3-500Z Home Brew problem almost solved and I need ideas.

Discussion in 'Amateur Radio Amplifiers' started by KM6CND, Jul 21, 2018.

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  1. W1BR

    W1BR Premium Subscriber QRZ Page

    Tuning at low power runs the risks of undercoupling as the drive is increased. That can blow band-switches, etc.
    W8JI gives plenty of solid advice regarding amplifier tuning:


    I'd also suggest making a simple chart once the amplifier control positions are known for each band... that allows presetting the controls before the amp is keyed and drive applied, followed by a quick repeaking as needed.

    KM6CND likes this.
  2. AI3V

    AI3V Ham Member QRZ Page

    If tuneing a amp with 20 db less drive than saturation could "blow a band switch"

    Just the pauses a between words would destroy a amp?

    The advice being given re: full power baby, is like starting your car after a engine swap (where you don't know squat about the new motor), and advising him to keep the throttle wide open at all times, and adjust for Max smoke out the tailpipe.

    He'll get smoke alright.

  3. W1BR

    W1BR Premium Subscriber QRZ Page

    Reread what what was said, slowly.... the part about as drive is increased. Tuning an amp at low drive and advancing the drive results in an under-coupled condition as the amp is driven harder. That can result in extremely high RF voltages in the tank. You can twist words anyway you wish, but that is a fact. Any competent ham should be able to tune a grounded amp if they follow the designer's procedures.
  4. AI3V

    AI3V Ham Member QRZ Page

    That not how it works,

    You test a new amp at low power untill you know it's ok at idle.

    Thinking you will developed "extremely high voltages in the tank" when running a kw class amp at a few watts to test is ignorant.

    Quit giving new guys bad advice.


    Watch out for the "advance the load" people that don't bother to measure the DC input, rf output and therefore efficiency and dissapation of there amp.
  5. W1BR

    W1BR Premium Subscriber QRZ Page

  6. W1BR

    W1BR Premium Subscriber QRZ Page

    Gawd, you have severe reading comprehension problems.... straight from W8JI's webpage (which he highlighted in RED):

    Never tune, peak, or dip the amplifier at reduced drive power, and then attempt to operate or attempt to suddenly apply full drive! If you are going to make a mistake, make the mistake by having the loading control too far open or unmeshed...not too far closed or meshed! At least with the loading control too far open, you will not cause an arc, blow out a bandswitch, or damage a tube grid. You have slightly more time for mistakes and corrections when the loading capacitor is open too far than too far closed.

    I think any ham with reasonable intelligence will recognize that is the advice I essentially gave. If it is wrong, go argue with Tom.

    AF7XT likes this.
  7. KM6CND

    KM6CND Ham Member QRZ Page

    Sat morning. The good news is I can return my 2nd Diploma from the School of Hard Knocks. I swapped tube # 3 with tube #2 (I completely killed tube #1) to see if it's dead or alive, and it's doing fine.

    On 40M with 50 watts of drive on SSB, I see peaks up to 400 watts, can't complain. Moving along very carefully.
  8. KM6CND

    KM6CND Ham Member QRZ Page

    Maybe it's just human nature to resist the idea of tuning up at full drive when you KNOW your amp is not tuned. Even with 20 watts of drive, the tuning cap arced. Everything looks and acts proper between the radio-amp-tuner-antenna.

    I'll have DYMO labels all over the face of the amp with baseline settings for each band.

    Next is to install a fan just above the tube. The cover for this amp is punched aluminium screen, the fans will rest above the tube.
    W1BR likes this.
  9. W1BR

    W1BR Premium Subscriber QRZ Page

    There is no problem with tuning up at reduced drive... the problem comes when one reaches FIRST for the drive control to increase power. A prudent ham advances the loading the control slightly before increasing drive level. That helps to ensure ample loading and alleviates soaring tank voltage conditions.

  10. AF7XT

    AF7XT Ham Member QRZ Page



    simply not good form OM

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