[Amps] Strange behavior (at least to me)

[Jim W7RY]

Almost sounds like a gassey or intermittently shorted tube?

73
Jim W7RY


-----Original Message----- 
From: Steve Bookout
Sent: Wednesday, May 27, 2015 5:52 PM
To: TexasRF at aol.com ; amps at contesting.com
Subject: Re: [Amps] Strange behavior (at least to me)

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 at 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 at 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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