13-12-2013, 03:36 PM
(12-12-2013, 04:48 PM)pwdrive Wrote: I have never used a receiver with the diff capacitor arangement and I'm wondering how it compares.
The Pye MM that Alan's got uses that system, I wonder if any other UK manufactures used it?
I had a Pye Model G radio (mains operated, 3 valve+metal rectifier) table radio in the 1970's, this had a differential condenser for reaction control.
There was some slight de-tuning as reaction was increased, though I don't know how this would have compared against what it would have been like if the reaction control was just a simple variable capacitor. Obviously just disconnecting one set of plates would have shown, but I was only 13 at the time and didn't know enough.
I'd understood that the idea of the differential condenser was to give the RF on the detector anode, somewhere to go. With anode to the moving vanes, one set of fixed connected to chassis, the other set to chassis via the reaction coil, the total capacitance from anode to chassis would have been nearly constant - assuming the impedance of the reaction coil was low. So the anode would have been heavily slugged for RF.
Without the differential condenser, if reaction was reduced, the RF voltage at the anode would have risen and particularly with a triode, damping on the input circuit would have greatly increased due to Miller effect. One solution would have been to have a permanent fixed RF bypass capacitor, but then it might have been difficult to get enough reaction.
