28-01-2017, 02:12 PM
(27-01-2017, 04:42 PM)Nowhere-Man Wrote: 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.
