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

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

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: L-MFJ
ad: Left-2
ad: MessiPaoloni-1
ad: Subscribe
  1. KM6CND

    KM6CND Ham Member QRZ Page

    I may have a spelling disorder, but I'm happy to see progress.

    The blades are not bent or exceptionally dirty. Maybe I just should not turn the dial with a high amount of drive... done it before with no issues though.
  2. KM6CND

    KM6CND Ham Member QRZ Page

    After all this, I'm now sure the second tube I thought I fried is likely just fine. I definitely killed the first one though.:(
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You have to be able to "tune up" with "full drive," you can't just guess where the settings should be; and they will change if you change frequency much or even change the load (antenna).

    Because tuning settings, especially LOAD, will change a lot with applied drive and output power, I don't even bother "tuning up" anything at reduced power if I have some protection mechanism in place to prevent excessive grid current. I hit it with full drive, make quick adjustments (normally 5 seconds or less), and "all done." My tubes last a very long time.
  4. KM6CND

    KM6CND Ham Member QRZ Page

    Works for me. The arc problem is solved, so far, so good.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    Tuning up at low power, to me, is a waste of time; although with tubes having no grid dissipation capability at all (like many tetrodes and pentodes) you have to be careful. Power triodes generally don't have this problem, but when "tuning up" I watch grid current and output power, and nothing else. Don't care about excessive plate dissipation with most of them, as tune-up times are just seconds and then my operating modes are low duty-cycle so I'll never damage a cathode or plate.:)
  6. KM6CND

    KM6CND Ham Member QRZ Page

    Thanks Steve, you've been a great help in the past.
  7. AI3V

    AI3V Ham Member QRZ Page

    Thing is Steve his amp has about 10 db of gain,

    Irregardless of input level, after all it's a LINEAR amp.

    So to tune a amp that you haven't tuned before, start with very low drive power, maybe a watt.

    Key the amp, but don't apply drive, note plate voltage and current. Calculate dissapation.

    Should be way less than 500 watts per tube.

    If So we can leave the amp on forever without damage.

    Now apply a watt or so of drive, monitor output with perhaps a 25 watt meter.

    Adjust amp controls to see output power peak, you should see about 10 times input if tube is grounded grid and in good condition.

    You should see a definite peak with the "plate" capacitor, at less that rated power the "load" capacitor may peak at one end or the other (it's like you "run out" of capacitance)

    Take another look at plate current, it shouldn't have gone up much, if at all.

    If everything looks ok, increase drive, maybe to 5 watts or so, watch output increase, continue to increase drive untill you can "load" to desired plate current at the resonance dip.

    As you increase drive, limit key down time, pay attention to the DC input and rf output, and don't overheat your tubes!

    Output power should be north of 500 watts per tube.

    Power should peak at the same setting as the plate capacitor "dips" and you should see the grid peak.

    You may find some settings of the load capacitor that cause much more plate current at the dip, with no corresponding increase in rf output, in other words causing unnecessary heating in the tubes, always calculate dissapation when deciding how "heavy" to load a amp in addition to imd concerns.

    Check grid current to see if it's in spec.

    Finally check amp input vswr.

    Easy peasy, start low and work your way up.

    Last edited: Jul 26, 2018
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, but that just wastes a lot of time and takes long key-down time while ramping up drive.

    I've absolutely never found that to be necessary with power grid tubes.

    It's impossible to overheat serious transmitting tubes with excessive plate dissipation when tune-up times are five seconds.
    VE1FAR likes this.
  9. KM6CND

    KM6CND Ham Member QRZ Page

    Thanks again for all the comments and peer review.

    At the present time, I select a band and tune with the dummy. This gives me a baseline to work from. I switch to the antenna and fine tune to get a 1:1 match.

    With 50 watts in, CW, I can see 500 or so watts on the meter with no reflection. On SSB, voice peaks are around 500 watts.

    So far, so good.
  10. AI3V

    AI3V Ham Member QRZ Page

    Once again your "advice" will destroy tubes.

    You see Steve, nobody has actually seen this amp work -unlike when somebody sent you a amp for "review" and all you have to do is "turn all knobs for Max"

    Randomly applying rf to a amp you don't even know how to calculate plate dissapation?

    That's some really really bad advice.


Share This Page