Finished 6L6 transmitter

Discussion in 'Homebrew and Kit Projects' started by K4EM, Feb 13, 2012.

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  1. AF6LJ

    AF6LJ Ham Member QRZ Page

    I would be concerned about tweaking the feedback cap, doing so could expose the crystal to too much current and cause it to fail.
    I would concentrate on the output circuit.
    I got so wrapped up in the circuit.....
    I forgot to mention how very Cool your transmitter and power supply looks.
    You did a real nice job.
  2. N3HEE

    N3HEE Ham Member QRZ Page

    I now believe the lower power is by design. You are using a regulated screen supply for the purpose of getting a nice clean cw note. This lowers the screen voltage which in turn reduces the power. This is a trade off of course.

    I don't have a regulated screen supply and my screen voltage is higher hence my output is higher. However, I use a DDS (digital) VFO to drive my 6L6 so I don't have to worry too much about getting a clean cw note. My keying is super clean with the VFO. I remember now that when I was using a crystal the cw note was awful with 10 watts output.

    Take a look at this article.

    "Here is my newest transmitter. It uses a 6L6 with a 6X5 rectifier and a 0D3 voltage regulator tube on the screen. The power output is only about 5 watts. It can go almost twice that by pulling out the 0D3, but the keying is not as good that way. "

    Your output power is pretty close to K4GC's notes. I wouldn't worry to much about it. Nobody's going to notice a difference in your signal over a couple of watts. When you get your VFO hooked up you could experiment with the screen supply to see if you can get your power up a little. But then again not much difference between 3, 5 and 10 watts. You need ten times more power for a 2 S unit increase!

    Use it and have fun with it. I have a ball with mine!
    Last edited: Feb 14, 2012
  3. K5UOS

    K5UOS Ham Member QRZ Page

    The crystal is a good thought. I have used homebrew VFO's forever so my crystal experience is limited.

    There are crystals on the market that I doubt will last long in a circuit with current as high as this circuit will see -

    The crystals on Brian's site are old FT243 cases stuffed with smaller HC49. The high B+ (>~400VDC) can even be a strain on the older, tougher vintage FT243 crystals.

    Any setting of C1 (feedback circuit trimmer) might not prevent a weak or fragile crystal from heating with the high B+ in this circuit.

    I have never put a crystal under extreme high Ep conditions so I don't know what the effect on drive would be.

    Where did the crystal come from?

  4. K4EM

    K4EM XML Subscriber QRZ Page

    The crystals came from a hamfest; I did try them in my Knight T-60 and they seemed to work, the signal looked like crap on my Flex though. I read the article you mentioned, I might try pulling the OD3 and make some measurements and see what i get. I have a couple of new 6V6's I can try, they are in a guitar amp I built. I'll do some more testing over the next several days and see what I get. I had looked at those crystals at af4k's site but at $14 a piece, i'll wait and scrounge the hamfests a little more.
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    Good point on the regulated screen supply.
    I wonder if the screen supply is unregulated if the tradeoff for higher power would be chirp in the signal.
  6. N3HEE

    N3HEE Ham Member QRZ Page

    Crystals are cool but expensive. I was tired of being rock bound so I spent the money to build the N3ZI DDS VFO. Money well spent. Let me know if you go that route. There are a couple of things you will need to do to get that VFO strong enough to drive your transmitter. I spent quite a bit of time and energy on that part.

    I imagine there will be chirp and stability issues when you pull that regulator tube out. I had LOTS of chirp at 10 watts. I have NO chirp with my DDS VFO. My rig is built bread board style on a piece of wood. Let us know how you make out.

    I'd like to have a QSO with you sometime to hear what it sounds like.
  7. N3HEE

    N3HEE Ham Member QRZ Page

    I get 341VDC on the plate and 254VDC on the screen key down. Drawing 63MA total (plate & screen) key down. Reading 10 watts output on 160 meters.
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    I remember your transmitter here on the Zed I followed it's progress with considerable interest.
  9. K6JJR

    K6JJR Ham Member QRZ Page

    Beautiful work, Dave. I built a 6146 xmttr last year and Im having a blast even with 30w output.
    Yours looks MUCH nicer tho..great job! I hope you get your output up a bit! 73!
  10. WB4CKM

    WB4CKM Ham Member QRZ Page

    I am curious about those two 10-watt 100 ohm resistors. Are they your 50-ohm dummy load? They appear to be the wirewound type, and so would have very high inductive reactance (i.e., impedance) at HF, much much greater than 50 ohms. For example 50 uH manifests as 2200 ohms impedance at 7MHz. If you are measuring RF output power delivered to such a reactive load, this might explain the low power measurements that you reported.
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