>
>Rich:
>
>In any screen grid amplifier, plate current is determined by the combination
>of control grid and screen grid voltage. Varying both the control grid and
>the screen grid certainly makes testing much more difficult and also adds
>another variable that can screw things up, but these variables do not mean
>that it cannnot work.
>
I have always agreed that the DAF config. works.
>What happens if the screen grid is zero volts. No distortion will be heard,
>because there will be no plate current. Splatter is most likely to occur
>when the input signal causes a flattening of the plate curve.
input signal alone is not going to cause emission saturation.
Saturation also requires screen amplification, which means a moderate
amount of screen potential must be applied.
>We have
>previously discussed the spikes that are transmitted in many exciters at the
>beginning of transmission - especially when alc is used.. These spikes
>saturate the plate and severe splatter occurs.
I have measured these spikes. I do not believe the claims of several
hundreds of watts. It is my opinion that such claims were an attempt by
Mr. Rauch to toss off the AL-80 / SB-1000 bandswitch arcing problem in
the open 160m padder contacts during intermittent regeneration @155MHz.
. (photos on Web site)
>However, in a G2DAF amplifier, screen voltage is at zero during voice lulls.
>When drive is applied, voltages to both grids increase. If the screen
>voltage lags a bit, that would not cause plate saturation.
There are other ways to produce distortion than emissive saturation.
> Your friend Norm
>hung a 1 mf cap on the screen - thus keeping the screen fully charged.
>Then, he ran a substantial amount of grid current.
As I understand it, G2DAF ran fairly substantial grid-current.
>Splatter would and did
>occur. I do not know how your other friend hooked his up, but I expect it
>was an "improved" model of the original circuit.
>
but of course
>A few years ago I heard a new extra class ham, almost 50 miles away,
>splattering 200 khz away from his fundamental. His transceiver was a
>FT-1000D and his amplifier was a new Alpha. The splattering was caused by
>the fact that he was relying on his ALC to limit everything, and the spikes
>at the beginning of his speech overloaded everything. This was not the
>fault of the equipment, it was operator error. Should I condemn all
>grounded grid amplifiers because they can be used improperly? That is what
>you are doing to the G2DAF circuit.
>
>Distortion is measured against peak output, and is caused when the plate
>current does not faithfully follow the input. I wrote a hypothesis on how
>the screen voltage can vary in a screen grid grounded cathode amplifer yet
>have linear output. I confirmed my hypothesis on my test bench. Lars has
>run tests on his amplifier, which were also consistent. But, you have
>ignored those results.
I only believe what I measure on the air, prior to announcing my
presence.
>
>The Collins KWM2 and 32S1 does not use a "regulated" screen supply, yet it
>produces an excellent signal.
They use RF-NFB.
>Bill Orr regulated his and reduced distortion
>even further, but because of rf feedback, The Collins provides an excellent
>signal even though it does not use the "regulated screen" that you say is
>essential.
The screen supply in the Collins 30S-1 uses a resonant-choke filter. The
dynamic regulation is superb.
>
>I should get one of the QBL tubes and copy the circuit of Lars and put it on
>the air over here, and see if you can tell when I have my grounded grid amps
>on or when I am using the "other" circuit.
It has been my experience that DAF true-believers tend to run virtually
zero grid-current during announced distortion measurements. As I
understand it there are still some Cold Fusion true-believers at the
University of Utah.
>
later, Colin
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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