Turning the 6DC6 into a 80m preamp for a reciver.

Discussion in 'Homebrew and Kit Projects' started by KC9OXP, Feb 28, 2009.

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  1. W0BTU

    W0BTU Ham Member QRZ Page

  2. KC9OXP

    KC9OXP Ham Member QRZ Page

    I just completed the testing phase of it. Works EXCELLENT! My static without it is a S1 with it, S5. Radio Havana Cuba is usually a S9, is now a S+20. As soon as I find some wire to solder in for the power supply of it, I will post pictures. My SX-117 has never received so well. With my wire attic antenna, seems to even have reduced the hum I get from that antenna. I am thinking about in the future installing a relay that will jump the in\out when powered off and making and installing a broad cast band nerf.

    It draws 36mA with a led power on light.

  3. KC9OXP

    KC9OXP Ham Member QRZ Page

    I put a short clip of it functioning on you tube. I can't seem to remember the BB code for embedding you tube videos, so its a link. Pictures may come after I get some more testing done and my picture camera batteries charge.
  4. W0BTU

    W0BTU Ham Member QRZ Page

    Hi Michael,

    Great! :) You made my day.

    I saw the video, and wouldn't mind seeing a photo of the preamp.

    Now all you need to really have fun is a good receiving antenna. Here's a site for starters, to get you thinking:

    And exactly what do you mean, broadcast band "nerf"?

    73 Mike
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Way to GO!

    Way to go Michael.

    Communications electronics are fun.:D

    Does the 5109 get warm with 39 mills?
    Last edited: Mar 15, 2009
  6. W0BTU

    W0BTU Ham Member QRZ Page

    Preamp current vs. IMD

    I'm glad you asked this question, as it finally motivated me to compare the two schematics to see the reason that W7IUV's preamp draws so much more current than mine.

    It appears that Larry has changed the value of R5 (in his schematic) from 75 to 10 ohms. I would say that this mod would explain the 89 mA current (and the need for a bigger heat sink). In an environment where there were other strong out-of-band signals, I would guess that Larry's mod would make the preamp less susceptible to IMD problems.

    My schematic (based on the ARRL book) also uses a 100 ohm resistor in series with the +12 volt input.

    The value of C3 is also different (.1 vs. .01).

    The back-to-back paralleled diodes between the input and ground could also cause problems near a high-power transmitter. But I lost several transistors during thunderstorms before I added those diodes.

    Here's W7IUV's reply to me concerning the preamp (with his kind permission to post it here):

    > Re the resistor: I can't ever recall using a 75 ohm
    > resistor there. Of course I've been messing with this
    > circuit for 30 years so it's possible that I did use
    > something like that at one time. But I am absolutely sure
    > that I never published that value. That's why I
    > don't want anybody to post the schematic on their web
    > pages. If it's not on my web page, it's not a W7IUV
    > preamp. Period.
    > The circuit shown on my web page is correct as is and is
    > the only one I am currently recommending. There are two
    > resistors in the emitter. One is 4.7 and the other is 10.
    > The 10 is bypassed.
    > Changing the 10 will alter the current without changing the
    > gain. Changing the 4.7 will change the gain and current at
    > the same time. These listed values are optimum for a 2N5109
    > with 12.0 volts on the collector. If you do not use a 2N5109
    > the current may be different! 2N5109, 2N3866 and MM2009 all
    > have DC gains that are very similar. Using a NTE sub may
    > cause "undocumented" operation.
    > If you do not want to have maximum performance and the heat
    > bothers you, simply drop the supply voltage to 10.0 volts
    > and keep the same resistor values. The gain will drop a tiny
    > bit, the IMD performance will drop a couple dB, but the heat
    > will drop about 40%.
    > Re destroying transistors: the preamp I am using on 160 has
    > far more than 60,000 hours of nearly continuous operation
    > with no "protection" of any kind. The power is
    > always on it and the antenna is always connected to it. No
    > matter if I'm TX'ing into an antenna 50 feet away or
    > there is a local lightning storm, it is always on!!!
    > If you experience failures with this preamp, you are doing
    > something wrong.
    > Also, putting diodes on the input is counterproductive. Why
    > go through the bother of building a high IMD performance
    > preamp and then deliberately put IMD generating devices on
    > the input. Does that sound smart to you? It doesn't
    > sound smart to me. At the very least you should be using 2
    > or 3 diodes in series. A single silicon diode junction will
    > start to rectify at less than 0.5 volts peak, which is
    > typically far below what a local BC station will produce at
    > the input without a filter.
    > 73, Larry

    Excellent idea about 2-3 diodes in series!
    Last edited: Mar 15, 2009
  7. KC9OXP

    KC9OXP Ham Member QRZ Page

    I will have to do 2 post to get the pictures...
    I got the web site saved. The way I use the word "nerf", in the same way online multiple player based games.. If you talents or specs get nerfed, they where reduced or taken away. So a broad cast band nerf would reduce or eliminate the band. I have attached a schematic from the 2009 ARRL Radio Handbook. I just don't know if I can use ceramic instead of polystyrene capacitors.

    The transistor gets slightly warm in caparison to room temperature. I used heat sink compound and bend the heat sink so it was a press fit. I believe the way its set up now it could be safely run with out it, but I'll leave it on. I will post a schematic of the way I layed it out. Didn't have the exact resistor for some of it. So I figured a 10% tolerance on the values had acceptableness. I also found the output was stable from 7 to 16 V.D.C. So I am working on installing a gel cell 12v battery in the case for portable use. I love electronics, that's why its my hobby and schooling :D

    The reason for the heat is that resistor change. I adjusted my var resistor to see if any thing changes, besides for current draw this is no audible difference in output. I still haven't finished the fine details in the connections, but I'm dropping the shunt relay. Waste of power when I just could direct connect it. Also might have put the diodes wrong in the picture but ah-well point is understood.[​IMG]

    Last edited: Mar 16, 2009
  8. KC9OXP

    KC9OXP Ham Member QRZ Page

  9. W0BTU

    W0BTU Ham Member QRZ Page

    That would be a high-pass filter or a broadcast-band rejection filter.

    You can substitute disc ceramic capacitors for polystyrene. But you can't always substitute polystyrene for disc ceramic.

    Sounds like you live close to an AM station, and it tends to overload the preamp and/or receiver. Is that the case?
    Last edited: Mar 16, 2009
  10. KC9OXP

    KC9OXP Ham Member QRZ Page

    Ya I think I live fairly close. The SX-117 don't overload, but I have a damaged Grundig receiver that overloads so bad its not usable unless the signal is stronger then the broadcast. Even before it was messed up, it would over load on British am broadcast bands. But it didn't overload on 180M+. As far as I can tell the preamp doesn't get overloaded by it.
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