>
>
>-----Original Message-----
>From: measures <2@vc.net>
>To: AMPS <amps@contesting.com>
To: <amps@contesting.com>
>Date: 25 March 2000 11:54
>Subject: [AMPS] Is screen potential important?
>
>
>>
>>In a G2DAF amplifier, screen potential wanders from zero to approx. max.
>
>
>It doesn't wander. It follows the rf envelope.
The ''envelope'' consists of RF cycles. However, RF can not exist across
the screen's RF bypass capacitor. In other words, the grid sees RF and
the screen can not.
>
>>Some say that screen potential can wander about with no serious
>>consequences. Please consider the constant-current curves for the 8171
>>posted at the beginning of my Web site. One set of curves is for a
>>screen potential of 500v. The other set is for 1500v on the screen.
>>Assuming a peak grid potential of +10v, with1500v on the screen, peak
>>anode current is c. 12a. However, with +10v on the grid and 500v on the
>>screen, .peak anode current is 3a.
>
>But screen voltage is set by rf (=grid) voltage. If +10 grid gives +1500
>screen it won't suddenly or randomly give 500V screen instead.
The charge on the screen RF-bypass cap. can not go from 0 to 1500 volts
in one input cycle. After a sequence of +10v-peak drive cycles, the
screen/cathode bypass cap. eventually becomes charged to a level of
1500v. The peak anode current is then 12a. As the screen cap. was
becoming charged, it reached a level of 500v. At this point, +10v peak
drive cycles would have produced 3a anode current peaks instead of 12a .
Does this sound like linear amplification?
>constant in the screen system is in the order of usecs, the fastest edge in
>a SSB signal is in the order of 100us - to all intents and purposes, the
>screen will follow the rf envelope.
All ''edge''s in a SSB signal are individual RF cycles at the fundamental
frequency. The variables are the amplitudes of RF cycles and their
offset from the carrier frequency.
>I don't think you could get +10 grid and 500 screen at the same time with a
>normal SSB drive signal.
The charge on the 8171's screen, RF bypass starts off at 0 volts and ends
up at 1500 volts. At some point, the charge on the RF bypass cap. had to
have been 500v.
The fix is keep 1500v on the screen at all times. RX/TX bias is handled
by an electronic grid bias switch. (Figure 4 on my Web site). This
arrangement provides a constant gain of c.20db. Simply put, the DAF
circuit only provides constant gain when the input signal is constant and
maximal. 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. .
>
later, Steve
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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