Building a Mono Band Transmitter

Discussion in 'Amplitude Modulation - AM Fans' started by AC0OB, Jun 25, 2017.

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  1. W6MQI

    W6MQI Ham Member QRZ Page

    Phil,
    From part III
    Just to make sure I understand correctly the choke value is selected using the formula so we end up on a frequency that matches the operating frequency of the transmitter? So for example if we plan to operate at 3.5Mhz a 3mH would be the best selection? Just want to verify "frequency sensitive resistor".

    Dave
     
  2. K4KYV

    K4KYV Subscriber QRZ Page

    Looks like a broadly-tuned resonant circuit, which would work OK for a single-band rig if it was broad-tuning enough to cover the desired frequency range. Another possibility would be to make it completely non-resonant, with enough inductance to pass the desired frequency and no resonance anywhere near the band of operation as indicated by a grid-dip meter. However, sometimes a non-resonant, passive-untuned circuit won't deliver enough RF voltage to fully drive the next stage. If it won't cover the desired frequency range with uniform output, a tuned circuit could be employed, using a coil and variable capacitor or a slug-tuned coil and fixed capacitor, but that would be one more tuned circuit to adjust when changing frequency.

    A well-known example of the broadly tuned circuit is in the output of the popular tube type VFOs from the 50s and 60s, like the Heathkit VF-1, Knightkit and Johnson 122. They use a piece of coax a few feet long between the VFO and the transmitter, with a slug tuned coil to broadly resonate with the capacitance of the coax. The slug is adjusted to peak in the middle of the band, hoping to maintain enough output to adequately drive the first stage of the transmitter across the entire band. This is usually somewhat of a compromise; to get adequate drive at one end of the band the drive may fall off at the opposite end.

    Tube amplifiers are more difficult than solid state amplifiers to broad-band.
     
  3. N2DTS

    N2DTS Ham Member QRZ Page

    Why does it have a band switch on the final for 80, 40 and 20 meters?
     
  4. W6MQI

    W6MQI Ham Member QRZ Page

    Was wondering that myself?
     
  5. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    From the OP

     
  6. K4KYV

    K4KYV Subscriber QRZ Page

    A different plate choke L1 will have to be switched in for each band.
     
  7. KM1H

    KM1H Subscriber QRZ Page

    Great thread but WHY a 6DQ5? Granted it is a fine tube but becoming pretty scarce, at least in this area. OTOH the 6BG6 and 6DQ6 are pretty common NOS and ignored at hamfests; the older 6BG6G can handle a lot more than the specs as it is really a 6L6G with a plate cap. It looks prettier also IMO than the slab sided GA:D:p

    Carl
     
  8. KM1H

    KM1H Subscriber QRZ Page

    Which L1?
     
  9. K4KYV

    K4KYV Subscriber QRZ Page

    The one shown in the schematics in Part III (see attachment in post # 15).

     
  10. KM1H

    KM1H Subscriber QRZ Page

    150 uH is adequate for all 3 bands but Id use about 2X for the extra XL on 80. Series resonances can be checked with a GDO but seldom a problem on those bands and physically small chokes.
    In ancient days without knowing better 1-2.5 mH was common.
     

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