>
>
>-----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.
Only with voice modulation. With two-tone modulation, the DAF circuit can
be fairly linear with minimal grid current. .
>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.
Like about 100F with a trickle-charger?
>As I recall, when Norm did
>this, it resulted in the most splatter that his amp produced.
As I recall, the splatter level was pretty much constant with several of
his modifications to the DAF screen circuitry. . The basic problem is
that there was no way to keep the screen bypass capacitor adequately
charged during voice modulation. In the end, Norm gave up on modulated
screen potential and went with a shunt voltage regulator for the screen.
Did it matter? The result was c. 10x less splatter.
>Theory and practice in agreement - whatever next?
>
Indeed. Theory indicated that screen potential needs to be fixed. When
this was put into practice, the problem was fixedand they lived happily
ever after. I would have been sad if Norm had not succeeded in making
his DAF amplifier linear. The ''next'' is that Norm goes to the land of
the silent keys with r-e-s-p-e-c-t.
>
later, Steve
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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