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Why Aren't There More Aspies Into Electronics?

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Pinky and his brain
21 hours ago, Dr-David-Banner said:

Then, I reasoned the filaments really are an essential 2 volt short circuit. It's just positive meeting negative across a thin wire so it gets hot and glows. So, did polarity matter? I mean, you can use A.C. to heat a filament which is why ceiling lights just go straight to the A.C. supply.

You can use both AC and DC to heat a directly heated tube, but using AC is more complicated. You need a special centre tapped transformer, where the centre tap is grounded, and the two outer wires connect to the heater. And you need some extra circuitry to balance the heater and to avoid hum on the output. 

And since you need only 2 Volts, it will be hard to find a transformer with only 2 Volt AC on the secondary. You would need to have someone make it specifically for you.

 

So using DC is the easiest way to heat a directly heated tube.

So find a good powerful transformer with around 5-6 Volts out, and then make a simple regulator for it. A simple zener controlled transistor and a few caps could be all you need. Just remember to put a heat-sink on the transistor. At several amperes load, the transistor will get hot.

 

On an indirectly heated tube, it does not matter what you use. Both AC and DC will work equally good. And polarity doesn't matter.

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Dr-David-Banner
2 hours ago, Pinky and his brain said:

You can use both AC and DC to heat a directly heated tube, but using AC is more complicated. You need a special centre tapped transformer, where the centre tap is grounded, and the two outer wires connect to the heater. And you need some extra circuitry to balance the heater and to avoid hum on the output. 

And since you need only 2 Volts, it will be hard to find a transformer with only 2 Volt AC on the secondary. You would need to have someone make it specifically for you.

 

So using DC is the easiest way to heat a directly heated tube.

So find a good powerful transformer with around 5-6 Volts out, and then make a simple regulator for it. A simple zener controlled transistor and a few caps could be all you need. Just remember to put a heat-sink on the transistor. At several amperes load, the transistor will get hot.

 

On an indirectly heated tube, it does not matter what you use. Both AC and DC will work equally good. And polarity doesn't matter.

Part of the mystery might have been solved last night. I now suspect possibly my small lamp isn't meant to always light up as a scale lamp does. The manual states (in faded print) the bulb is a fuse intended to blow if there's a short circuit. It's designated "F1". It may be easier not to worry about the bulb and just carefully trace the heater pin circuit so it will get two volts. I could also insert a modern fuse. Possibly fuses in the Thirties were few and far between so a lamp bulb would have been used.

In the past the bulbs I came across were simply scale-lamps.

One thing to watch here is the H.T. ground and L.T. ground are not directly connected. It was usual to place resistance between the two grounds around 800 Ohms. The L.T. was the ground directly connected to the steel chassis. H.T. negative was directly fed from the 100 volt battery Sometimes the resistances would be tapped at this negative junction to provide a biasing circuit.

Edited by Dr-David-Banner

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Dr-David-Banner
2 hours ago, Pinky and his brain said:

You can use both AC and DC to heat a directly heated tube, but using AC is more complicated. You need a special centre tapped transformer, where the centre tap is grounded, and the two outer wires connect to the heater. And you need some extra circuitry to balance the heater and to avoid hum on the output. 

And since you need only 2 Volts, it will be hard to find a transformer with only 2 Volt AC on the secondary. You would need to have someone make it specifically for you.

 

So using DC is the easiest way to heat a directly heated tube.

So find a good powerful transformer with around 5-6 Volts out, and then make a simple regulator for it. A simple zener controlled transistor and a few caps could be all you need. Just remember to put a heat-sink on the transistor. At several amperes load, the transistor will get hot.

 

On an indirectly heated tube, it does not matter what you use. Both AC and DC will work equally good. And polarity doesn't matter.

I do have a transformer I could use but, as I said earlier, it's not clear whether I'll dare risk a test. Part of the insulating plastic cracked with age and so the wires were just a bit disturbed.

On the topic of transformers, the cleverest idea to date was to build sets without any transformer at all. The classic AC.DC design actually took the live and neutral from the mains straight into a Bakelite set. Dropper resistors were used to derive 120 volts from live and about 5 volts for the heaters while dropping 100 or so volts. I'm not sure whether this is legal today but the design works well and saves the weight and bulk of a transformer. The tube heaters are wired in series and the rectifier is only half wave. These sets were very popular in the U.S.A. and also with a  classy design in Bakelite. You do need a very hefty mains dropper resistor (or in some cases a dropper line chord was used).

One major gripe has always been some of these sets had a hot chassis. Plugged in and with the switch off, the chassis could be totally live and any grub screws in contact with it. Below is a typical transformerless set.

Anyone who wants to "have a go" at this kind of work can easily find an old Vidor battery set from the late Fifties.

 

48.jpg

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Dr-David-Banner

In electronics, I try to do theory as well. In fact, maybe I'm more theoretical than "practical". I try to keep some sort of a balance.

