[AMRadio] Home-brewing construction considerations

D. Chester k4kyv at charter.net
Thu Jan 17 22:05:24 EST 2008

> Is there a difference in feeding each of the inside sockets on the
> jack-bar with B+ from the top of the RF plate choke, vs having the two
> coils tied together, in the middle, and feeding B+ there?
> I understand that the potential is the same, but for the sake of having
> less stray capacitance and a 'cleaner' RF environment, which would be
> better, and please... tell me why?

> -Geoff/W5OMR

I don't think it makes much difference, particularly on 160-40m and I have 
seen it done both ways.  Actually, tying the two halves of the coil together 
in the middle would introduce less stray capacitance and inductance to the 
coil.  I think the reason for the split coil and separate connectors for 
each half is to give you the option of  metering each tube separately with 
the series fed circuit in which the coil is hot with HV and carries the DC 
plate current to the final.  Also,  those coils can be used in a  link 
coupled antenna tuner, and the split coil gives the option of series feed 
using a conventional split stator capacitor.

As for the vacuum capacitor, the tank should still remain balanced if care 
is taken with the layout.  I suspect it would be more sensitive to unbalance 
due to stray capacitance and differences in lead length, than with all the 
capacitance in a split capacitor.

Don k4kyv 

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