[Amps] Strange behavior (at least to me)

[Steve Bookout]

Hello Jim,

This is something that should be easy enough to do and does make some sense.

Will wait till next week to deal with it as I will be very involved in 
the WPX CW contest this weekend.

Thanks.

73 de Steve, NR4M

On 5/28/2015 14:50 , Jim Hargrave wrote:
> Steve,
> It sounds like a bad  connection  on the variable capacitor. It
> could be external or inside the bottle.
> Depending on the construction of the capacitor, you may or may not
> be able to fix it. In some construction they use finger stock on the
> variable shaft. I have had some success in positioning the shaft
> facing up and spray a little Tri-flow lubricant on the shaft and
> rotate it back and forth  until it goes In a little.
>
> 73, Jim - w5ifp at gvtc.com
>
>>> -----Original Message-----
>>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of
>>> Steve Bookout
>>> Sent: Wednesday, May 27, 2015 6:47 PM
>>> To: amps at contesting.com
>>> Subject: [Amps] Strange behavior (at least to me)
>>>
>>> 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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>