One-tube AM-QRP experiments

Discussion in 'Homebrew and Kit Projects' started by W0RIO, Nov 8, 2018.

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  1. W0RIO

    W0RIO Ham Member QRZ Page


    Here's a fun project that I've been wanting to build for several years now, a 1W AM transmitter based on
    the 6U8A tube. I call it the "AM Chickadee".

    It is not specifically for ham radio use, but could certainly be used for in-town communications on 40 or 80 meters.
    Note the modular external PI filter, I use that on my Little Chickadee QRP CW transmitter as well.

    I've always wanted to be able to make an AM transmitter that could be picked up by my numerous
    antique short wave tube radios so that I can play vintage music through my vintage radios.
    It is "legal" as long as the 1W signal is sent to a dummy load, there is enough leakage to pick up
    a nice signal through my house.

    I experimented with both screen and plate modulation and finally settled on plate+screen modulation as
    the best sounding type for this setup. If it can sound good with music, it should work great with voice.

    The schematic and circuit details are here:
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    When my brother and I were yutes, we built a phono oscillator from one of those all-in-one electronics had a single 6sn7 in the middle. We found we could transmit a couple of blocks with a really long wire in the back 40. It was a lot of fun to be Wolfman Jack for a while. Also check out Bob Heil's pine board project. That puts out super clean audio.

  3. K6BSU

    K6BSU Ham Member QRZ Page

    Plate + screen modulation was the standard circuit for most commercial AM transmitters. I am surprised that a filament transformer would have sufficient power bandwidth for good audio.
  4. W0RIO

    W0RIO Ham Member QRZ Page

    It sounds good to me, my high frequency hearing is not that great, nor is the response of the various radios I'm using
    to monitor the signal. The default radio that I have used for monitoring is the chassis from an old Crosley console radio
    with a single 6V6 output tube, I'm using an external 3-way speaker of medium quality.

    As for the modulating iron, it's an old filament transformer, built in the same style as most tube output transformers
    with the primary coil in the center and the filament coil on top of that. Contrast that to some of the newer style split-bobbin
    filament transformers with two side-by-side windings.
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    Filament transformers make very good transformers for going from 500-ohm / 600-ohm audio output impedances on receivers to speakers from 3.2-ohm to 8-ohm. It doesn't make that much difference if the speaker impedance is 3.2-ohms or 8-ohms.

    The 120 VAC primary is connected to the higher impedance source and the filament secondary is connected to the speaker. Especially for communications quality audio, the filament transformer works very well. Even for AM broadcast music the transformer works pretty well.

    Glen, K9STH
  6. W0RIO

    W0RIO Ham Member QRZ Page

    I spent some more time working on the AM Chickadee transmitter this weekend.
    First, before N2EY gets a chance to remind me, I added the RF safety choke to the
    output of the pi filter. That removes the high voltage DC components from
    the antenna terminal and makes it easier to monitor the modulated AM signal at that point
    using an oscilloscope. Without the choke, the superimposed modulation waveform makes the
    signal jump all over the place.

    I decided to move the oscillator coil to an insulated RCA jack on the top of the box.
    (warning, there is exposed high voltage on the jack)
    That makes it easy to change coils for operation on different frequencies.
    I had crystals for 7.3 Mhz and 10.88 Mhz and have wound some toroids that
    resonate at those frequencies.

    Finally, I found a 1.23 Mhz crystal that produces a signal in the AM BC band,
    I had an oscillator coil that I was able to resonate on that frequency.
    For AM BC operation, I can remove the PI output tank circuit and switch over
    to a ferrite rod antenna in parallel with an old BC tuning capacitor.
    The ferrite rod puts out a surprisingly strong signal.

    With those three frequencies, I can produce signals that can be picked up on all of my antique
    AM/SW radios. I had fun listening to oldies music on many of my tube radios this weekend,
    the XYL thinks I'm nuts ;-)

    The photo shows a few of the plug-in components and the ferrite loopstick antenna.

    AD5HR and W7UUU like this.
  7. AA7EJ

    AA7EJ Ham Member QRZ Page

    Are those boxes just electrical junction boxes?
    I hope the OM who posted question about " plastic or aluminum" is reading your post.
    IMHO this kind of experimenting is what amateur radio is all about.
    Yes, I do not own HT for 2 meters...
  8. W0RIO

    W0RIO Ham Member QRZ Page

    Yes. I've built quite a few tube projects on J-boxes. The smaller knock-out tabs are perfect for a 9 pin tube socket
    and the larger ones can fit an octal socket. You can cascade multiple boxes if you need shielding between circuit sections.
    For this project, I mounted the tube socket on a top plate that had one knock-out in the center.
    Here's what it looked like when I started the build:

    K6CZH, AA7EJ, AD5HR and 2 others like this.

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