[Amps] Parasitics & Filament Sag

[Jim Forsyth]

An easy answer to your question is a parallel circuit consisting of a series 
R and L in one branch and a series R and C in the other branch. This circuit 
has two resistances but your formula only provides for one.

Jim, AF6O

Unless the formula has gone through a radical change since my classroom
days, its always been Z= SQ.RT. R²+(XL-XC)², when at resonance = SQ.RT. R²=
R. Now where am I going wrong here?