[AMPS] UPDATE ON 3-1000Z OSCILLATION

[measures]

>
>So much for the "magic" of nichrome. The anode is loaded with 
>nichrome, and it is oscillating. 
>
€  The only person who claims any magic for nichrome (Ni Cr) alloy 
resistance wire is none other than Mr. Rauch.  He insists that the 
resistance of Ni Cr increases as frequency decreases.  (sic)  I know of 
no conduct or that has such a characteristic.  

>> For amps that use a significant run of coax between the input pi-
>> circuits and the socket, the coax must act as a high-impedance
>> quarter- wave stub at some VHF frequency, because it's shorted at the
>> other end by C2 of a pi-network. Would there be an advantage in
>> mounting the 10m pi-circuit right at the socket, so that its C2 acts
>> as a permanent VHF bypass? Obviously the networks for  the lower bands
>> would need to be modified to allow for this, but that's easy enough
>> using modern RF design tools.
>
>Might be a good idea, this doesn't usually change stability much 
>because the feedback mostly involves the grid and layout more 
>than anything else. But it sure affects efficiency and even IMD!
>
€  In our parasitic suppressor retrofit kit for the 3-1000Z, we provide 
VHF dampers to connect from the filament pins on the socket to gnd.  

>The cathode needs to see a low impedance at harmonics of the 
>operating frequency. 
>  
>> >e thing that someone mentioned was the size of the cabinet- It is
>> >> large. The tank compartment is 22"w x 16"d x 14"h . It was a nice
>> >> case I picked up at a hamfest. How critical is size? Could I have
>> >> possibly constructed a big VHF cavity? 
>> >
>> >No, that's non-sense. 
>> 
>> Ummm, not so sure about that... it does happen in big amplifiers, but
>> you'd also need to be unlucky with feedback paths and other in-circuit
>> resonances in order to get a VHF oscillation.
>
>Best to find out why the tube has coupling between the input and 
>output and happens to have two resonances at the same general 
>spot.
>
>By far the largest worry with the cabinet is ground path length and 
>ground loops.
>
€  In the Henry 2K-4 and 3K-A, the output enclosure appears to be a 
factor in intermittent oscillation near 95MHz.

>...

Cheers, Tom 

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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