UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Interesting Armstrong AM detector circuit.

Synchrodyne · 27th Oct 2017, 6:29 pm

My recollection is that Armstrong did it that way in its ST3 tuner and related equipment in order to minimize modulation rise distortion, which could occur in final IF stages to which full AGC voltage was applied when operating with large signal inputs. A non-AGC’d final IF stage relieved the burden on the AGC’d IF stage, so that it no longer found itself chasing its own tail, being required to deliver a large signal to the demodulator and AGC rectifier whilst back-biased to the very bent part of its characteristic. I suspect that it might have come about not as an ab initio idea, but following a search for a productive way to utilize a spare half-ECC85.

Somewhere I might have a reference to this circuit feature – I’ll look and revert.

Cheers,

27th Oct 2017, 6:29 pm	#3
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	Re: Interesting Armstrong AM detector circuit. My recollection is that Armstrong did it that way in its ST3 tuner and related equipment in order to minimize modulation rise distortion, which could occur in final IF stages to which full AGC voltage was applied when operating with large signal inputs. A non-AGC’d final IF stage relieved the burden on the AGC’d IF stage, so that it no longer found itself chasing its own tail, being required to deliver a large signal to the demodulator and AGC rectifier whilst back-biased to the very bent part of its characteristic. I suspect that it might have come about not as an ab initio idea, but following a search for a productive way to utilize a spare half-ECC85. Somewhere I might have a reference to this circuit feature – I’ll look and revert. Cheers,