Bauer Power On

Discussion in 'Amplitude Modulation' started by KA4KOE, Apr 21, 2017.

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  1. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    If the change in power level is accomplished by decreasing or increasing plate voltage, the impedance shouldn't change. If it is modulating linearly, any change in plate voltage should result in a proportional change in plate current. The change in RF output power should be directly proportional to the change in DC input power to the plate of the final.

    Think of it this way. When you plate modulate a final, the plate (and screen) voltage are changing all the time, from zero to twice or more the resting plate voltage, throughout the audio cycle. If the impedance changed with plate voltage (and thus power level), the impedance would be continually varying in step with the audio as the transmitter is modulated. You would have "impedance modulation" of the final as well as plate modulation. This would undoubtedly result in undesired phase modulation and other forms of distortion, and would indicate severe modulation non-linearity.

    The final amplifier impedance should remain constant if the only thing changing is the plate and screen voltage, as the output power level is changed. OTOH, if the power change is achieved by adjusting the loading on the final (with the plate and screen voltages remaining constant), the impedance will change as the power output level is varied.

    In some cases, the grid drive may be varied proportionally with the plate voltage as well, since for example, it would take less grid driving power to fully drive a class-C final running at 1000 volts @ 250 mA than to drive that same final at 2000 volts @ 500 mA. Hence, the Gates BC-1T/G/H gimmick of using a tertiary winding on the modulation transformer to modulate the driver stage along with the final. This improves modulation linearity, so that changes in plate current more closely follow changes in plate voltage, and consequently, changes in rf output power more closely follow changes in DC input power to the plate(s) of the final.
     
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Agreed.

    The basis of my statement was that there would not be a truly linear relationship since the loading would affect the Final's Plate current.


    Phil
     
  3. N2DTS

    N2DTS Ham Member QRZ Page

    How is the screen voltage set?
    Reducing the plate voltage without reducing the screen voltage is risky.
    If its off a dropping resistor from the plate voltage, I guess it would track.
    The problem with some designs is they were expected to be run a particular way.

    I use a fixed adjustable screen supply with a high value dropping resistor. That allows a wide range of change before the screen overload relays trip.

    Anyway, you can get a feel for the Q by how sharp the dip is, depending on cap value and reduction drive.
    A really sharp dip at rated power output is high Q.
    Is the rig neutralized?
    Plate dip does not change grid current?
    Or, dip is at peak power output.

    Were most broadcast transmitters neutralized?
     
  4. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    If they used the 833 triode in the Final, most were neutralized.

    If they used the 4-400 tetrode in the Final, most were NOT neutralized.


    Phil
     
  5. KA4KOE

    KA4KOE Ham Member QRZ Page

    Screen and B+ are both varied at hi/low power.
    B+ High 3100V/B+ Low 1550V
    Sg High 470V/Sg Low 270V
     
  6. N2DTS

    N2DTS Ham Member QRZ Page

    1550 volts is really low for 4-400's.
     
  7. N1BCG

    N1BCG Ham Member QRZ Page

    Is it on the air?
     
  8. KA4KOE

    KA4KOE Ham Member QRZ Page

    This week is what I'm thinking.
     
  9. W7TFO

    W7TFO Platinum Subscriber Platinum Subscriber QRZ Page

    BC rigs usually had cutback options to fixed power levels for op at night according to licensed values;

    250 or 500 for a 1kW, 500 or 1kW for a 5kW, eTc.

    Not to be confused with a tune/operate function.

    50 Pages!

    73DG
     
    AC0OB likes this.
  10. KA4KOE

    KA4KOE Ham Member QRZ Page

    IMG_2290.JPG IMG_2292.JPG IMG_2293.JPG IMG_2295.JPG Sleeve installation and coaxial cable run. Connectors tomorrow. I'm plum tuckered out from plundering around in the attic space behind my shack wall above the garage. The ceiling holes are right behind the transmitter. The far left one is for coax, and the other is for audio, monitoring, and control lines. The stainless plates were entirely unnecessary but I wanted it to look nice. The conduits were attached to the studs with U-clamps behind the gypboard.
     
    KD2ACO likes this.

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