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Old 30th Sep 2015, 9:25 pm   #1
Goldieoldie
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Default Valve heater voltage

Hi all,
Why was 6.3 volts selected for the heater voltage on many valves ,why not just 6 volt ?Also same question for other valve voltages 4 volt etc
I understand the higher voltage heaters for series applications ,that makes sense
Cheers Pete
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Old 30th Sep 2015, 9:49 pm   #2
winston_1
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Default Re: Valve heater voltage

6.3v is the voltage of 3 lead acid cells in series over much of their discharge cycle.
Why 4v rather than 4.2v I don't know.
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Old 30th Sep 2015, 9:53 pm   #3
mickjjo
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Default Re: Valve heater voltage

6.3 volts was chosen mainly as it was the average voltage of a 6 volt lead acid battery, discussed in this old thread:-

https://www.vintage-radio.net/forum/...ad.php?t=50285

Regards, Mick.
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Old 30th Sep 2015, 10:58 pm   #4
broadgage
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Default Re: Valve heater voltage

Quote:
Originally Posted by winston_1 View Post
6.3v is the voltage of 3 lead acid cells in series over much of their discharge cycle.
Why 4v rather than 4.2v I don't know.
My theory is that 4 volts was two lead acid cells in series, and relatively small cells at that. The type of cells that you take to the shop for charging.
For cells that are small in relation to the load current, 2 volts per cell on discharge is a fair average.

6.3 volts became popular at about the same time as early motor cars, most of which used a 3 cell lead acid battery for lighting and ignition.
These batteries were often of substantial capacity and 2.1 volts per cell would be a fair average on slow discharge.
Motorists either removed the car battery to power the radio, or more commonly purchased a second battery and swapped them over as needed.

Non motorists still tended to use 6 volt vehicle batteries for other purposes as due to mass production they were relatively cheap.
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Old 30th Sep 2015, 11:11 pm   #5
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Default Re: Valve heater voltage

Early battery valves, usually 2 volt filaments but were also available in 4 and 6 volts, were designed to use accumulators on discharge. Later 1.4 volts for dry cells under the same conditions. As said, 6.3 and 12.6 volt heaters seem to have their origins in the on-charge voltages of accumulators.
For mains usage, any voltage could be used subject to design limitations (insulation, ruggedness etc.) In the UK 4 volts was common for many years. Rectifiers tended to use 5 volts (see this thread: https://www.vintage-radio.net/forum/...d.php?t=119861) until the internationally common 6.3 took over. There have been valves of just about any voltage for particular uses, even 220-240 volts.

Edit: Cross posting again.
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Old 6th Oct 2015, 12:14 am   #6
Synchrodyne
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Default Re: Valve heater voltage

As already recorded by others upthread, the 6.3 volt number was evidently carefully selected for use in systems fed by 3-cell (“6-volt”) lead-acid batteries, taking account of battery discharge curves, likely I²R losses in wiring and so on.

Likely a similar careful approach was used to determine the optimum heater voltage, namely 26.5 volts, for use in valves designed to operate in systems fed by 12-cell (“24-volt”) lead-acid batteries. Some examples of such valves were the RCA 26A6, 26C6 and 26D6, and the Sylvania 26BK6.

26.5 volts is higher than the 25.2 volts that one would arrive at from simply multiplying up from the 6.3-volt/3-cell battery case. Possible reasons for this that come to mind are that with the higher battery voltage and consequent lower heater currents, I²R losses in the wiring would be lower, and so the valve heaters would typically see a higher proportion of the battery voltage than with the 3-cell case. Also, the lower current heaters may have been more fragile, and less able to handle overvoltages, this pointing to a nominal rating that was a higher proportion of the expected maximum.

26.5-volt valves that were counterparts to members of the mainstream receiving series with 6.3 V, 300 mA heaters had 70 mA heaters, which implies that they had relatively “thin” heater wiring.

With valves designed for use in systems fed by 6-cell (“12-volt”) lead-acid batteries, the situation may have been different. The 12-cell precedent, with 26.5 rather than 25.2 volts, suggests that a simple doubling of the 3-cell voltage may not have been optimum, yet that is what was done, with 12.6-volt heaters being used. On the other hand, linear interpolation between the 3-cell and 12-cell cases, as a first approximation to capture the departure from direct proportionality, would give 13.0 volts. Here though, 12.6 volts may have been a something of a force-of circumstances choice. It already existed as a standard heater voltage by virtue of being used for the 150 mA series-string counterparts to 6.3-volt, 300 mA standard receiving valves. Thus, when 12-volt car radios were required in the USA in the early 1950s, existing 12.6 V, 150 mA receiving valves were pressed in to this service, with the gaps being infilled by new 12.6-volt releases, such as the 12AQ5 and 12V6GT, both in 1953 as far as I know. Use of 12.6 volts also enabled series-pair connection of existing 6.3-volt valves. In favour of this approach, as well as its obvious simplicity, was that the 150 mA series-string valves were probably designed with quite robust heaters that could well handle the putative slight overvoltage.

Returning to the valves with 26.5-volt heaters, some of these, the 26A6, 26C6 and 26D6 at least, were designed to work with 26.5 volts on the anodes (and screens, where applicable) as well as with “normal” voltages for these electrodes.

Cheers,
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Old 6th Oct 2015, 1:24 am   #7
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Default Re: Valve heater voltage

I design equipment for operation from vehicle batteries - both "12v" and "24v" and it's interesting to see the voltage range required to be accommodated.

12v systems need to operate down to 9v so that in the event of a generator failure, you've still got radio, transponder, flashing beacons and instruments for long enough to get yourself back home. Double that for 24v of course. And then there is overcharging which can get up to 16.5v/33v under fault conditions. then generator failure and field energy dumps can give you 100ms surges to 30/60v, and load switching can make 80v spikes. The figure for ground vehicle systems using similar alternators are about the same.

I have to design single range systems which can run from 9v to 60v and just use it whatever voltage it is within those limits.

At least the thermal mass in valve heaters would average out the shorter transients, but taking a long term overcharge as a boiling battery attempted to hold down a generator voltage was more threatening to them.

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Old 6th Oct 2015, 10:25 am   #8
turretslug
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Default Re: Valve heater voltage

I wonder if attempting to cover the full possible range of 24/28V (depending on your nationality!) systems with consistent operation became too much of an ask for heater strings as airborne equipment became more and more complex, explaining the appearance of 19V regulators.

And 19V reared it's head again with rather more modern stuff....
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