[Amps] Strange behavior (at least to me)

[Steve Bookout]

I do have another tube that I can swap out, but hasn't been used in 
YEARS.  May want to let it 'cook' without HV for a couple of days.

Steve

On 5/28/2015 04:34 , Jim W7RY wrote:
> 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
>>
>>     _______________________________________________
>>     Amps mailing list
>>     Amps at contesting.com
>>     http://lists.contesting.com/mailman/listinfo/amps
>>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>