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Re: [Amps] Strange behavior (at least to me)

To: TexasRF@aol.com, amps@contesting.com
Subject: Re: [Amps] Strange behavior (at least to me)
From: Steve Bookout <steve@nr4m.com>
Date: Thu, 28 May 2015 00:52:20 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Hi Gerald,

One of the guys brought up the idea that the vacuum relay may have reached the end of it's life, but we didn't check it out at that point. It was NOS when I got it 30 years ago, and no telling how many cycles it's gone thru in the many THOUSANDS of QSO's that were made using that amp.

Thanks for the data point.  We will check it out.

73 de Steve, NR4M

On 5/28/2015 00:46 , TexasRF@aol.com wrote:
Hi Steve, you might try putting a short strap across the vac relay terminals for a test.
It sounds as if you might be loosing the plate load in an erratic manner.
73,
Gerald K5GW
In a message dated 5/27/2015 6:47:20 P.M. Central Daylight Time, steve@nr4m.com writes:

    Hello all,

    Since it's gotten quiet here, i thought I would get the collective
    'gray
    matter' working again.

    I am having a strange problem with a homebrew 8877 amp. It has worked
    almost flawlessly for 30 years with the same tube and still easily
    puts
    out legal power.  4000 volts at a load of about 550 ma., driving with
    about 22 watts.

    During the night shift in CQWW (I believe) the amp started acting
    up and
    was replace with a spare.  It is a 40 meter mono band amp.

    I was told that there was no output but 'the meters still were
    'jumping
    around''.  I opened it up expecting to find an obvious issue with the
    vacuum relay on the output.  Power has to go SOMEWHERE, right?   All
    looked as good as it ever did.

    Just before Dayton, myself and two budddies, fired up the amp, after
    once again looking inside.

    This is what I am seeing:

    We tuned it up with 10 watts drive into a oil filled dummy load
    and got
    700-750 watts out with a few hundred mils of plate current and
    about 10
    mils of grid current.  Unkeyed it and then rekeyed it. Upon rekeying,
    the 100 mil grid meter slammed and the grid trip circuit did it's
    job.
    I have it set for 75 mils of grid current. Wound up reducing the grid
    capacitance so that the max grid current would be more reasonable.
    After trying it on/off again and again, when it goes 'nuts', the grid
    current stays on the scale at about 60 mils and about 'just a little
    bit' when it seems to conduct normally.  When it acts up, if I
    turn the
    TUNE vacuum variable just a wee bit, I get my plate current as I
    should
    and the grid current goes down to normal.  My multi turn counter
    on the
    TUNE cap is marked '0 to 100' on the knob skirt and a 'wee bit' is
    from
    88 to 96, just 8 marks, or 8% of one turn.  This is on a 375 pf vac
    variable cap.
    Taping on the chassis would make the problem come and go, but not
    every
    time and not easily.    Try it again and I would have to turn the cap
    back the other way to resonate it (seemingly.)

    In my mind, it seems like something in the RF side of things, since
    changing the cap brings it back from the 'dark side'. Keying the amp
    with no drive and swinging the caps back and forth showed no
    instability.  I also took all the connections in the tank circuit
    apart
    looking for loose joints and corrosion.  None found. Reseated the
    tube.
    Works fine every time till it doesn't.

    It's doing the same thing as I think I would expect to see if I
    manually
    de-tuned the tank circuit with the input cap.  It also showed plate
    current during the 'event', although lower than normal, so where
    is the
    power going?   Quiescent plate current is about 120 mils.

    Any ideas out there?  I can supply pics if needed.  Maybe I should
    record the event and put it on you tube with a link.

    Look for some of you this weekend in the WPX CW contest.

    73 de Steve, NR4M

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