Just as an information about my DAF amp, the screen voltage is derived by a
voltage quadrupler circuit, not a voltage doubler. Screen voltage on peaks
at maximum screen current is around 650 volts, a good value for my QBL
5/3500 tube.
The RF input is across a 200 ohm resistor and I use tuned input circuits
for all bands presenting a steady load to the exciter at all power levels.
Detailed pictures can be seen at www.qsl.net/sm2cew and there is a link
there to SM3BDZ's webpage with all the plots he made when he put his amp on
the test bench.
Ciao,
Peter SM2CEW
At 14:56 2000-10-27 , you wrote:
>
>Regarding the 3/2 law, Rich asked?:
>
>"In your opinion, what does Eimac say, Colin"
>
>In responding to your same request in December of 1999, I said:
>
>"The 3/2 power law simply states that if you change all of the
>interelectrode
>voltages applied to a power grid tube by a factor of 2, that the current
>will change by a factor of 2.8.
>
>In the G2DAF amplifier, the grid voltage and screen voltage vary in relation
>to the drive, while the plate voltage remain constant. One unique feature
>of the G2DAF circuit is that the ratio of grid to screen voltage varies
>depending upon the drive as a result of the inefficiency of the half wave
>doubler circuit. The more screen current that is drawn, the lower the
>relative voltage applied to the screen.
>
>The 3/2 power law is a simple conversion process for changing all of the
>voltages on a screen grid power tube. It neither proves nor disproves the
>theory of the G2DAF design.
>
>However, if you put a 2 mfd capacitor on the screen, overdrive the grid, use
>a 4CX250 tube and yell into the mike as Norm did, then you can splatter."
>
>I also quoted directly from Eimac's "Care and Feeding" book in a follow up
>messagel:
>
>"the screen voltage is not critical for most applications and the value used
>has been chosen as a convenient value consistent with low driving power and
>reasonable screen dissipation. If lower values of screen voltages are used,
>more driving voltage will be required on the grid to obtain the same plate
>current. ... The value of screen voltage can be chosen to suit available
>power supplies or amplifier conditions."
>
>Upon reflection, my opinion of the 3/2 law remains the same today.
>
>Colin, K7FM
>
>
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