I have a few issues lately over capacitors. It so happens that capacitors have higher reactance at low signal frequencies and very low reactance at high signal frequencies. By "reactance" I mean "resistance". Inductor coils are totally opposite. They have high reactance at high frequencies. The coils are called "chokes" for this reason.

A couple of nights ago it sort of hit me I'd not quite grasped the whole concept of how capacitors function. I think the point of confusion I have is around the area of low pass/high pass filters and basic R.F. H.F. bypass capacitors.

Question: If a bypass capacitor has low reactance to an Audio Frequency, it's commonly accepted in the books it allows the said frequency to pass through the capacitor easily in order to "decouple" the D.C. and A.C. circuits.

What it doesn't tell you is the specifics. Does this kind of circuit behave similar to a low-pass filter?

As I recall from one electronics textbook I once got from the library:

If a 400 Kilocycle frequency is passed through a capacitor filter, reactance is high and the output frequency hardly differs from the input signal (in amplitude).

If the 400 Kilocycle is speeded up to 4 Megacycles with the same capacitor, reactance is very very low and the output signal loses amplitude. It's said you almost get a short circuit.

All this makes me wonder if the bypass capacitors also filter the incoming signal so the output amplitude is "attenuated" as they say.

I used to get heckled on regular sites for asking these questions but I've never had a problem identifying any sticking point. Put simply there are a few issues I don't fully grasp as the books I have don't seem to have picked up on any similarity between low pass filters and bypass systems.

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Pinky and his brain

If a circuit is a "Low-pass" or a "High-pass" filter, depends on the configuration. If you make a simple series filter, the capacitor will work as a high-pass filter, and a coil will work as a low-pass filter. If it's a "By-pass" filter, it will be the other way around.

I made a primitive drawing to show you. It's just a quick hand drawing, so excuse the quality. :P

56dad1866c46b_serielfilter.thumb.jpg.d64

56dad2106f56e_bypassfilter.thumb.jpg.582

 

These types of filters only work at low frequencies. As soon as you move into HF area, things start to get much more complicated. I don't remember the official limit, but I think as soon as you go beyond 500kHz, things start to get critical. The theoretical behaviour should be the same, but in reality none of the components made today, behave as perfect capacitance, perfect inductance or perfect resistance. And those imperfections are creating problems, as soon as you move into HF area. A typical capacitor also has internal resistance, and internal inductance. Those values are small, but they matter at very high frequencies. Same behaviour also exists for coils and resistors. Even a straight piece of wire has resistance, inductance and capacitance. At low frequencies, none of these "extra" values matter, but at HF it does. (resistance matter at low frequencies, thin vs. thick wire. But that's all).

Hope this is somehow helpful to you. The frequency stuff is among the more difficult things to understand in electronics. :)

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Dr-David-Banner
On ‎05‎/‎03‎/‎2016 at 0:51 PM, Pinky and his brain said:

If a circuit is a "Low-pass" or a "High-pass" filter, depends on the configuration. If you make a simple series filter, the capacitor will work as a high-pass filter, and a coil will work as a low-pass filter. If it's a "By-pass" filter, it will be the other way around.

I made a primitive drawing to show you. It's just a quick hand drawing, so excuse the quality. :P

56dad1866c46b_serielfilter.thumb.jpg.d64

56dad2106f56e_bypassfilter.thumb.jpg.582

 

These types of filters only work at low frequencies. As soon as you move into HF area, things start to get much more complicated. I don't remember the official limit, but I think as soon as you go beyond 500kHz, things start to get critical. The theoretical behaviour should be the same, but in reality none of the components made today, behave as perfect capacitance, perfect inductance or perfect resistance. And those imperfections are creating problems, as soon as you move into HF area. A typical capacitor also has internal resistance, and internal inductance. Those values are small, but they matter at very high frequencies. Same behaviour also exists for coils and resistors. Even a straight piece of wire has resistance, inductance and capacitance. At low frequencies, none of these "extra" values matter, but at HF it does. (resistance matter at low frequencies, thin vs. thick wire. But that's all).

Hope this is somehow helpful to you. The frequency stuff is among the more difficult things to understand in electronics. :)

Thanks. We'll discuss this later in more detail. I've had a hectic day and only 10 minutes remain of internet time.

I agree, yes. I don't know about transistors but there were issues with inter-electrode capacitance with triode valves. This was a bit complicated but has to do with the grid - anode capacitance at higher frequencies as you'll have read. It's probably the same with transistors but I'm not sure how the land lies there. Anyway the screen grid was used and finally pentodes. My radio has a few pentodes and beam tetrodes.

I'll talk more of this later. I did make a start with carrier waves to get a bit more depth. If you think about it, a 1 Megacycle radio wave that has a 1 Kilocycle AF wave superimposed is a big frequency variation. I know the capacitors are used to bypass R.F. and so on but I was a bit curious about the amplitudes involved.

