[Amps] RE: 4CX250B Screen Supplies

[Steve Thompson]

On Saturday 24 July 2004 12:05, R. Measures wrote:
> 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.
I'm not talking about the screen voltage and the RF cycle - I'm talking about 
the screen current and audio frequencies that arise from the two tone test.

If I wind the carrier power up and down, the screen current goes up and down. 
If I wind the power up and down at 2Hz the screen current goes up and down at 
2Hz and the meter needle will follow it. If I wind the carrier power up and 
down at 1kHz the screen current goes up and down at 1kHz but the meter needle 
indicates steady 'average' current because it can't follow the changes, not 
because the current is actually constant. The regulation of the voltage at 
the audio frequency does matter. It's tricky winding the power control up and 
down at audio frequencies manually, two tones does it more effectively.

Steve