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

[Richard]

>> 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.  
>
>What do you have plugged into the receive antenna jack on the MP, 
>anything? I've had problems with transceivers oscillating when using 
>separate receive antennas because of poor external antenna port 
>isolation. Those problems are very power level sensitive. 
>
>It's also somewhat common for solid-state PAs are unstable running 
>into narrow filters. Did you try removing the filter between the amp 
>and the radio?
>
>It's virtually impossible to be in the amp, 

€   813s don't need a VHF suppressor because they have virtually no VHF 
gain.   However, this is undoubtedly not the case for the 8877.  The last 
time I talked to Mr. Rauch on the telephone, he told me that no amplifier 
he designed has ever had a VHF parasitic oscillation.  Methinks this 
could be another case of Not Invented Here syndrome.  Denial of reality 
is a consequential malady.  

cheerz, Tom

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