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Why Aren't There More Aspies Into Electronics?

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Dr-David-Banner

I think there is something I misunderstood above and it dawned on me today. This was what I stated earlier about disconnecting the aerial wire from the grid of the first HF tube and connecting it to the anode of the following tube. I think F Camm must have been referring to the top cap of the second tube. This was normally bonded to the grid. I say that because the aerial signal amplitude is merely millivolts. You can certainly apply such a signal to a grid if the grid is below zero volts and it will act as a gate, modulating the main cathode anode current. The term used was amplification factor. This would be one volt negative increase on grid equals up to,say, 20 volts on the anode to restore the current value (prior to the bias). However if millivolts were applied to an anode with 160 volts already on it, I can't see how anything would happen. So Camm must have meant a top cap (the ones people used to touch to get a hum). I do know the diodes on DDT tubes were below zero volts so probably I misunderstood.

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Dr-David-Banner

That is now something new I learned that was simple but somehow I never picked up on it. Maybe because most detectors in my diagrams are Triodes or Pentodes. The key is to be found in the diode load and polarity. Where a simple diode tube is used, it seems the load resistance of about 0.5 Meg or around is between the cathode and chassis. The + volts are on the cathode side of the resistance and negative the other side with the anode itself connected to ground via the tuning circuit. For some reason unless this is pointed out I tend not to understand. Such a small matter of the voltage being at the cathode side has escaped me. I had always viewed anodes as being all at high voltage which they are in amplifier tubes. Simple diodes were rarely used as detectors and virtually all sets fed the signal to a grid.

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Dr-David-Banner

The cathode voltage is where they tap off the AF. If this sounds complicated it isn't. To make a simple receiver, all you need is a big aerial. Couple the aerial to an HF coil and variable capacitor with one end of the tuning to ground. Feed the other end to the anode of the tube. Run a big resistance load from cathode end to chassis, Put a 0.1 mF or so capacitor shunted across the resistance. Apply voltage + at cathode side of the load resistance and - to chassis side of R. With phones you should then get the AF tapped off at that point.

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Dr-David-Banner

Currently been working on LC constants and calculation of transformer inductances. This involves a calculation based on diameter, radial depth, length and number of turns. There is another formula to callculate the turns to inductance ratio so you can make your own winding. It is ok to do with a calculator but the maths formula is long winded. This is for inductance windings with no core.

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Dr-David-Banner

I've been on an American electronics site which is friendly but it seems the standard is way back from the sixties. Most posts are kind of superficial ("I found such and such in the garage and anyone got a schematic?") Americans had really good tech education in the 1960s but so far on the forums of today, it lacks depth. Electronics remains a very important subject and it should be taught in schools. How do diodes convert AC to DC, , how do transistors amplify, how fast do radio waves travel and so on. For me what worked was doing it all the old way. I have radio sets from the 1940s and some of them work. They are simple but durable. To work on them you need basic theory but not rocket science. I guess the current shortage has been caused by over reliance on digital mass production overseas so EU and USA tend to just import electronic products - all churned out to a set pattern. No more need to learn TV repair. No actual jobs for repair of radio, TV or turntable sound systems. Leading countries in tech are now China, Japan with Russian hugely improving weapons electronics. 

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