[R-390] replacement filters

[D. ball]

Ok, now I move one to replace this 4kc filter. So here goes some ideas. 
There are newer replacements out there as mentioned but they do have the 
lower input impedance. From what i can tell there are two things to over 
come. First is the match on the input. I assume you could use an op-amp 
rated for that frequency as a buffer. You would have to power it off a power 
supply derived from the 6 volt heater lines. This could work but may degrade 
the overall IMD performance of that path, this could also be proven with 
test eqpt. The second is the AGC voltage that seems to be feed thru the 
output coil to the grid of the 6BA6W. I would think that a 1meg ohm resistor 
from the AGC source across the new filter output would work.
For some reason I do not feel output impedance will matter because you are 
feeding such a high impedance input to the tube. The voltage level should be 
the same with no load from the tube to pull it down. I think all I am 
concerned with here is matching the output voltage of the new filter with 
the old ones and finding something that will have the same spec. as the 
original design.

thanks
Joe
KE1MB

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