[Amps] T'bolt ideas

[2]

>
>Rich said:
>
>>AB2 grid-driven makes rotten splatter with 4-400s.
>
>Not necesarily. But you do need a well regulated screen supply, a 'stiff'
>bias supply that doesn't move with grid current (mine moves under
>50millivolts), some RF NFB in the filament circuit, and about 3600volts on
>the plates.
>
>Pappenfus explains why AB2 can be more linear than AB1.
>
**  So why doesn't either Eimac or Amperex recommend AB2 grid-driven for 
the 4-400A?  The two tetrodes Eimac recommends for AB2 grid-driven are 
the 4cx1500B and the 4cx600, and even then, the specified grid-current is 
under 0.4mA.  However, both of these tubes are fairly unique in that they 
have a striped oxide cathode that is designed to dispence thin ribbons of 
electrons between the grid wires in order to minimize electron current 
therein.  The 4-400A does not have a dispenser type cathode.  When the 
grid-cathode potential begins going positive, more and more grid-current 
begins to flow and the grid exhibits a progressively decreasing 
resistance.  Thus, the net grid terminating resistance begins decreasing, 
and the SWR the transceiver sees starts increasing. 
-   SWR is not much of a problem with old tube-type radios, but this is 
not the case with modern solid-state radios.  For example, The typical 
grid-termination R for 4-400s is 200-ohms.  A pair of 4-400s draw c. 60mA 
when the grid-cathode potential is +20v.   This presents a c. 330-ohms of 
additional load to the driver at the positive peak in driving potential.  
330-ohms in parallel with 200-ohms is c. 125-ohms.  At this point, the 
SWR is 200/125 = 1.6:1.  Most solid-state radios begin to throttle down 
with an SWR of 1.2 or more.  
Editorial -- For SSB,  AB2 cathode driven is good because it has built-in 
RF-NFB to clean things up.  AB2 grid-driven (except with a 4cx1500B or a 
4cx600 and little grid-current) is a good way to get tarred and feathered 
at the next Ham convention.  
>
cheers, Peter 

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