Fun with the heathkit MT-1 Cheyenne transmitter...

Discussion in 'Amplitude Modulation' started by N2DTS, Dec 26, 2020.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I have had the transmitter sitting around for a while and decided to try yet another modulator setup.
    The audio compartment is very small and it pays to keep the parts count low.

    I decided to try this circuit:
    [​IMG]

    It works great and has a low parts count, but I have to yell into the D104 to get 100% modulation.
    I then tried a D104 with a preamp in it and its still a bit on the low side, not a lot of extra gain.
    Maybe the second stage has little gain?
     
  2. W8KHK

    W8KHK Ham Member QRZ Page

    The second stage is a cathode follower, therefore it has negative gain, but high current output capability. There should be no reason that the first stage of the 6DE7 needs a low impedance source. I suggest reconfiguring the second stage of the 12AX7 as a common cathode, and you can save your voice.
     
    N2EY likes this.
  3. N2DTS

    N2DTS Ham Member QRZ Page

    Yes, I was unsure why it was designed that way. Some sort of negative feedback in that cathode setup?
     
  4. W8KHK

    W8KHK Ham Member QRZ Page

    The output of the cathode follower is developed across the 100K resistor. The 680 ohm resistor develops the negative grid bias.

    Tie the bottom of the gain pot to ground. Use anywhere between 3 and 4 kohms cathode to ground, and anywhere between 100K and 220K plate to B+. Then take your coupling to the 6DE7 grid from the plate of the 12AX7 pin 1. You will have your gain back.

    It makes no sense to put a cathode follower there if more gain is needed. There is no magical feedback going on in that stage, just a classic cathode follower.
     
    N2EY likes this.
  5. N2DTS

    N2DTS Ham Member QRZ Page

    I put a 470k to B+, a 3000 ohm cathode resistor to ground and a .05 cap to the next stage.
    Plenty of gain and it sounds good.
    I only have about 550 volts for HV so power output was 5 watts carrier at 120% positive modulation.
    700 or 800 volts would make it sing out to 10 or 15 watts.

    These transmitters have a very nice and stable vfo in them.
    The chrome knobs look great but they have to go, they are very small and slippery.
     
    N2EY likes this.
  6. N2EY

    N2EY XML Subscriber QRZ Page

    Yes, the use of a cathode follower there is a mystery. Also, note the 0.1 uF capacitor between the first and second stages, while all the other coupling caps are .005 uF. Why? If more low end is wanted, shouldn't all the caps be, say, .01?
     
  7. K1APJ

    K1APJ Premium Subscriber QRZ Page

    When you figure that the effective resistance of the volume control is probably better than 10M, due the bootstrap connection, the .1 uF becomes even sillier.

    And, I need not mention, there is no controlled carrier effect in this whole circuit, although that may be the intent.

    Definitely not an "engineered" design.
     
    N2EY likes this.
  8. N2DTS

    N2DTS Ham Member QRZ Page

    Did a VFO alignment, this one does not track as well as the other one I had, but it is stable.
    I run the filaments off a 12 volt supply to keep hum out of the PTT and audio stages.
    I added 100K resistor to the 5763 screen circuit and still get plenty of drive, even on 10 meters.

    You need to really filter the power supplies as there is no mod transformer to act as a choke.
    I will build up a nice well filtered power supply for it, 650 volts, 300 volts, -120 volts.
     
  9. N2DTS

    N2DTS Ham Member QRZ Page

    I worked N8YHP on 3885 on the MT-1 using a little mcHF sdr plugged into the RX port and got good reports from David
    with 5 watts carrier.
    The transmitter worked fine.
    Very easy to tune up, put in CW, tune for max power (20 watts), switch to phone and away you go.
    1.5 ma grid drive works fine.
    VFO seems very stable.
     
  10. K0IZ

    K0IZ Ham Member QRZ Page

    Easy gain increase: stick a 10uf or so bypass cap on the 1st stage cathode.
     

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