[Amps] RE: 4CX250B Screen Supplies

[R.Measures]

On Jul 23, 2004, at 7:47 AM, Steve Thompson wrote:

>> Also, with a 2-tone test, it would seem not to matter whether the
>> screen-V is regulated or unregulated since screen-I is constant and
>> therefore screen-V would not vary unless the electric-mains V
>> fluctuated.
>> end
>
> Doesn't the screen current vary as the rf output varies from zero to 
> peak at
> the beat frequency between the two tones?

  Hello, Steve ---  Yes, because there is zero screen-I until the 
instantaneous anode-V dips down close to the screen-V -- i. e., the 
anode is no longer positive enough to take away all the electrons that 
pass by the screen.   For instance, with an 8171 @ E-screen = 1500v, 
during the RF cycle, the screen current varies from zero to about 900mA 
even though the average screen-I is c. 120mA.  From the characteristic 
curves, it can be seen that most of the screen-I flows when the 
instantaneous anode-V dips under 2kV.

> The meter reading will be steady
> because of inertia.

The screen's RF bypass-C keeps the instantaneous screen-V constant 
during the RF-cycle.
end
>
> Steve
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org