[Amps] al-1500 parasitic?? or some other problem?

[Richard]

>This problem has developed sometime in the last 6 months or so, but has
>taken some time to pin down as it seemed intermittent and was being
>blamed on other things.  It did not exist when I first installed the
>amp.
>
>First, the amp is on the 20m position of my station, there is an ice
>bandpass filter between it and the ft-1000mp exciter.  There is a pair
>of stubs after it.  I have bypassed the stubs with no change in the
>symptoms.
>
>What I am seeing is this... with the radio set for 14024.5 I tune the
>amp and all appears normal.  Except when going above about 800w a strong
>spur suddenly appears on 21036.9, almost exactly 1.5 times the
>transmitter frequency.  The spur can be moved by tuning the 20m radio.
>It is not moved by adjusting the tune or load controls on the amp.  With
>the amp tuned to 1500w output on 20m the spur can be turned on and off
>with fairly small adjustments of the amp tune control (5-10 degrees
>max), it does not change over fairly large adjustments of the amp load
>control.  The onset point of the spur is not stable, sometimes it
>doesn't start until the amp is up to almost full power, sometimes when
>turning the tune control the start and stop points are not the same.
>But once started the spur is very stable and its frequency is not
>affected by the tune or load controls, not what I would expect from a
>parasitic.  When keying the radio with CW it creates large clicks on
>15m, probably because of some instability when suddenly increasing
>through 800w.  There is also no discernable change in grid current when
>the spur starts and stops as I would expect from a parasitic.
>
>Maybe unrelated.. in one test I had my 6m radio turned on and when I was
>turning the spur on and off by adjusting the amp tune control.  When the
>spur stopped on 15m I could hear it sweep through 6m, with some practice
>I could tune it to the 6m frequency using the amp tune control.  This is
>more what I would expect of a parasitic.  This was very weak and
>probably only detected because the 6m antenna is only about 5' above the
>20m one.
>
>Any ideas where to start with this one???
>
€  The AL-1500 is the only mfg 8877 MF/HF amp on this planet that does 
not use a parasitic oscillation suppressor.  A friend modeled the 8877 on 
a computer program for determining amp stability.  In a typical HF layout 
with c. 120nH between the anode and C-tune, the program predicted 
marginal stability above 100MHz.  My guess is that this has something to 
do wth the 8877's high VHF gain. An autopsied 8877 from an AL-1500 is 
shown in figure 24 on my Web site.  This tube was damaged by 
gold-sputtering from the gold plating on the grid.  Although it doesn't 
show up in the photo, most of the gold loss took place at the grounded 
end of the grid cage.  To me, this suggests that the parasitic was in the 
UHF region.  We sell a parasitic suppressor retrofit-kit for 8877 
amplifiers.  In the interest of R and D, I will send you a retrofit-kit 
gratis if you will install it and report back to the group.
>
cheers, David.

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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