[R-390] replacement filters

[John Kolb]

You could indeed terminate a low output impedance filter
with a 2 K resistor to ground and capacitor couple to a 1 M
resistor between grid and AGC line. The output side of the
filter would be happy. If, however, you were using a 7:1
stepdown ratio transformer on the input side (100K:2K match),
you would be loosing 86% of your IF voltage unless you step
it back up with a 1:7 transformer at the output side.

The loss isn't that bad, however. the "N" series filters
used in the 390A are spec'ed at 25 db loss max. Modern
mechanical or ceramic filters will be more like 6db or less
loss. With modern filters, you might end up inserting a resistive
pad to make the replacement filter have as much loss as the
other filters in the set.

JOhn
<http://members.cts.com/king/j/jlkolb>

On Mon, 12 Apr 2004, D. ball wrote:

> 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 po=
wer
> supply derived from the 6 volt heater lines. This could work but may degr=
ade
> 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 resis=
tor
> from the AGC source across the new filter output would work.
> For some reason I do not feel output impedance will matter because you ar=
e
> 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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