The good news is I seem to be on top of my work and suppose I stand a half-decent chance of success. I located the aerial lead yesterday from the grid to the variable condenser and fixed capacitor. I have the H.T. wires and power capacitor sorted so I'll need to trace the speaker wires soon. The only issue I really have is the heater circuit still.

 

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Dr-David-Banner
On ‎05‎/‎03‎/‎2016 at 0:51 PM, Pinky and his brain said:

If a circuit is a "Low-pass" or a "High-pass" filter, depends on the configuration. If you make a simple series filter, the capacitor will work as a high-pass filter, and a coil will work as a low-pass filter. If it's a "By-pass" filter, it will be the other way around.

I made a primitive drawing to show you. It's just a quick hand drawing, so excuse the quality. :P

56dad1866c46b_serielfilter.thumb.jpg.d64

56dad2106f56e_bypassfilter.thumb.jpg.582

 

These types of filters only work at low frequencies. As soon as you move into HF area, things start to get much more complicated. I don't remember the official limit, but I think as soon as you go beyond 500kHz, things start to get critical. The theoretical behaviour should be the same, but in reality none of the components made today, behave as perfect capacitance, perfect inductance or perfect resistance. And those imperfections are creating problems, as soon as you move into HF area. A typical capacitor also has internal resistance, and internal inductance. Those values are small, but they matter at very high frequencies. Same behaviour also exists for coils and resistors. Even a straight piece of wire has resistance, inductance and capacitance. At low frequencies, none of these "extra" values matter, but at HF it does. (resistance matter at low frequencies, thin vs. thick wire. But that's all).

Hope this is somehow helpful to you. The frequency stuff is among the more difficult things to understand in electronics. :)

I was revising this last night. I have a 1940 book on radio engineering and it breaks the bypass capacitors into alternating currents on D.C. power lines, A.F. filtering and R.F. filtering. Very typical capacitors will be screen grid bypass, anode bypass and so on. I imagine the R.F. frequencies are going to be much faster.

What you said about higher frequencies summarises a problem with triode tubes. It's why the tetrode was invented. Finally the pentode. I keep forgetting the basic theory behind it so I briefly looked at it again and, in short: Very high frequencies caused small capacitance between the anode and grid. This was a snag as the capacitance was intended to be at the anode and cathode. Thus, they came up with the screen grid idea. This was to insert an electrostatic shield between the anode and grid. That cancelled the stray capacitance but, of course, there was a need to apply voltage to the screen so the electrons would continue to flow to the anode. I think it was around 1 megahertz or so where the stray capacitance was an issue.

I find different books give different angles. The most technical book I have is the AAR Handbook 1960. This is a huge book intended for HAM Radio constructors. I have to keep re-reading it as there is information that tends to seep in over time. Another book is more down-to-earth engineering. This latter book was written in the hungry Thirties and updated before the war.

Today I found there was a fault in the electrical supply. It could have been dangerous. I discovered I have a hot neutral in the mains circuit and no voltage on the actual hot wire. It turned out to be a faulty extension cable that took power to my boat. Someone must have mixed up the neutral and live. On changing the cable for one I knew to be working fine, neutral was once again at zero in my domestic circuit. This could have been very dangerous before, given the time I spend working on old radio sets. Those ACDC sets are especially dangerous due to not having any transformer. They connect neutral to the chassis so I must have had a "hot chassis". I guess the lesson to be learned is never trust your A.C. House supply and it goes to show how many simple mistakes can lead to serious faults.

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Dr-David-Banner

Hmmm, where do you buy a 50 Microfarad capacitor these days to take around 400 max volts? 50 Microfarads is a pretty big capacitor!! It turns out this was the usual value for cathode bypass on most output tubes.

There is a site called Just Radios in Canada that stocks 50 mF capacitors but, of course, the postage is where it becomes a bit costly. I did try Farnells but can't seem to find that value.

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Pinky and his brain

The problem is that 50uF is no longer a standard value. Today Capacitors and resistors follow a decade standard. It's either going to be E12, E24, or E96 standard. There are other standards too, but they are not used much. You can see the decades here: E decades

So you have to settle with either a 47uF or a 56uF. But that's not a big problem, because the precision on capacitors is usually +-20%. So they will always be a little off the value anyway. I think you can feel quite safe just using a 47uF or 56uF instead.

On Farnell, Mouser or Digikey you can find many different caps, with a value of either 47 or 56 uF and voltage rating between 400 and 450 Volts. Remember a higher voltage rating is not a problem, only a too low rating is a problem.

Here is a link to a search on Farnell: Farnell caps

:)

 

Edited by Pinky and his brain

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Little Guy

I was into electronics when I was young and built crystal sets and then tube sets. I got kind of lost once transistors came along. Built short wave radios but never got Morse code due to tone deaf. Still today in the US, Morse code is no longer a requirement so I got my general ham license after 50 years of first studying the ARRL books. Later, I got back into electronics and started fixing and building computers.

Today though programming fulfills the same need to use my logic pathways in the brain. I program web applications for a living now sometimes several thousand lines all out of my head.:wacko:

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