configuration for the modulator driver of my temporary Micro-Iron AM Transmitter, a completely modified AF-67. The stock 3:1 Class B Driver Iron was changed to a UTC Linear Standard LS-21. This Iron cannot take any DC on it's Primary at all, so a Parafeed Input configuration was implemented. The modulator driver 12B4A plate is choke loaded using a 100H @ 40mA Plate Reactor with a 3uF polypropylene coupling capacitor to the LS-21 Primary. 520VDC is supplied to the reactor and the 12B4A idles at 38mA. It's screaming, but the 12B4A is tough.
     
  4. AC0OB

    AC0OB Subscriber QRZ Page

    Well, I appreciate all of the comments but I think a couple of schematics would certainly help to suport the text, aka, a picture is worth a thousand words.:cool:

    I think many people are graphic-driven, especially those new to Homebuilding and modifying vintage gear.:)

    Pheel
     
    N6YW likes this.
  5. W8KHK

    W8KHK Ham Member QRZ Page

    Here is a reference schematic with some of the guidelines and caveats for folks wishing to implement the parafeed coupling network as referenced by Mike, WZ5Q. He mentions the careful selection of the capacitor value, and the theory is explained here, referencing the classic schematic diagram.

    parafeed.JPG

    PFcap.JPG

    Reference Radiotron Designer's Handbook, Edition 4, page 519 circa 1952
     
  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    That seems like it would be horribly inefficient, since the load resistor would have to dissipate something on the order of half the audio power...but it might be a reasonable compromise with frequency response.
     
  7. KL7AJ

    KL7AJ Ham Member QRZ Page

    Replacing RL with a high reactance inductor would be much gooder.
     
  8. WZ5Q

    WZ5Q Ham Member QRZ Page

    Here ya go Pheel. ;)

    af67.809.mods1.jpg
     
  9. AC0OB

    AC0OB Subscriber QRZ Page

    Kool! Screen driven sweep tubes.

    But where did you find room to stuff a 100H@>=50 mA choke?

    Also curious as to why not run the 809's plate at 520V instead of 370V? Higher Mod percentage perhaps?

    Pheel
     
    Last edited: Jul 22, 2019
  10. WZ5Q

    WZ5Q Ham Member QRZ Page

    I have all the iron outboard on a shelf.


    2_little_frankie_near.jpg

    3_little_frankie_audio_iron.jpg
    4_little_frankie_audio_iron.jpg
     
    W2VW likes this.

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