[Moon-net] 70cm 4cx250b amp question.

Jordan outposter30 at shaw.ca
Sun Nov 9 20:47:37 CET 2008


Hi again group.... thanks for all the suggestions and info on the supply 
side of my 'project' here, now on to the RF section....

This is a cavity type amp, and the builder strayed a little from the 
original plan and for the most part I have had it working, but it's been a 
few years now....

Going through the grid section, I will add a 33 to 36K 10W resistor from 
screen to ground, within the cavity compartment, Does orientation matter at 
all..?
It will be as close to the chassis as I can put it, but the best direction I 
can lay it is parallel to the copper tubing used for the grid ckt ....

In the plate section, I removed the load capacitor, and it has been 
modified, or the stators melted off..!   I can see lots of overheating of 
the rotor plates as well..The remains of the capacitor is marked 5M11 , 
which I think is an EFJ number, and looks like it was maybe 5-35pf or 
so...???
I see he used a rather short length of #12 wire for the coupling loop , 
about 1" of it paralleled to the plate tube and another 1/4" to the cap at 
one end, and the same length going to the N output connector at the other 
end....It has also been over heated...

I will move the location of the loop and the variable capacitor to move it 
away from the tune capacitor, as I think there has been arcing due to the 
1/8" spacing between edges at some points....

Should the variable have such a range, or was it due to the length of the 
coupling loop that so much capacity was needed...?

The designs I've seen for tuning an amp at 432 would provide say from .5 to 
12 pF....

Any thoughts...???
 Thanks for your time...73...Jordan VE6ZT 


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