Five Volt Rectifier Valves.

[Daimlernut]

Was there a reason why 5V was chosen for rectifier valves and not 6.3V? Seems an odd arrangement having all the signal processing valves at 6.3V and rectifier obviously on a separate winding but 5V or am I missing something - Alan

[Boater Sam]

Just a guess, it may be because the rectifiers were still using direct heated cathodes a lot longer than most other valve types, obviously you can't mix them on the same secondary winding.
Early decimalisation? If we still made valves would they all be 10v?

[Heatercathodeshort]

I have wondered about this too. I can only think that it came from the USA as they had directly heated 5V rectifiers a while before we did. The type 80 comes to mind, a UX based 5Y3GT.

English rectifiers had 4V heaters such as the U10. The 5U4G was an American valve available before it's UK equivalent, the U52 again with a 5V heater. The popular 5Z4G was another American designed rectifier with a 5V heater but this time indirectly heated.

The odd one is the Mullard AZ31 octal and the AZ1 side contact. 4V and directly heated.

Mazda produced 4V rectifiers on the early B4 and B5 bases and a few on the Mazda Octal base but I think these were all indirectly heated. [UU6/UU8 etc] [Did Mazda produce a directly heated rectifier? ]

The only Octal based 6.3V rectifier suitable for connection in a common heater rail is the 6X5GT [EZ35] another American design.

We wil never know! John.

[Radio Wrangler]

6.3v came from 6v vehicle batteries (with a dynamo system charging) so there were applications for 6.3v heaters without there being a heater transformer. A directly heated 6.3v filament in a rectifier wouldn't be much use here, so no need to change from whatever value they were.

High power valves, big transmitting jobs tend to run low voltage, high current heaters so the heaters are durable. This may have had a bearing on early heater voltages. Then the filaments in direct heated rectifiers seem to be lengths of ribbon, so maybe there is a tradeoff between emissive area and heater voltage for a there-and-back-again filament that favours 4 or 5 volts over higher voltages.

David

[Daimlernut]

Yes of course the American connection they used a lot of 2.5v valves (sorry tubes) so 5v just happens to be double and half the current requirement. - Alan

[broadgage]

Early valve heaters were indeed often powered from 6 volt lead acid batteries of vehicle or similar types.
6.3 volts however represents the average voltage on light discharge such as an amp or two from a large battery.
If the battery was on charge via a vehicle dynamo or otherwise, then the voltage would be about 7 volts to 7.5 volts.

A 6.3 volt nominal heater would still have a reasonable life at 7 volts, but for prolonged use a dropper resistance to lose about 1 volt would have been advisable.

I have seen military radio equipment intended for vehicle use, this used four 6.3 volt heaters in series from a nominal 24 volt vehicle battery.
A relay powered from the engine ignition switch inserted a series dropper resistance in the heater circuit when the engine was running.
No fine or close voltage regulation was involved, just direct connection to the battery with the engine stopped, or via series resistance with engine running.

[M0SOE_Bruce]

Many of the early American valves had 5v filaments with valve types like 00A, 01A, UX201, UX280, etc. At that time radios only needed a 5v filament secondary to power all the valves. For some reason when 6.3Vvalves were introduced many of the rectifiers remained 5V.

[G6Tanuki]

5V seemed to be the 'standard' for directly heated rectifiers long after it _needed_ to be (and long after filamentary cathodes had become obsolete eldewhere - why did they persist so long in rectifiers?)

I suspect 5V was indeed 'twice 2.5V', 2.5V being what a lot of US 'tubes' used in the days before 6.3V took over (I recall having a first-generation HRO receiver with 2.5V valves).

Mullard/Philips seemed to have a bit of an ongoing thing about 4-volt D.H. rectifiers - and stuck with 4-volts even after WWII in their (admittedly indirectly-heated) B8A 'rimlock' AZ41: http://www.r-type.org/exhib/aaa0973.htm

[Leon Crampin]

The AZ41 is directly heated.

