[ARC5] [BoatAnchors] BC-375: STABLE AM QSO on 7160 KC

[Arden Allen]

> ........It may NOT be thermal changes
in the structure of the 211 tubes.  It may be
thermal changes in the Osc. tank coil components,
because removing the DC current from the MO tank
settled it right down. ..........

Several points to note, Dave:  The tube is ostensibly running class C as it 
is grid leak biased.  Meaning the tube's Miller capacitance is see-sawing 
all over the place.  However the tank circuit swamps that capacitance to 
keep the frequency on the "spot" so to speak.  Therefore the tank circuit 
components are responsible for the majority of the drift.  With today's high 
selectivity receivers that's a more noticeable problem.  Nothing takes the 
fun out of CW like a drifty VFO or MO as in your case.  So why did switching 
to shunt feed improve things?  The DC component of the tank coil current is 
a small fraction of the RF circulating current thus you haven't reduced coil 
heating by much.  My guess is the choke coil is adding a contrary reactance 
that helps cancel the tank circuit drift.  Choke coils are not purely 
inductive.  They have idiosyncratic resonances so their reactance does not 
climb smoothly with frequency.  Just my lip pull for the day.

Arden Allen
KB6NAX

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