10m output is low, 3-500ZG, Henry 2K Classic

Discussion in 'Amateur Radio Amplifiers' started by N6YFM, Sep 26, 2016.

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  1. N6YFM

    N6YFM XML Subscriber QRZ Page

    I finally have my recently acquired Henry 2K Classic hooked up and under
    test with a Bird 694 KW dummy load and Wavenode WN-2d SWR/Power meter.
    All bands are in spec, but power out at 10m is down by half.
    (I also notice on 10m that I can hit rated plate current, but grid current is lower than
    the other bands, along with the lower output power)
    Exciter input is the same and exciter SWR is fine.

    On the schematic, a grounded grid 3-500 amp is dead simple. I can't see anything
    in the output tank circuit that is not common to all bands, except a
    75 pf 8kv doorknob capacitor for 10m that gets switched in.

    Does low output on 10m typically indicate soft tubes?
    (What does this term mean anyway? I am fairly new to tube amps)


  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Who did the 10m modification? The 2K Classic shipped without 10m active, and missing the 10m tuned input circuit entirely (they supplied that as a kit on request by a licensed ham).

    When you drive the amp on 10m if the grid current is low (say below 250mA) my first guess is the 10m tuned input circuit has a problem or may simply need re-tuning (L12, accessed on the side of the RF deck).

    What does the SWR measure "looking into the amplifier" (that is, measured at the transceiver, between the transceiver and the amp) on 10m? Should be very low, nearly 1.0:1. The Henry circuit worked well if it's all intact and tuned properly.

    Edit: I see you're in Torrance. This isn't far from Henry, and the amp shop there still does repair Henry amplifiers (the shop is in L.A., near Venice Beach -- a couple miles from the store).
  3. N6YFM

    N6YFM XML Subscriber QRZ Page

    The transceiver (Exciter) SWR looking into the Henry Amp on 10m was 1.3, so I used the tuner built into
    my rig. So you are thinking it may simply need the input circuit retuned, especially since the grid current
    is low?

    Yes, I am not far from Henry's store, but I love electronics so would be inclined to first remove the RF deck
    and inspect that tuning circuit. Thanks for the pointer. It is less expensive to inspect that first.

    With regard to someone saying that the tubes may be "soft"; If the output was down half due to the tube
    age (these are 1999 RFparts.com 3-500ZG tubes that mostly sat idle), would the pair still draw 250ma grid
    but simply not put out the power?

    Thanks again for the help.

  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    What is the indicated grid current on 10m when you drive it hard (this amp can take 100W drive no problem)?

    Always adjust the PLATE tuning for MAXIMUM grid current, then the LOAD control for maximum output power, then PLATE for MAX grid current again, then watch the Ig and if it's >300mA, crank the LOAD more clockwise (increase loading) to bring Ig back down to 300mA or less without sacrificing output power. I never look at plate current, it doesn't matter much. The amp can be tuned entirely by watching grid current and output power.

    If after tuning the Ig cannot reach even 250mA, the input circuit may well be mistuned or at fault.

    Who installed the 10m modification? It's possible to do it wrong.
  5. N6YFM

    N6YFM XML Subscriber QRZ Page

    Hi Steve:

    Not sure who did the 10m mod. The amp was given to me a few weeks ago by an Extra that is getting
    out of radio. He acquired it from an S.K. in the neighborhood. So we are not sure how many
    owners had this thing, or who did the mod. Everything else in the bottom console looks stock
    and on the top side of the RF deck looks stock. I plan to remove the RF deck sub-chassis so I can
    turn it over and inspect the input switching and tuned circuits. I am really enjoying going over this
    thing with a fine tooth comb, inspecting, measuring, and testing. Rather fun.

    Thanks for the above tips on tuning. I expect to get some more free time to work on this in the next
    week, and I will keep you posted as to what I find with the input circuit.

  6. N6YFM

    N6YFM XML Subscriber QRZ Page

    Hi Steve:

    Using a 12 foot RG8x jumper from the transceiver to the amp input, I measure SWR of 1.4 feeding the amplifier at 10m.
    I also noticed that this time (5 days apart) that I drove it and measured 10m power output, it was 80 watts higher (now almost 400 watts,
    this is in low power mode, where 20 meters maxes out at around 750 watts).
    I have not pulled the RF deck yet to inspect the input circuit.
    All other conditions were identical, same numeric settings for tune and load.
    By the way, the Henry instructions say that in low power mode (2,000 volts B+) to not exceed 600ma plate current.
    So I am driving the amp to that value while tuning, and then noting approx 750 watts for 80m through 20m at 600ma plate
    and approx 250ma grid.

    For the 10m being low, but starting to come up higher now when I use the amp more,
    Is it possible that this is simply soft tubes from sitting for years? Maybe they are starting to getter?

    I am also curious about the input SWR. Is 1.4 really a big deal? I would not expect that to knock down the power very much?
    Tomorrow, I will spend a little more time heating the dummy load. I want to start building a chart of input power, swr, band,
    output power, grid ma and plate ma, tune and load setting.

    Anyway, pending your advice about the input circuit having 1.4 SWR, if I get a third run at "heating" the plates and the dummy load,
    and 10m starts to put out yet another higher number, I will assume soft tubes.


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