Leon.

[G6Tanuki]

Noted.

And even stranger as to why such a quaintly-obsolete valve was ever issued!

[Leon Crampin]

Probably to avoid re-design or a change to transformer specifications when sets evolved to rimlock from octal valves.

There were many small set makers after the war who may have preferred to avoid the cost of a design change when the octals became obsolete. However, I think the AZ41 was more widely used on the European mainland than here. There was also a GZ41 produced by somebody - the last of the 5V heaters...

Leon.

If you ever put an AZ41 on a valve tester, note that one or two of the "IC" pins are in fact a filament centre tap - you could wire it as a 2V rectifier.

[glowinganode]

I'm sure 2.5 V was chosen to limit the current gradient along the length of the filament of a mercury arc rectifier. Since a fwr consists of two separate rectifiers the filaments could be run in series giving 5 V.
Rob.

[Daimlernut]

Its interesting Ive been restoring a National FB7 with seven 2.5v valves and a total heater current requirement is in excess of 9 amps from an external psu, the cable is critical you need 2.8v output to get 2.5v at the receiver so I can see why they abandoned 2.5v valves in the USA

[kalee20]

That's credible for the mercury vapour rectifier - the voltage drop across anode-cathode is fairly low (15V ?) so a few volts gradient along a directly heated cathode would cause a seriously skewed current distribution.

I guess the balancing act is minimising the voltage drop along the cathode, without giving rise to a ridiculously high current, the magnetic field of which would also affect current flow!

[broadgage]

Are you certain that mercury arc rectifiers HAVE a filament ? The ones that I have seen are cold cathode. The cold(ish) pool of mercury being the cathode, and 1,2, 3, or 6 anodes being provided.

The mercury pool must be kept both reasonably cool, and also cooler than the rest of the valve, this is usually achieved by an upward blowing fan in the larger sizes.

Unlike a thermionic rectifier, a mercury arc rectifier has a minimum operating current below which the arc goes out and has to be re-struck. This is done by means of an auxiliary electrode.

[TonyDuell]

I know the device that Broadgage is refering to, but there were also hot-cathode (filament?) mercury rectifiers. The one that sprang to my mind was the GU50, see here :

http://www.r-type.org/exhib/abh0058.htm

[turretslug]

Also the 866 as another good example of a hot-cathode mercury vapour rectifier with low-voltage, high-current filament;

http://www.r-type.org/exhib/aab0018.htm

[G6Tanuki]

There's a difference between mercury-arc rectifiers [which are generally high-power beasties - often steel-cased and water-cooled] and mercury-vapour rectifiers [which are lower-power, have heaters, and can happily fit inside a glass bulb]

The 866 is a good example of the latter: http://www.r-type.org/exhib/aab0018.htm - think of these as a traditional thermionic rectifier where mercury-vapour helps to reduce the forward resistance. They were a pain because of their tendency to flashover if you applied HT to the things before they were properly up to temperature. There was a range of "thermal delay" valves [bimetal strip heated by a coil] designed to control this.

[glowinganode]

Yes you're quite right, I did of course mean mercury vapour rectifiers, the 866 being a very good example, also the 83 fwr.

Rob.

[trh01uk]

I suspect you have the reason in your original post: ".....and rectifier obviously on a separate winding......"

I think that is the point. Its about getting designers to do what they ought to do, and not cut corners and save money - or at least give their bosses reasons why they could not save money. If the rectifier valve had had the same heater voltage (i.e. 6.3V) as all the other valves, then there would have been a temptation just to run it on the same winding. Once you have it at 5V, you are strongly encouraged to put it on a separate winding. And that is important, because you really want the rectifier heater to be floating up at HT voltage to keep the heater-cathode voltage difference of the rectifier near zero. Similarly, you do not want all the other valves to have heaters up at HT potential.

I say "strongly encouraged" because the real misers could have tapped the 6.3V heater winding at 5V and thus still put them all on the same winding.......


Richard