local oscillator

Posted on Feb 4, 2014

Used Zener Diodes to derive the stabilized voltage for the Hartley oscillator (although I did find a couple of suitable voltage stabiliser valves (CV287) whilst sorting through a box of valves this morning). Whilst sniffing around in that box of valves mentioned above, I was adding to my catalogue of valves `in stock`, in which I like to include links to a data sheet. Whilst doing that,

local oscillator
Click here to download the full size of the above Circuit.

I noticed something I had never realized before. The ECC81 used in the local oscillator is also widely know by the equivalent name 12AT7 (as mentioned in describing the BFO). What I never twigged before is the obvious fact that the valve should use a 12V heater supply (obvious because I knew all this from the 6SK7s etc used in my Paraset - I was considering replacement with 12SK7s to make battery operation easier). Accordingly, I had wired up the heaters for the two 12AT7s in this project for only 1/2 the rated heater voltage - amazing that they still worked at all (and another possible explanation for the fun and games I had with the BFO, as described here. ) Note to self (and any others who happen to be reading): this is also true for the double triodes ECC83 / 12AX7 and ECC82 / 12AU7, widely used in audio frequency gear. . you can see the coil at bottom left. You can also see that I only have a single gang capacitor - which is going to be a problem (I need a twin-gang device; one for tuning the LO and one for tuning the resonant circuit at the input). That will have to be dealt with in the future - for now the carol service from from King`s has just kicked off.

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