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[AMPS] Screen stabilization in 30S-1?

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Subject: [AMPS] Screen stabilization in 30S-1?
From: (Rich Measures)
Date: Sun, 28 Feb 1999 07:01:43 -0800
>Hello Rich

>>>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
>shunt configuration. 
>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
>single FET.

?  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,  

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