Hi Ian
> If it's cathode-driven, it isn't a G2DAF - it's somebody else's even
> weirder creation.
Then weird creations have been more common than the original
idea in what I've come across.
> The original G2DAF amp was grounded-cathode, grid-driven (across a 300 ohm
> resistor) with a voltage-doubler (2 * 6U4GT) to generate the screen
> voltage. G2DAF's stated operating condx for two 4-125s at the UK power
> limit of 400W PEP output are:
> Note the screen and control-grid voltages. These are so far "off the
> map" for the 4-125 that you're completely on your own.
I'd say they are!!!
> Note also that the input loading resistor was 300 ohms, not 50, so the
> original G2DAF design was only suitable for tube exciters with a flexible
> pi-tank output. Some people have modified the circuit for a 50 ohm load
> using a step-up input transformer, but that may not develop enough screen
> voltage, especially at 30MHz where stray capacitance may be a problem.
Let me ask another question.
The rectifiers heavily "load" the exciter in order to supply screen
power.
Was his original circuit as bad as the later offshoots where a peak
storage system was used?
What was the time constant of the screen source?
> Also the efficiency of the RF rectifier-multiplier diodes may become
> poor at higher frequencies. Many people have used more modern rectifier
> tubes, but a low heater/cathode capacitance would be critical.
Damper tubes would sorta fit, but maybe a filament choke would be
better. But no matter how you solve the rectifier capacitance issue,
you are still stuck with non-even exciter loading and the God-awful
changing voltages on the screen and no grid bias.
I'd wager on top of that you are stuck with phase delay in the
screen voltage.
> All in all, it's probably quite difficult to develop enough screen
> voltage. In that case, there's a very strong temptation to increase the RF
> input to get more screen volts, which grossly overdrives the control
> grid... with the results that Rich, Tom and others have noted.
Grid driven zero biased tetrode with a screen that waffles around
almost certainly would give those results without excess drive. In
all likelihood, anything that turns the screen "on" is too much drive.
73, Tom W8JI
w8ji@contesting.com
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