Hi Peter,
It has been told that low screen grid voltage will give a low
distortion at IM3 IM5 etc. The statement is not always true.
When you change the dc parameters during operation you can see
IM3 products come up and down with output loading, operation
point versus grid dc, driving power and stability of all dc
voltages like screen grid plate etc.
All environment parameters take the chance to distort.
In other words, PA 1 generates IM3 -30dB with 250V at screen,
PA 2 generates IM3 -36dB at 300V, also opposite, it depends.
Look at technical descriptions. Same tube will give -30dB
from manufacturer X and -37dB from a second PA builder Y.
Higher screen grid voltage will help increasing tube emission
may be it's a pull out tube after a long run.
The higher voltage increases the plate current strongly.
Best is higher plate voltage and higher screen grid voltage.
Peter I stop here because it's a long story when anybody will
tell it without simplification and mistakes.
Have a nice time and good luck with this fine tube.
73, Uwe, DL9NC
> Hi all, I am new to this list and need some help.
>
> In various ( linear SSB ) 4CX 1500B PA designs I have seen various Vg2
> voltages from + 225 V until +300V.
>
> As far as I could recall, the lower the Vg2 the lower the IMD3 (?)
>
> What could be the reason for designing a ( SSB linear ) 4CX1500B circuit
> with Vg2 at 300 V ?
> End of all, one of the reasons to choose for a 4CX1500B is the relative low
> IMD3.
> What are pro's and con's of such relatively high Vg2 ?
>
> EIMAC's " Care and feeding.. " does not resolve this question for me .
> Who could ?
>
> Thanks for any explanation.
>
> Peter, PE1E
>
>
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