dc vs ac vacuum tube filament winding for reliability/lifetime/durability?

[coppercone2]

I read the traditional debate of DC vs AC in vacuum tubes, but I have not seen any talk about the steady state field present as a result of DC through a heating coil.

Can it cause something with the offgassing/metal vaporization depositing in a peculiar way? I know that they need to be inrush controlled for the sake of strain in the metal or whatever it is exactly.

[Gyro]

If you're talking about AC vs DC on the heater, then DC is heating is mostly audiophool - it is only relevant when very low levels of hum pick up are required - eg. Mic amps. Even then, tubes like the EF86 were constructed with bifilar-wound heaters so that AC could still be used.

[coppercone2]

no I mean the dynamics of the gas and vaporizing of the electrodes in the presence of the DC field. Does contamination do anything funny.

 AC at 60Hz is pretty slow though, but it should still randomize it. Higher frequencies should effect it even less.

Like can the filament migrate towards one side (polarity of the DC) and short some shit out or increase contamination in a certain spot?

[Circlotron]

Have a read up on vacuum tubes and design techniques for old timey undersea analog telephone repeaters. They had to run a squillion hours without quitting. Might be something relevant there.

[LaserSteve]

For high current gas tubes, ie Ion Lasers, we use AC or equal duty cycle "spread spectrum" flipping DC  to distribute the 10 to 35 amp arc over the cathode. Otherwise the arc will etch the 75 to 125 watt (depending on model) directly heated cathode at one end weld. Some rare apps (mostly semiconductor inspection)  use the slowly flipped DC to avoid having a strong sixty Hertz  spectral line in the laser's amplitude spectrum.

Steve

[coppercone2]

hmm.

That makes sense for an arc. Does anyone know how the vapors from the metal behave? Is it just a neutral gas? Can you get charged particles some how?

[GeoffreyF]

In almost all normal tubes made in the past century (really the past century) the filament is not the Cathode.  Instead it heats the Cathode which surrounds it.  Therefore, AC or DC do not matter much.  I've actually been on the planet long enough to have built vacuum tube sets of various kinds.  If memory serves, the tubes were heated with 6.3v AC.   It's been awhile though.

[coppercone2]

Well if a charged particle some how is made then it will follow the field to whatever direction. I don't know if this could happen. If its a heating coil nearby (like a bulb) it might direct a charged particle some where it does not belong in more frequency then usual.

Also you have gravity inside of the tube. AC reverses direction periodically so it might allow something to fall down to the bottom vs get directed into something else its attracted to?

[amyk]

Also you have gravity inside of the tube. AC reverses direction periodically so it might allow something to fall down to the bottom vs get directed into something else its attracted to?

are you trolling?

Yes, electrons have mass and can be affected by gravity. No, in practice the electrostatic and electromagnetic forces are many many orders of magnitude higher.

[floobydust]

I think with DC filaments you get ion-migration and shorter filament life, just like with a tungsten light bulb.
AC gives a longer life. But open filaments due to old age I've never seen.

For indirectly-heated cathodes, they are insulated from the filament by 'something' that cares about polarity.
This is if you have heater to cathode bias to lower noise.
6X4 datasheet
Heater-Cathode Voltage:
Heater +ve: Total and DC Peak 200V
Heater -ve: Total and DC Peak 450V

[coppercone2]

Also you have gravity inside of the tube. AC reverses direction periodically so it might allow something to fall down to the bottom vs get directed into something else its attracted to?

are you trolling?

Yes, electrons have mass and can be affected by gravity. No, in practice the electrostatic and electromagnetic forces are many many orders of magnitude higher.

I mean if some how a small particle flakes off of something and gets charged. I thought I read about that before with flaking. I think the wear occurs kinda like chipping of concrete, you get some dust but occasionally you get something larger being thrown off. I think this can some how result in electrically charged 'dust'. I thought with DC if that kind of generation occurs it would possibly be thrown more to one particular direction. I don't think the filaments always evaporate ideally into a gas like you think. I noticed in high power equipment like (old) argon lasers you actually end up getting crap that basically rattles around the bottom of the vacuum tube. If you get those kind of particles it means you get everything in between from nanosized particles to what we would consider dust by our standards. As it breaks off I imagine there is small point wehre its not connected to the circuit and there is particles rubbing against each other. I don't know if in that instant you can consider them to be separated by a vacuum (at that point in time in that tiny area) either as its fracturing. 

[vk6zgo]

In the real world, as it operated for many decades, ac operated indirectly heated heaters in valves (tubes) was the overwhelming standard.
They were used in low signal level ampliers with no ill effects.

Occasionally, a piece of test equipment would use DC for the heaters, but, in that case for signals several orders of magnitude lower than any audio or RF equipment.

Portable equipment usually used directly heated filament type cathodes, because they were more efficient emitters, &  the designers were struggling to get the most performance possible for the longest battery life.

One place where indirectly heated tubes were widely used with DC heaters was in car radios.

The use of DC was obviously because it was available at a voltage useable with 6.3v heaters, indirectly heated tubes were used because they offered higher performance, & also because the "DC" from cars is quite noisy

This latter was likely to be injected via the directly heated cathode & lift the noise in the signal passing through the amplifier stage.

One of the standard tests for indirectly heated tubes was "heater to cathode leakage ".
If this was out of spec, tubes were discarded, & replaced with new ones in  Broadcast, Comms & other critical work.

Some, if not many tubes available today are "pulls" from years ago, which have been tidied up, (maybe) tested for emission, put in new boxes & sold.
If these have out of spec' heater/cathode leakage, (which many do), & are put into audio amplifiers, they will almost certainly cause hum.

If the heaters are fed with nice regulated DC, such tubes are probably quite useable, so the urban myth has developed about running tubes with DC heaters.

Lifetime/durability?
With new, in spec tubes, the lifetime with ac heaters was normally many thousands of hours for the less stressed ones, rectifiers & power output tubes had shorter life expectancy.

Car radio tubes seemed to have similar lifetimes to those in domestic radios, which was a bit worse to a lot worse than those used in Broadcasting/Comms.

In conclusion, I would say there is no difference, between ac & DC, all the cathode sees is heat.
Open circuit heaters are fairly rare, most cathode faults are when the cathode material loses emission, or the insulation between heater & cathode fails.

[LaserSteve]

On the arc based lasers,  sputtered material defies gravity and heads for the coldest, most neutral place it can find, the critical mirrors and windows on the end of the tube

As lasers run horizontally , most of the crud ends up above the cathode condensed on the cathode end bell. It buries a considerable amount of gas when it lands.

Subtle design steps are taken to prevent the windows from being fogged, the emission requied being too great to use an indirect cathode with a seperate heater. The arc current returns through a center tap on the secondary of the cathode transformer to balence the ion bombardment on the cathode.

On dc excited broadcast tubes and thyratrons using direct cathodes,   usually the most positive end of a hot cathode is wired as to be toward the plate to minimize ion bombardment.

Steve

[David Hess]

For common indirect heated cathodes there is some reliability advantage to biasing the heater positive with respect to the cathode which prevents electron emission from the heater from damaging the insulation between them.

https://archive.org/details/GettingTheMostOutOfVacuumTubes