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[AMPS] PA Recommendation Wanted

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Subject: [AMPS] PA Recommendation Wanted
From: (Rich Measures)
Date: Mon, 9 Jun 97 09:37:33 -0700
>Rich Measures wrote:
>>5.  The 4CX1000A has a maximum grid dissipation rating of 0W!!.  With the 
>>anode cooler removed, the 4CX1000A's grid wires are so delicate they are 
>>virtually impossible to see without magnification.  
>Funny that Eimac allow 5mA on peaks, if you read the whole data sheet.
I don't see the point.  Does this mean the their 0w rating is bogus?  
>>To limit grid 
>>current, I would use a grid supply whose output R was about 100k ohms.  
>Only if you can promise never, never, never to run any grid current at
>all! With an output resistance that high, the slightest trace of grid
>current puts you in IMD hell. ...snip....

Nah.   As a bit of grid current begins to flow, grid-rectification develops a 
small E=I*R potential across the series R, which adds a bit to the voltage from 
the grid bias supply.  Slightly higher bias V makes it harder for the overdrive 
to produce grid current.  It's a self-limiting DC negative feedback process.  
The ZSAC temporarily decreases a bit, but it doesn't come crashing down as 
 A knowledgeable operator would tweak the grid bias V a bit higher (so that no 
grid current could flow on voice peaks), and put the ZSAC back to where it's 
supposed to be with the screen V adj. I would be tempted to re-check the screen 
current under max. drive condition. 
 If what you and Mr. Rauch profess is true, Ian, Eimac and Svetlana should 
caution designers to use "stiff" grid bias supplies in Class AB1 amplifiers.  
However, they do not.    
 If what you and Mr. Rauch say is true, Eimac erred by stating that up to100k 
of series R was acceptable for the 4cx15,000A.  Consider the spec. sheet for 
the 4CX35,000C.  Again, Eimac recommends limiting the grid R to 100k ohms.  
(note:  the 4CX35,000C appears to be a serious device.  2.9kW filament; anode 
rated at 20kV/15A.  This thing makes X-rays, folks!)

.. ¿¿Which way is up??  

R. L. Measures, 805-386-3734, AG6K   

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