DX-100/Apache Audio/Modulator Improvements

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

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I recently helped an email acquaintance upgrade his DX-100 modulator system, which I affectionately call the, “Pre-Apache.”

    While the two rigs share very similar circuitry, the DX-100 uses the 12 volt equivalent of the 807, the 1625, while the Apache’s modulator tubes are EL-34/6CA7. The 6CA7 takes less control grid drive than does the 807 in Class AB2, so we dispensed with the 1k grid parasitic resistors and replaced them with 100 ohm resistors which are just as effective but increase grid drive.

    We regulated the modulator screen voltage with the zeners he had on hand since without regulation the screen voltages can surge to 350 volts or more shortening the life of the tubes.

    The capacitive coupling at the driver-to-transformer primary removes any problems with DC core saturation and its ensuing distortion.

    With the component values shown in “[]” the Peak-to-Peak voltages for 100% modulation were as follows:

    12BY7 control grid Pin 2: 6 volts p-p,

    12BY7 Plate Pin 7,8: 133 volts p-p,

    Full Primary of 124D: 130 volts p-p,

    Full Secondary (Grid-to-Grid) of 124D: 195 volts p-p.

    The Modulator transformer was wired as per the original schematic, except for the soft-knee limiter.

    Attached Files:

    WD4IGX and N6YW like this.
  2. W8KHK

    W8KHK Ham Member QRZ Page

    Interesting updates to the driver stages. I am curious as to the advantage of back-to-back electrolytic capacitors coupling the 12BY7 plate to the driver transformer primary. As the plate would always be positive with respect to the transformer primary, with the cold end grounded, I would think one capacitor would be sufficient. In addition, if there is leakage current in the capacitor with the positive terminal toward the plate, there could be a reverse polarity charge on the capacitor with the negative terminal toward the plate. Perhaps this is an un-intended carry-over from a previous circuit design?
  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    No, this is simply way to arrive at a non-polar (bi-polar) capacitor without having to pay for the price or size of a non-polar (bi-polar) capacitor. :cool:

    A single 10 uF@450V polarized cap will work almost as well. :)

  4. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    Feedback covers a multitude of sins:
    One of the innovations of Harris broadcast transmitters in more recent incarnations is the application of inverse feedback derived from a DETECTED RF envelope...and fed back to a low level audio stage. The neat thing about this method is that it compensates for any non-linearity in EITHER the audio or the modulated RF stage. It would be a great idea to experiment with this in the AM ham universe. p.s. It's best to use a FULL WAVE detector or even a synchronous detector to derive the feedback signal.

    WD4IGX, N6YW, W2VW and 1 other person like this.
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Inverse feedback from the modulated to the low-level stages is not a recent innovation. In the older Gates, RCA, Raytheon, Bauer, etc, a ladder network fed the AC audio levels back to the 600 ohm input transformer or AC and DC was fed to the cathode of one of the low-level audio stages.

    For diode detected audio feedback, the full wave detector has to be linearized so it doesn't contribute its own artifacts.

    But for Amateur radio, who really needs 0.5 % distortion figures?

    N6YW likes this.
  6. W8KHK

    W8KHK Ham Member QRZ Page

    Yes, I have used back-to-back electrolytics to implement a non-polarized capacitor many times, for example, in audio crossover networks where there is no DC polarizing voltage. But in this application, there clearly is a polarizing voltage. I asked what is the advantage of having the two capacitors instead of just one. You say that a single non-polarized capacitor works "Almost" as well. Why "Almost"?

    And you did not address the risk factor on the opposite polarity capacitor. I am not convinced I would want to implement in that manner.

    When using electrolytics for audio coupling capacitors, they certainly do not excel at higher frequencies. A better approach, in my opinion, would be to parallel the electrolytic with a smaller value mylar or other non-polarized capacitor to handle the higher frequency audio. Please describe the specific advantage, and how it applies to DX-100 audio. Inquiring minds want to know! Thank you!
  7. N2DTS

    N2DTS Ham Member QRZ Page

    I have thought about feedback from the rf side to audio for years, but never found the actual need for it.
    I also never had much luck with global feedback and limit current stuff to feedback within each stage if needed.
    I built an AB1 4-125 modulator off an old qst design which uses no feedback but looks like a lot of 1940's broadcast transmitter
    designs, a pair of 6SJ7's driving the grids of the 4-125/250/400 tubes with NO feedback.
    Sounded fantastic until the mod transformer gave up.
    The 4-125's like 2500 volts on the plates but the mod iron liked 1500 volts 75 years ago...
    That is why I am looking for a T368 mod transformer, it will handle the voltage.

    Feedback cleans up distortion, but good design should be low enough in distortion to do without.
    Pure signal IS feedback from the RF side and surely is the cleanest signal yet produced on the ham bands.
    Only a few people could likely HEAR the difference I suspect...
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    See simulation comment below.

    Use whatever design philosophy with which you feel comfortable.

    I never said they excelled at high audio frequencies and we're not designing for Audiophile quality audio, and the main reason I stated for using this design was to prevent any DC saturation in the driver transformer. And another consideration is that if there is no DC current through the driver transformer's primary winding, there is less heating of that winding from I^2xR losses.

    The back-to-back arrangement has been simulated by Stu, AB2EZ, and these are his comments, in part:

    After doing a little research, on line, about the physics and chemistry of electrolytic capacitors... I decided to do a simulation to see how two electrolytic capacitors... placed back to back would behave....

    ...The simulations are shown below. V(n001) is the voltage across C1 (from left to right), and V(n002) is the voltage across C2 (from right to left). The location of the ground is not relevant, except to provide a reference point for producing the voltage displays. The applied voltage source is a 100V (amplitude) 60Hz sine wave.

    As predicted, the combination of the back-to-back capacitors...acts like a single capacitor whose value is 0.5 x the capacitance of either of the two back-to-back capacitors. More important: neither capacitor experiences a voltage in the wrong polarity.

    I've never tried this in practice... but it looks like a great idea for creating a non polar capacitor out of polarized electrolytic capacitors.

    So I suggest you present your own Apache/DX-100 driver design (schematic) and simulate the back-to-back capacitance situation as well and show us how it won't work.

    Also, please study the Inovonics 222 processor schematic and tell us what kind of coupling capacitance arrangement is shown between amp IC1A and IC1B.


    Last edited: Jul 20, 2019
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  9. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    I certainly agree, and this has always been the philosophy in broadcast transmitters. In my recent CQ column "Analog Adventures" I make the statement, "Inverse feedback covers a multitude of sins, but you need to be careful because it can create some totally original ones. " :)
  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page


    For example if you design low level audio stages with sufficient gain and do not by-pass the cathodes, then each stage will have its own local distortion reduction/feedback mechanism.


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