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Discussion in 'Amateur Radio Amplifiers' started by W0PMT, Jun 27, 2019.

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  1. W0PMT

    W0PMT Ham Member QRZ Page

    I have a 2K-3 with a upgraded power supply and 3-400 tubes. I have a scope on the way. How much drive on ( AM ) can I use Safely and Cleanly on the Henry with out Stressing the Tubes.I can lower the rf deck to accept 3-500z tubes if necessary. I can drive the Henry with a Johnson Ranger or SDR with qrp linear.
  2. WA5VGO

    WA5VGO Ham Member QRZ Page

    The issue is efficiency. You’ve got to over couple the amplifier by at least 6dB. This means, when there’s no modulation, the amplifier will only be about 30% efficient. As you modulate, the amplifier will actually cool off as the efficiency rises. If the power supply can handle it and the amplifier has good cooling, you should be able to achieve close to 400 watts of carrier. At this power level the tubes will show quite a bit of color.

    If you don’t have enough headroom, the RF envelope will clip and you’ll have splatter.

    If you don’t know how to tune an amplifier for use on AM you need to study the process. It’s completely different than tuning one for the other modes.
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The 2K-3 has a very robust power supply, albeit an old-fashioned design (choke input).

    I wouldn't run it at more than about 300W carrier power, but you must "tune up" (CW steady carrier) at 4x that power level for it to actually work on AM and be linear enough to not distort, allowing for full modulation.

    As stated above by VGO, efficiency of linear amplifiers used on AM isn't good, but gets better during the modulation envelope peaks. At 30% carrier power efficiency (not a bad estimate, might be slightly lower), 300W carrier power output represents 1000W DC input power -- continuously. Yes, the tube anodes will be red, and get that way quickly. With 100% modulation, this can create 1200W PEP output power and the amp should be able to handle that assuming you tune it up at 1200W key-down power and do not "retune" for the lower carrier power you'd use for AM (by reducing drive).

    The 3-400Zs or 3-500Zs can handle "red anode color" indefinitely, but my only worry would be the filament pin seals and possibly the anode pin seals which can get very hot. The Henry uses a blower and chimneys, but if the tube pin seals get too hot they can ruin the tubes; I'd check for proper blower operation, clean and unobstructed air flow (free of dust and dirt) and possibly even go to the next higher speed blower which will produce more air. Maybe the blower's already been replaced, as after so many years they do wear out and replacements in old Henry amps are fairly common.
  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    Yes, 300 watts AM carrier is about right for long tube life. 400 watts carrier with 100% modulation put you over the legal limit.
  5. W2VW

    W2VW XML Subscriber QRZ Page

    The amplifier should be tuned exactly as you would for ssb use.

    Adjust the drive level on AM for peak power output similar to what it makes on ssb.

    If using a Johnson Ranger, it is a very bad idea to underload it for reduced drive. Lazy hambone way.
    W0PMT, K2XT and KD2ACO like this.
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Here is my 2K-4 setup. The 2K-4 circuitry is very similar to the 2K-3.


    Attached Files:

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