Odd Amp issue only on 20 meters

Discussion in 'General Technical Questions and Answers' started by N8FVJ, Nov 10, 2019.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    I use the AL-811 amp on 75 to 20 meters. It has three Cetron 572B tubes. 65 watts in on 75 & 40 meters produces about 800 watts PEP out and the meter is known to be accurate. On 20 meters 45 watts in makes 800 watts PEP out and amp appears to oscillate with 70 watts drive plus I get RF lip burns on mike grill shield. No RF burns with 45 watts drive and amp operates normal. Appears an oscillation at higher power input? This never appeared in past and amp needed the 65 watts drive for 800 watts PEP out on 20 meters. Antenna match is less than 1.5 to 1 SWR on 20 meters.
    Also, producing a AM carrier I hear a squeal on 20 meters and no squeal on 40 meters.
     
    Last edited: Nov 10, 2019
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Is it one of the later models that has the grids directly grounded, or an earlier model that doesn't?
     
  3. N8FVJ

    N8FVJ Ham Member QRZ Page

    Early model. Should I remove parts and directly ground the grids? I can use 1/4" braided wire if necessary.
     
  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    I thought an early model, is not and has the grids grounded directly.
     
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    You have "RF in the shack". Unlikely to be a fault of the amplifier, especially since it is only on one band. Antenna SWR has nothing to do with RF problems.

    Look at your "antenna system" i.e. antenna type, location, orientation, distance to shack, Common Mode suppression methods, grounding/arresting system, feedlines arrangements. Inspect the antenna system for physical condition.

    -What antenna? How fed? How far from shack? What Common Mode suppression methods have you taken; e.g. chokes, baluns, etc?

    [p.s.] You are driving that amplifier mighty hard.
    Your mentioned output: 800W
    Ameritron AL-811 rating: 600W
    572b
    tube Plate Dissipation Rating: 160W x 3 tubes = 480W.

    Perhaps one shouldn't be surprised at problems that arise when driving the amp at +160% of rated capacity of the tubes! ;)
     
    Last edited: Nov 10, 2019
    N8AFT and KP4SX like this.
  6. N8FVJ

    N8FVJ Ham Member QRZ Page

    EFHW 8010 antenna feed about 30 feet from shack and 130 feet of wire away from shack. 50 feet RG-8X coax, no lightning arrestor. Choke on amplifier 120 volt line, 1/4" ground braid to all equipment including Astron PS. 800 watts PEP has 1500 volts on plates at peak output from tubes. 572B plates show no red color at this output. Uses same power transformer as AL-811H that is rated 800 watts PEP out. 480 watts plate dissipation not exceeded at 800 watts PEP out as amplifier efficiency is likely 63%. SSB duty is only 20% anyways.
     
    KA0HCP likes this.
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Well, we would not be surprised to have CM current on an unbalanced end fed antenna.

    First suggestion: Add a 1,500W rated choke/1:1 unun to your feedline, just before it enters the shack.

    -General housekeeping: e.g. Straighten all cables, coax; coil up excess.
    -Consider adding an additional choke/1:1 unun at the amplifier output.
    -Move antenna feed point further from shack.
    -Minimize lengths of coax jumpers
    -You could add mix 31 snap on chokes to jumpers between components if desiring belt & suspenders approach or for quick and easy things to try.
    -Consider adding an external ground/surge protection point and/or grounded entry panel for feedlines.
     
    WB5YUZ likes this.
  8. N8FVJ

    N8FVJ Ham Member QRZ Page

    Thanks, good to know it is not the amp. For the time being I will reduce power to 500 watts PEP out on 20 meters. No issues at that power level.
     
    KA0HCP likes this.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Actually, at higher voltage (typically 2400-2700V) the 572B is rated 320W power output each. Three in parallel, with high enough anode voltage, can handle just about 1kW output power without exceeding any ratings.

    However, in the AL-811 the anode voltage is lower, in the 1700V range, so that much power is not to be expected.
     
  10. N8FVJ

    N8FVJ Ham Member QRZ Page

    Some think AL-811 or AL-811H are rated with the amplifier capacity in mind. In the case of the AL-811 series it is the 811A tubes that limit the power and everyone knows 500 PEP out is the safe maximum for three 811A tubes. However, the 19 lb large transformer is the limiting factor and 800 watts PEP out with the transformer capacity in mind is no issue using 572B tubes. I can actually get 1K PEP 0ut with 80 watts drive on 75 meters and three 572B tubes, but that is really pushing it. Likely starting to 'flat top' the signal out.
     

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