[AMPS] Is screen potential important?

[Steve Thompson]

snipped for brevity

-----Original Message-----
From: measures <2@vc.net>
To: Steve Thompson <rfamps@ic24.net>; AMPS <amps@contesting.com>
Date: 27 March 2000 14:26
Subject: Re: [AMPS] Is screen potential important?

>>>The charge on the screen RF-bypass cap. can not go from 0 to 1500 volts
>>>in one input cycle.
>>
>>No, but neither can the rf drive from a SSB exciter go from zero to full
in
>>one cycle.
>
>This is precisely whan happens.   The first cycle out of the exciter can
>be anything up to the limit of the Final Unit.


Surely not in practice. There is some rise time in the envelope - during
that time, the amplitude of the rf cycles gradually increases from zero to
the intended level. The envelope risetime is inversely related to the
modulation bandwidth. When the signal has been through a 2.5kHz wide filter,
the time is around 100s microseconds.

>Simply put, the DAF
>>>circuit only provides constant gain when the input signal is constant and
>>>maximal.
>>
>>That's not how the measurements for the QBL5-3500 amp look.
>
>The only splatter measurement that matters is on the air, using two
>double-filter receivers, Unannounced.  .


So you can't tell if it's the amplifier, the exciter or the operator
adjusting things wrongly that's causing whatever you measure. I prefer to
know what the drive signal is doing so I can make sure it's the amplifier
I'm testing.

>>
>>>When the screen's RF bypass cap. is not fully charged, the gain
>>>is not what it should be.  //  note - In a typical HF, 8171,
>>>grounded-screen amplifier, the total cathode/screen RF bypass capacitance
>>>is c. 0.05uF.  .
>>
>>With that much C it's probably not a good candidate to try in a DAF
system.
>
>With a more common 4000pF of screen bypass, would the DAF circuit be able
>to apply RF to the screen?


Ahh - is this where confusion arises? RECTIFIED rf is applied to the screen
and the screen cap stores it after the rectifier goes reverse biassed. Take
an example of 2000pF, 50mA screen current and 2MHz frequency - what's the
droop between rf pulses topping up the capacitor? What's the droop at 20MHz?

I calculate that the time constant for charging the screen capacitor is in
the regions of a microsecond, in which case if the rf  *envelope* has rise
and fall times of 100s us, the screen voltage, to all intents and purposes,
follows it.

Steve


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