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For AM BC engineers: plate voltage?

Discussion in 'General Technical Questions and Answers' started by NM7G, Aug 11, 2018.

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  1. NM7G

    NM7G Ham Member QRZ Page

    Despite having a GROL, I've little experience with AM broadcast gear. For a transmitter using 4-400A/B/Cs in the PA (either pair or single tube), what plate voltage is most commonly used by transmitter manufacturers? I have tube data sheets. Please don't quote tube ratings. What anode voltage is typically used, plate modulated?

    How much is carrier Pout allowed to drop before tube(s) are pulled and replaced?
  2. AC0OB

    AC0OB Subscriber QRZ Page

    In most cases, 1kW AM transmitters use QTY. 4, 4-400's; 2 in the ClassB push-pull modulator and two in parallel in the Final and with Pi-L network circuits.

    Final Plate voltages run an average of 3.150 volts at approx. 470 mA total plate current.

    Modulator Plate voltages operate around 3,000 volts at 300 mA.

    Maximum PO is 1.1kW.

    Finals were changed when the power dropped to about 85% of licensed power. Mod. tubes were changed when the modulation percentage fell below 85%.


    Last edited: Aug 11, 2018
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  3. NM7G

    NM7G Ham Member QRZ Page

    Super! That's exactly what I needed, Pheel. 73

  4. K8JD

    K8JD Ham Member QRZ Page

    Tube Rotation
    I remember one place I worked at had the same type tubes in the 20 kW FM final and the 5 kW AM final and modulators !
    They would rotate them from the FM transmitter when it's power dropped 5 % , to the AM final where they would be able to run full output, Then after that when they dropped 5% they would go into the modulator stage where they would still work for a while at full modulation !
    KK4CUL and KB4QAA like this.

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