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Homebrew HF amplifier mis-match

Discussion in 'Homebrew and Kit Projects' started by KM4HGU, Jul 15, 2017.

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  1. KM4HGU

    KM4HGU Ham Member QRZ Page

    I ran across a schematic for an amp using an 813 tube. I had pretty much all the components needed in a box of junk. So, I decided to give it a go.
    Got it all assembled, and fired it up. No majik smoke. burned in the tubes, and shot the HV to it. Still no majik smoke, and the diodes giving a nice purple glow. HV tested to 1kv. Shut it down.
    So far all seems well. I don't know why, but I decided to hook up my antenna analyzer to the RF input to make sure that there would be a 50 ohm impedance, prior to hooking up the driver. The analyzer shows
    66 ohms, and swr of 1.5:1. Hmmm, sez I. I fire up the filaments, and key it, impedance jumps to about 75 ohms, swr off the scale. I know that the analyzer doesn't have enough power to drive the amp, (something in the mw range) But I decided to give it a go anyway, and shot the HV to the plates. Of course there was no change.
    But my concern is, that with the amp off, and dummy load connected, the analyzer is showing me a 66+/- ohm impedance. And an swr of 1.5:1. Which is what my transmitter will see when hooked to the amp.
    There's something between the RFin, and the RFout, causing this, but I can't find it.
  2. SM0AOM

    SM0AOM Ham Member QRZ Page

    You cannot measure the actual input impedance of especially a Grounded-Grid amplifier with an antenna analyser.
    The input impedance is non-linear and highly drive dependent, especially if there is no cathode tuned circuit.

    To interpret your measurements, it is necessary to know more about the circuit, but if you get a "cold" impedance of about 66 ohms, you are seeing the circuit losses.

  3. G0JUR

    G0JUR Ham Member QRZ Page

    If you mean you see 66 ohm and swr 1.5:1 without the amp keyed I would clean the relay contacts.
  4. KM4HGU

    KM4HGU Ham Member QRZ Page

    Thanx guys. That's what it is, "cold" @ 66 ohms, and 1.5:1, amp not even plugged in.
    I suspected the relay right off the bat, and even though it is new, I replaced it with another. (I have several)
    I guess I'll just hook it up to the xmitter, and see what happens.
    KG7SWP likes this.

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