Hullo All.

I decided to look at the Mixer valve system again between winding dud modulator coils and achieved a smoothly working example.

As suggested by Radio and Hobbies I used a pentagrid valve but with some logical circuitry to have control over what I was doing.

In this article I am using the prototyping chassis that has many more parts in it than required purely so I can change individual stage operating conditions. I have also drawn a more compact circuit to suggest a better way if someone is silly enough to want to build an example. I can see why audio enthusiasts were so excited by the audio bandwidth achieved in an era of really poor audio performance. When properly tuned in the frequency response goes from the floor all the way to the ceiling and would have been a revelation if the user was lucky or rich enough to have an audio amplifier to match! Of course you have to be blind to the elephant in the room shortcoming, the almighty screaming heterodynes as you tune in to a station, but if the circuit had ever become commercial i'm sure a press button slug tuner like on the car radios would have been employed.

In this era where AM stations have disappeared and the few remaining play compressed digital material its all academic but fun to see what our Dads and Grandads experienced.


Cheers, Fred.

The Synchrodyne AM Tuner - The Mixer Valve Tuner


Yes, we have "errata"...………………...somehow I got "6SL7" lodged in my brain while writing this article and obviously a 6SL7 is not a pentagrid valve! it is a double triode and not quite the same thing. Just shows you how you can make a mistake and just carry on and not notice. I'll blame the medication i'm on. Of course I used a 6 SA 7 as mentioned else where. Sorry folks. If you see 6SL7 just mutter '6SA7 idiot 'and every thing will be alright.

Document uploaded.