It depends on the particular type of tube used. With a 4CX1000 I experienced
similar readings to what Bill notes. However when the screen current was in
the negative region, tuning the plate tuning would show further negative
screen current at maximum output and plate current dip. But when more drive
was applied and the screen current went into the positive region then it
behaved as Bill describes. Screen current peaks as output peaks and plate
current dips.
Watch the grid current!! With the 4CX1000 just a little grid current, less
than 1 ma, is the point to keep the drive at for adjustments. Then tune
plate and loading for screen current wanted per the tube spec. heavier
loading (less load capacitance) will lower the screen current. Tune plate
capacitor last for a screen current peak. It is very sensitive. More so than
output peak or plate current dip. Keep the drive down when tuning so as not
to exceed maximum screen current and maximum grid current while tuning.
Other tubes may behave differently as to negative screen current. I had a
PL172 tetrode similar to the 4cx1000 in the same amp. When keyed with no
drive the screen current would go slightly positive, 4-5 ma,. As drive was
applied it would always go positive. The amp was loaded up so as to indicate
around 30 ma when grid current started to flow. This was the maximum power
output point. The screen current never went negative in any condition on
this tube.
Negative screen current is common with some tubes as in the 4CX1000. I have
seen some articles where they indicated that maximum output on that tube
happened with negative screen current of -15 ma and never going positive.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Bill, W6WRT
> Sent: Saturday, April 18, 2009 9:18 AM
> To: Amps@contesting.com
> Subject: Re: [Amps] screen grid current question on gu-84b
>
> ORIGINAL MESSAGE:
>
> On Sat, 18 Apr 2009 08:08:08 -0400, "ka1mwp" <ka1mwp@comcast.net>
> wrote:
>
> >
> >Thanks. Let me know if I am explaining this correctly, I tend to run on
> >LOL
>
> REPLY:
>
> It sounds ok so far. I have not used a GU-84b but I have used a
> 4CX1000A and they should be similar. With zero drive, the screen
> current is a little negative. With increasing drive, the screen
> current goes more negative at first, then as drive increases it
> becomes less negative and becomes positive as full power is
> approached. At full 1500 watts output, the screen current is about +25
> mA. Tuning is simple - adjust the plate tune cap for max positive
> screen current and adjust the load for max RF out. The point of max RF
> out also coincides with the +25 mA point, so basically, I just adjust
> the tune cap for a screen current peak and adjust the load cap to make
> the peak +25 MA. Once you know the values for your particular amp, you
> can do the tuning without even having a power output meter.
>
> Piece of cake.
>
> 73, Bill W6WRT
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