[AMPS] 4-400 operation parameters and such

Rich Measures measures@vc.net
Tue, 5 Aug 97 19:21:18 -0700

>I'm further along in my building of my first grid driven amp and could
>use some input.
>Could someone give me some idea of what voltages I should be looking at
>and maybe the currents as well.  
>I have the parts to build a solid 4100 volt supply which is CCS at 750
>mils.  Have tested it and all seems well.
>Running 600 mils with a pair of 4-400's is well within plate current
>specs and will give me about 2400 input ( not considering any E drop due
>to load).
>Any suggestions what to run for screen Voltage?   I can do 400-500 volts
>easy with parts I have.  
A screen supply of 750v to 800v is good for Class AB1 linear service, 
especially with 4.1 kV on the anode.  The problem with using lower screen 
V is that the peak emissive capability of the 4-400 cathode can not be 
realized without driving the tube into the (non-linear) grid current 
region.  Perhaps you could use a fullwave v doubler to secure the higher 
voltage.  (It takes little avg. screen current for a pair of 4-400s.)  
>How about grid bias?  
around 135v of bias for operate, 200v for cutoff.  Using a grid term. R 
of 50 ohms, you would need a driver that can produce 185w.  Using a grid 
term. R of 200 ohms, driven through a 1 to 4 step-up (bifilar) 
transformer, you would need about 50w of drive.  
-  For best stability, I would directly ground the screen terminals and 
connect the screen supply between the fil. CT (cathode return) and 
chassis-ground.  A grounded screen circuit is shown on my Web site:    
There is also an electronic-switched Class AB1 grid bias supply and a 
short-circuit proof regulated screen supply.  . 
cheers, Steve

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

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