What's your component removal system?

[pwdrive]

I should have said learn not learning...

Lawrence.

[Nowhere-Man]

" learn about voltage and current behavior with resistance, reactance and impedance, ohms law formula is your friend here as it can be applied to all those three, just change the term,...."

I think you indicate the voltages in transformers are always induced by the change in field. Therefore not DC. In the case of the output transformer, the primary of course is used as the optimum load of the output valve. The secondary will have a turns ratio matched to the speaker.
I seem to vaguely recall Chas Miller commenting on this in his book but I no longer have the copy.
However, yes, I am familiar with capaciive reactance and inductive reactance formulae, phase, voltamps and power factors. I do marine electrics as well so some of it I learned in that area.
One interesting sum I use for transformers is my so-called "magic number sum".
If primary has 400 turns but current unknown
Secondary has 2800 turns and a known current of 0.2 amp
Multiply 2800 by 0.2 equals 560
The number 560 applied to the turns ratio of the primary must indicate the primary current. So dividing 400 into 560 gives 1.4
The kicker is the stepped down winding with the lower voltage will have the higher current and all induced voltages will oppose each other.
Not that it's not common knowledge but I like to experiment with the theoretical stuff.

[ppppenguin]

That's a very long winded way of working out the current in the other winding. It's simply the turns ratio multiplied by the current in the first winding.

Iprimary = (2800/400) * Isecondary

Hence 0.2 * 2800/400 = 0.2*7 = 1.4

Similarly for voltage except the winding with more turns will have the higher voltage.

All this assumes a perfect transformer with no leakage inductance or resistive losses.

[Terry]

... ohms law formula is your friend ...

I highlighted an important aspect of Ohms Law that is often overlooked in this recent post.

Ignore it at your peril ...

[pwdrive]

Yes I understand that, that's what we were told at college, but to me, it is in part irrelevant, the temperature I mean.

Lawrence.

[Nowhere-Man]

It's long winded but experimentation allowed me to develop some more complex calculations. I've been known to do exam questions (mostly dated ones) but I also try to make alternative calculations. I was doing some with air cored transformers and found some interesting ratio patterns I still have scribbled down. Some of my maths wasn't learned in a book but just derived from a calculator. Just for fun I tried to work out how long it would take to get to Europa (the moon) at the speed of light and then at rocket or shuttle speed. Then you wind up getting into relativity.
Actually I enjoy the theoretical side maybe more but I'd like to have some practical application to it.

[Terry]

...  it is in part irrelevant, the temperature I mean.

It may be more relevant than you think, Lawrence.

Have you ever considered why incandescent light bulbs virtually always fail at switch on?

Quite simply, the cold resistance of the filament is much lower than it is at its normal working temperature, thus the inrush of current when cold is much higher than the bulb's rating might suggest. When the wire for the filament is manufactured there will always be some small part of it that is marginally thinner than the rest so, at switch on, the voltage drop - and current - will be higher than other sections with the same unit length. Thus there is a tendency for a minuscule portion of the wire to vaporise, rendering that part to get progressively thinner with use and increasing the unequal current distribution during every switch on cycle. Eventually the switch on surge is so high that the filament melts and you hear the characteristic 'ping' as the coiled filament springs apart.

A valve heater acts in the same way but, as the ratio between the cold and operating temperatures is much lower than with the white hot heat of a light bulb, the inrush current is much better controlled and open circuit valve heaters are very rare indeed. However, whilst this is fine when heaters are fed in parallel, thus controlling the current with the fixed voltage, the same is not true of series heater chains. 

No two valves will exhibit identical switch on characteristics and without precautions, one valve would always receive the lion's share of the excess current. So a thermistor is added to the equation.

Having a high resistance when cold and a low one when hot it reduces the current flow at switch on to a very low level, gradually increasing as its temperature rises during which time the valve heaters are brought gradually up their nominal operating point without any excess current or voltage drops.

[pwdrive]

Yes, I understand all that stuff, I used to repair TV's, but R still equals V/I.

Lawrence.

[Nowhere-Man]

Just touching the theme of filaments Terry raised: Consider the very first radio filaments would run on only 2 volts. That's a good way from 6.3. It suggests to me that such early filaments would be pretty delicate. If they run on just 2 volts to emit electrons, they must be fairly thin. I read that some filaments have to be kept within 10 per cent of voltage and that may possibly include under voltage too (although I'm not sure).
I have heard accounts of some of the suitcase type 90 volt battery HT powered radios needing precise LT voltage (or they are history). I am not sure if the average 1930's battery portables had a bit more leeway.
As to Ohms Law I tend to use the basics often enough not to forget but sometimes find I forget the sums for resonant frequency or even capacitive reactance. I am then forced to look it up and refresh my memory.

[Terry]

Consider the very first radio filaments would run on only 2 volts. That's a good way from 6.3. It suggests to me that such early filaments would be pretty delicate. If they run on just 2 volts to emit electrons, they must be fairly thin. I read that some filaments have to be kept within 10 per cent of voltage and that may possibly include under voltage too (although I'm not sure).

You are overlooking the power consumed by the filament. As early cathode/filament coatings were not as efficient as later ones, overall, more  power would have been needed, resulting in higher currents at lower voltages. In the early days of wireless, not all homes had an electricity supply and there was no national standard for those that did, resulting in a mixture of different voltages in various areas plus, of course, some were AC and some were DC so it was simpler to rely on battery power. However, the filaments/heaters of early valves would have been prohibitively expensive to run on dry batteries so a single lead acid cell was used instead - this is where the 2V figure comes from.

In practice you would have bought two single cells - one to power the radio while the other would be deposited with a local dealer or garage to be recharged for a nominal sum.

As mains operated sets became more popular, 4V valves became common in the UK. In the US, however, they went one stage further and adopted 6V as the standard - probably with an eye to producing sets for use in cars which, in those days, tended to use 6V accumulators. But a single lead acid cell does not have an output of 2V - it is actually 2.1V nominally - although it would be very easy to lose the odd 100mV in the wiring but by the time you get to a 3 cell accumulator, those 100mV increases are starting to become noticeable, especially as the current demand would be much lower too, hence the 6.3V figure that we are all used to.

As for the accuracy of heater/filament voltages, you are quite right. Overrunning them with excess voltage definitely results in a serious reduction in service life. It it is much preferable to under rather than overrun them.

If you take the later valves developed for use with dry batteries you will note that, whilst a single cell is marked 1.5V the valves will be rated at 1.4V because that is the average during the working life of the cell, which will start off higher even than 1.5V before dropping very quickly and then tailing off as the cell becomes exhausted. Eventually one of the valves will stop working or they will all degrade the performance to the point where the only option is to replace the cell. 

In mains/battery receivers, where the voltage on mains operation can be expected to be stable it is better to underrun the filaments slightly at between 1.3 and 1.35 volts.

Mike Watterson is the expert in this field and wrote up the subject extensively on the old VRAT forum. If you wish to persue the subject I would recommend that you look back at his posts on there.