Driving An Amplifier

Discussion in 'Amateur Radio Amplifiers' started by N9CUC, Nov 5, 2016.

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  1. AC4BB

    AC4BB Ham Member QRZ Page

    Still got the invisible 811-H Ah. works great.
     
  2. N9CUC

    N9CUC Ham Member QRZ Page

    Dan at the Ham Station was able to get the 450ma plate current and 100ma grid current on 15M today down at his store. He feels, along with my electrical engineer, that high SWR is probably the reason I am having problems at my home QTH. So we have a MFJ antenna tuner coming and we are all keeping our fingers crossed that will solve my problem.

    r/Mike
    N9CUC
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    When checking an amp for proper operation a dummy load should always be employed. Not doing that, all bets are off.
     
    WZ7U, KD2ACO and AA7QQ like this.
  4. N9CUC

    N9CUC Ham Member QRZ Page

    So, the manual says to set the Load and Plate at various settings per band. Which I do on 15M. Then I cannot get the 450ma plate current and 1000ma grid current when I increase the drive. So, at that point, is it okay to increase the Load and Plate in order to get the 450ma plate current and 100ma grid current?
     
  5. KH2G

    KH2G Ham Member QRZ Page

    If your totaly new, be sure to have your amp connected to a dummy load while practice tuning. The amount of drive is specific to the amplifier. Your drivet (transciever) will normally be in cw mode for tune up. Can't say much more witnout your specific setup. Oh yes. On tune up, only tune for about 10 or 15 seconds then let up unless your about there. Off tune makes for hot and burned tubes if held too long. Enjoy.
     
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    First as others have said, test into a dummy load and in normal operation tune into a dummy load before loading into an actual antenna. IOW, you shouldn't need an antenna tuner to validate proper amp tuning and appropriate drive levels.

    That said adjust the plate Tune control to find a dip in plate current at the operating frequency. It is never appropriate to intentionally misadjust the Tune control to increase plate current. If you do so you are operating the amp intentionally tuned off resonance and your tubes won't last very long as they're forced to dissipate excess power that isn't being transferred to the output.

    The Load control should be adjusted for maximum power output and then advanced a bit further until the power drops about 10% below the peak power observed. That helps ensure linearity and clean signal from the amp. If you do that and don't see the grid or plate current levels that you expect then it's likely your drive level is low or your amp's tuned input stages are either not efficient (e.g. excessive swamping resistance) or not tuned properly. The two potential causes of low drive may be related. For instance if the amp has tuned input filters that are not tuned properly the rig may see excessive SWR and fold back its output power such that the amp doesn't see sufficient drive levels. And of course one or more bad tubes can result in several of these problems (high input SWR, low power out, low plate current). I hope your shop and engineer friend aren't suggesting a tuner between the rig and amp to ensure the rig doesn't see high SWR, there are better ways to solve amp input stage SWR issues.

    As others have posted if you don't have input stage problems like mistuned input filters and your rig sees high SWR looking into the amp then one or more tubes is likely bad. A tuner won't solve problems stemming from bad tubes.

    Good luck and don't intentionally adjust the Tune control off resonance nor adjust the Load control to try to achieve certain current targets if that doesn't coincide with proper tuning of the amp which generally means dip the plate current via the Tune control and advance the Load control till output power drops about 10% below peak output power. In the end, specific current targets aren't the goal. The goal is maximum output power in an appropriately tuned amp with a clean output signal, don't sacrifice power or linearity for specific current targets listed in the manual.
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    This step of the loading instructions is kinda stupid. This can change and be totally different if your antenna is not 50 ohms. It is just as easy to say "Pre-set load and tune to specified settings and set exciter for 20 watts output" Quickly rotate the tune control for a dip in plate current. Wait 15 seconds, Key down and adjust load for peak power output. Now add another 20 watts drive, adjust load and tune for maximum output. Continue this process until specified plate and grid current are met or until desired output is desired. Adjust drive according.
     
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    And if using SSB or AM, adjust LOADing & drive 'til flat-topping disappears on your monitor 'scope.
     
  9. N9CUC

    N9CUC Ham Member QRZ Page

    Picked up the MFJ-986 tuner at the Ham Station yesterday. Anxious to get it up and rolling to see if it solves my problems. Not sure what the next step might be if the 811H still wont tune properly.

    By the way, I didn't realize that MFJ-986 tuner was that long. Its about 18 inches long, 12 inches wide. I don't know how I am going to get it onto my shelves now???
     
  10. N9CUC

    N9CUC Ham Member QRZ Page

    No change.

    Nothing seems to work.

    Cant get the 450ma plate current or 100ma grid current.
     

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