On Thu, 26 Oct 2000 20:02:24 -0400 "Tom Rauch" <w8ji@contesting.com>
writes:
>
Hi Tom,
>
>Can you tell me how you got that number, for a zero bias 4-125A
>with a voltage doubler hanging off the grid driving the screen at
>some unknown voltage?
The original article states that the amp required 25 watts of drive.
17 watts, if I recall correctly, went into the 300 ohm resistor and the
balance went into the two grids.
I also took the Eimac 350V charts for the 4-125A and scaled them
according to the 3/2 power law by a voltage factor of ½, as per "Care &
Feeding... 1967 edition" p.98, just for a quick check. This gives a
chart for 175V screen V which should be very close to the voltage on the
screen, at the peak of the SSB waveform, in the DAF amp at full drive --
i.e. voltage doubling the peak drive V (100V), less some drop in the two
thermionic rectifiers; close enough for ham discussion.
>According to tables, at 20 volts positive voltage grid current of a
>single tube is 10mA. At 80 volts, around 30 mA....but that's with
>350 volts on the screen.
... with 175volts on the screen the 160V (chart) line will now be the
80V value and the current will be .35 of that corresponding 80 mA
(interpolated chart) grid value, which is about 30 mA. Figure 60mA for
two tubes which is 1333 ohms with your numbers. Close enough for radio
amateur work.
>With less screen voltage, control grid current is much higher in a
tetrode.
That is NOT a general rule. Some of the larger tubes run the other
way, plus there are the secondary emission effects which come into play
in the positive grid V region.
From the above example, it doesn't appear that the control grid
current vs voltage of the 4-125 changes significantly with a variation in
the screen V.
>Are we assuming the doubler draws no current, and does not load
>the grid system? Are we assuming the control grid resistance with
>somewhere near zero volts on the screen (since the doubler draws
>no current) is the same as it is with 350 volts of screen voltage?
The doubler takes about 5 watts. The details are all in the article
on Ian's website.
... and of course the impedances of both the control & screen grids
are going to vary with the applied screen voltage but, I don't care
enough about this "project" to figure those numbers at several points.
The maximum was sufficient for the input circuit loading discussion.
73,
Marv WC6W
*
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