Thanks everyone, great information, 
The sure did look like DC blocking, so I guess they would be effective at 
blocking RF as well.   The symbol threw me into a tail spin though, try to find 
that in a schematic symbol list! 
I would have thought that 50 or 75v even would be just fine, but maybe they are 
trying to build in a safety factor, or as you say, they don't come in smaller 
values. 
I think changing to DC would mean that the feed through caps couldn't be used 
as wouldn't they block DC?   I would have to change them to be more like a 
bypass cap to shunt RF to ground rather than block DC.   One of the things I 
was thinking with DC is that it is very easy to wire up a simple voltage 
regulator to dial in the filament voltage exactly.  That is a bit harder with 
AC.   Once I have a nice DC voltage regulator for the filaments, I could then 
easily power other circuits since (ideally) I will have stable voltage at the 
tube.   -- anyway, that is where I was going with that, if it makes sense. 
I am adapting this project somewhat.  One of my goals is to build it with the 
one tube as it is specified, but I also want the capability to add another 
3cx800 if/when I get one.   To that end, should I run 4 feedthrough caps, or 
can I just use the two and put the second tube in parallel with the first 
through the same feed through caps? 
Thanks, 
Paul (KG7HF) 
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