[Amps] AL-811H problems

[2]

>AL-811H up date:
>I  removed the top cover and found one tube was not lit.
>Removed all tubes and tested on a TV-7D/U.
>Found one tube was open.

When an 811A - or any tube-in a HF amp oscillates at VHF, the HF tank 
offers no load to the VHF energy produced.  This causes high 
grid-current.  Since current flow always produces a perpindicular 
electomagnetic force, since all grid current originates from the cathode, 
lateral stress is placed on the 811's W-Th filament.  A local friend who 
writes a Ham magazine review column received the prototype AL-811 from 
Ameritron for evaluation.  During testing. he heard a pop and the 
amplifier's RF output ceased.  He brought the amp over here to get my 
opinion.  I concluded that the 811s were kaput since I could see filament 
fragments in the bottoms of the glass envelopes.  A dipmeter found a 
sharp dip in the anode circuit at c. 95MHz.   Since this is above the 
811's natural grid resonance, I guessed that a VHF parasitic oscillation 
was possible because a g-g amplifier is unconditionally stable only where 
the grid can shield the cathode from feedback -- i.e., below the grid 
resonance.
-  It is my opinion that a tendency to oscillate at VHF might be overcome 
with lower VHF-Q suppressors.  

have phun

>One has a micromhos of 850 that is low.
>Two tubes have a micromhos of 1,300 -ok-
>
>Any one have two Svetlana 811A's pulls around this micromhos value?
>
>I have only had single tube amp's in the past, Will one bad tube on a
>muti-tube amp
>cause a SWR problem?
>
>Any thing else I am missing?
>Thanks de John KØCQW
>
>"John E." wrote:
>
>> I was on 80m CW the other night with my new to me (used) AL-811H, when
>> the
>> amp stopped working and the high SWR light came on the display on the
>> FT920.
>>
>> When I shut off the AL-811H the SWR went down to normal using 90w.
>>
>> Today started to by pass tuner, external SWR meter etc. I ended up with
>> the
>> dummy load connected into the output of the amp and started testing
>> again.
>>
>> Found the amp would stop transmitting and the SWR would go high, Changed
>> out
>> the jumper between the radio and amp.  no difference.
>>
>> Found after 45 sec of key down in to the dummy load and multiple keying
>> on and
>> off sending CW the amp would stop working.---Must be in the amp--
>> The amp has not blown any fuses and the HV runs at 1700v.
>>
>> I have no idea the condition of the tubes(811's),  the amp when working
>> will  put out 400w CW.
>>
>> Any thoughts?
>>
>> Thanks de John KØCQW
>>
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>
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-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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