-----Original Message-----
From: measures <2@vc.net>
To: Steve Thompson <rfamps@ic24.net>
To: <amps@contesting.com>
Date: 29 March 2000 01:05
Subject: Re: [AMPS] Is screen potential important?
snipped for brevity
>>Then I need to relearn Fourier analysis - are you running classes in the
new
>>maths?
>>
>[chortle] // So why enter a race attempting to rapidly charge and
>discharge the screen bypass C ? The evidence at hand indicates that
>constant screen potential is needed for linear amplification of all input
>levels. .The fact that DAFs typically exhibit c. 10x less splatter on
>the air during 2-tone tests suggests that something is missing during
>voice operation. In my opinion, the missing something is screen
>potential.
I'm not disagreeing that regulated screen makes for good linearity, and I
won't be surprised if the DAF system turns out to be relatively poor if I
ever get the chance to do some tests of my own.
This thread started when you described a situation to show that a DAF amp
could not be linear. My point was that the circumstances you described
simply don't occur with a normal properly adjusted SSB drive source.
I can think of a couple of ways of making them happen:
Apply a drive signal with much wider bandwidth, for example, an exciter
deliberately adjusted to splatter like crazy. In practice, the amp's
linearity worsening with such drive is hardly an issue.
or
add plenty of capacitance to the tube screen. As I recall, when Norm did
this, it resulted in the most splatter that his amp produced. Theory and
practice in agreement - whatever next?
>> What
>>is the time constant with 2000pF of screen capacitor? I'd reckon on 2us or
>>thereabouts. Is 50 timeconstants long enough to charge a capacitor?
>>
>sounds somewhat reverse-engineered .
No - an error on my part. I glanced quickly at the original DAF circuit
diagram and didn't spot the 5000pF on the rectifier side of the meter. It
doesn't make a material difference to the answer.
Steve
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