>>>That means the so-called "screen" supply has to provide several hundred
>>>mA of cathode current
>>? The cathode current in a 4cx1000a can be as high as 1000mA.
>>>as well as the few tens of mA for the screen. It's
>>>way beyond the current capabilities of normal screen stabilizers such as
>>>zeners, AG6K's series regulator or my Tetrode Boards, all of which are
>>>only good for a few tens of mA.
>>? The current capacity of the ag6k regulator is limited primarily by the
>>dissipation of the 6A/600piv/150w FET pass element.
>Your series regulator would obviously be a much better choice than the
>However, the unregulated input voltage would need
>to be high enough to avoid loss of regulation (drop-out) on negative
>peaks of ripple at 1A. This requirement could result in a rather high
>voltage drop across the regulator at more normal currents, and therefore
>high power dissipation - probably outside the Safe Operating Area of a
? Good points, Ian. . On the diagram I included a note about
paralleling more FET pass elements for more output current. The good
news about FETs is that the gate draws no current, like the grid of a
tetrode or pentode in Class AB1 service, and there is no
secondary-breakdown problem. // In the 30S-1, since the screen-supply
does double-duty as part of the anode-supply, I would test the
screen-supply at 1A / 300v.
>>>It would be possible to stabilize the screen-cathode voltage by radical
>>>re-wiring - essentially by leaving the existing two stacked supplies to
>>>float, and providing the screen voltage separately - but has anyone
>>>found a simpler idea that will work with the 30S-1?
>>? Such an undertaking is possible, but hardly practical. The stock
>>Collins screen supply has ok regulation. A problem with raising the
>>screen potential in the 30s-1's novel "super-cathode" circuitry is that
>>peak drive potential is added to screen supply potential. Thus, one must
>>be careful not to exceed the max. screen rating of 400v.
? Since the screen potential is a bit lacking, the most popular 30S-1
mod. is adding a 120v/120v 100w isolation transformer. The secondary is
connected in series with the existing screen supply winding to increase
the voltage fed to the rectifiers and resonant-choke filter, thereby
increasing the screen and anode potentials by approx. 100v each.
However, with RF drive applied, the peak screen potential then exceeds
the 400v rating. Thanks to the 3/2 power law, power output reportedly
increases to c. 1900w pep. To Eimac's credit, 8168s seem to take it, at
least on ssb.
- cheers, Ian
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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