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Argus25 · 7th Dec 2019, 1:20 am

The other interesting thing here is that since the Q of a parallel resonant circuit is Q = R.root(C/L), then lets say the V/C is in some mechanical position on the band and you switch in a lower inductance coil, to shift up to the next adjacent band, then the Q should increase because the inductance is lower.

So for the bands that use the same V/C, tuned to some point on the band, the circuit Q's of the parallel resonant circuits should go up for the higher frequency bands, but of course as you tune up the band by reducing the capacity of the V/C, the Q drops, all other things equal, for a parallel resonant circuit.

So any amount of feedback derived in an oscillator circuit, probably drops for this reason toward the high end of a band as the V/C values are lower, and this is often where the oscillations can drop out with a L/O, especially if the valve has gone gassy or has lost transconductance too.

It is fun waiting for this mystery to be solved.

7th Dec 2019, 1:20 am	#65
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: RCA AR88D Comms Receiver. The other interesting thing here is that since the Q of a parallel resonant circuit is Q = R.root(C/L), then lets say the V/C is in some mechanical position on the band and you switch in a lower inductance coil, to shift up to the next adjacent band, then the Q should increase because the inductance is lower. So for the bands that use the same V/C, tuned to some point on the band, the circuit Q's of the parallel resonant circuits should go up for the higher frequency bands, but of course as you tune up the band by reducing the capacity of the V/C, the Q drops, all other things equal, for a parallel resonant circuit. So any amount of feedback derived in an oscillator circuit, probably drops for this reason toward the high end of a band as the V/C values are lower, and this is often where the oscillations can drop out with a L/O, especially if the valve has gone gassy or has lost transconductance too. It is fun waiting for this mystery to be solved.