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Finished 6L6 transmitter

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

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  1. K4EM

    K4EM XML Subscriber QRZ Page

    I finally finished the transmitter, the external power supply was finished last year. As observed on my Flex 5000 the signal looks good and clean. I only have 2 crystals right now to test it with, a 80m & 40m crystal. This is the AI2G design that can be found on many sites. I tried it with a 6V6, 6L6GC and 5881; power output is lower than I expected. With the 6V6= 1.5w; 6L6GC= 3w; 5881=4w. Plate voltage is 420v & plate current is 20ma thats 9.5w and screen voltage is 148v. I suppose this is normal for a MOPA. I did add a small relay to switch between an internal dumy load and the antenna, the unused side of the relay is used for LED indicators.
    Dave
    k4em
     

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  2. KB9LXP

    KB9LXP Ham Member QRZ Page

    Looks excellent !!!!
    Any problems finding parts? That's my biggest gripe on ham brewing....finding parts.
    Mike
     
  3. WA6TKD

    WA6TKD Ham Member QRZ Page

    Looks very nice. From the looks of it, I suspect that power supply could handle a lot more then 10 watts?
     
  4. K8ERV

    K8ERV Ham Member QRZ Page

    You are transmitting 6L6s? Where are they going?

    Scotty wants to know.

    TOM K8ERV Montrose Colo
     
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Now, you need to go to a metal 6L6, invert it in water, put 1000 volts on the plate, and run it like some operators did in the 1930s and 1940s. :rolleyes:

    One had to run high speed CW because a short "dah" turned the plates red and a long "dah" melted them!

    Glen, K9STH
     
  6. K5UOS

    K5UOS Ham Member QRZ Page

    Dave,

    Is the +420VDC measured key down? This is not a MOPA. It is a keyed oscillator.

    I think there is something that can be tweaked to increase output. I have a 6V6 keyed oscillator with an output of 5W with approx +250VDC on the plate.

    My 6AG7/6L6 MOPA outputs 12W with +330VDC on the plate.

    I'm curious what your plate voltage is key up and down? What is the screen voltage key down?

    I would expect about 7W output from your rig with a 6L6. Nice construction.

    K5UOS
     
  7. N3HEE

    N3HEE Ham Member QRZ Page

    I also agree something isn't quite right. My 6L6 oscillator will do roughly 8-10 watts output with 330 volts on the plate. Not sure what my screen voltage is.

    Looking at your layout you may want to focus on the feedback cap values in the oscillator circuit. The run from the xtal socket to the tube can add enough stray capacitance to throw the oscillator off. That's where I had most of my problems getting mine to work properly. Also, if you are going to drive it with a VFO, if that's what the RCA jack is for, then you will need to bypass the feedback network with a .01ufd cap to kill the oscillator.

    Definitely a very nice looking build though!
     
    Last edited: Feb 13, 2012
  8. K8ERV

    K8ERV Ham Member QRZ Page

    Musta been a Chinese tube.

    TOM K8ERV Montrose Colo
     
  9. K4EM

    K4EM XML Subscriber QRZ Page

    Thanks for all the replies and info. The plate voltage not-keyed is about 430v; keyed it is about 420v. The screen is 148.9 non-keyed; keyed it is 148.2. The tubes I used are all used, these may be weak not sure, I do have some other 6V6's I will try out. I could replace the long wire from the crystal socket to the tube with some rg174 small coax. The RCA jack is for future VFO operation, a heathkit HG10.
    Dave
     
  10. K5UOS

    K5UOS Ham Member QRZ Page

    You are going to have a lot of fun with this transmitter. You should be proud. Lots of people start vintage receivers and transmitters but never finish or use them on the air.

    The circuit is really the Three Band Oscillator Transmitter for the Novice in the 1964 ARRL Handbook. This was/is popular and claimed 30W input 80,40,15M or about 17W output with a 6DQ6A.

    Also see the original in the Dec 1957 QST p34 by Lew McCoy - Three-Band One-Tube Novice Transmitter.

    Your Plate current (Ip) looks low for +420VDC. Below is the typical operating characteristics for a 6L6 metal type from NJ7P’s FB site. This is (for practical purposes) close enough to the expected operation for the 6DQ6A.


    6L6 - Class C Oscillator/Amplifier - http://www.nj7p.org/Tube4.php?tube=6l6
    Plate Voltage ................................. 375 V
    Grid No. 2 Voltage ............................ 200 V
    Grid No. 1 Voltage ............................ -35 V
    Grid No. 1 Current ............................ 3.5 mA
    Plate Current ................................. 88 mA
    Grid No. 2 Current ............................ 9 mA
    Driving Power ................................. 0.18 W
    Power Output (approx) ......................... 17 W

    It could be the lower screen voltage, a weak tube, the PI network or many other things like trimmer C1 adjustment.

    I think KB3KJS agrees that you should be getting at least 10W output.

    Good luck. I have at least one spare 6L6 and some new Sovtek 5881's I can send. I tried a chinese 6L6 and it worked fine in my MOPA.

    K5UOS
     
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