28-11-2012, 05:30 PM
Hartley osc.: my understanding is that it must feature a tapped coil - usually arranged with the cathode of the valve to the tap. That puts part of the coil in the cathode cct., part in the anode cct. - in terms of the R.F. cct.; the tap of the coil being the reference point for the R.F. cct. Consequently, there is a mutual inductive coupling between anode and cathode, and since the R.F. currents in cathode and anode are 180° out of phase, oscillation is enabled. The amount of mutual coupling is critical: enough is required to overcome the R.F. cct. losses; too much gives an over-drive situation, resulting in squegging. It is the position of the tap on the coil that determines the amount of positive feedback: ideally just enough and a bit more.
So, on that basis, L5 in Mr. Jones' cct. doesn't - at first - seem to appear as the inductance in a Hartley osc.: there is no tap on the coil. However, if the tap is considered to be at the 'top' of the coil, producing maximum +ve feedback then that might just work if the -ve bias on the grid is large enough (considering the gm of the valve); that bias being provided by the auto-bias by-passed resistor in the cathode.
Al.
So, on that basis, L5 in Mr. Jones' cct. doesn't - at first - seem to appear as the inductance in a Hartley osc.: there is no tap on the coil. However, if the tap is considered to be at the 'top' of the coil, producing maximum +ve feedback then that might just work if the -ve bias on the grid is large enough (considering the gm of the valve); that bias being provided by the auto-bias by-passed resistor in the cathode.
Al.
