>
>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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