DHT heaters . and their property"s

[v4lve lover]

i wanted to explain something about directly heated tubes . more exactly their coatings .

as far as i know there are three different main coatings used in DHT filaments

* barium coating (oxide) coating
* barium -strontium (oxide) coating
* thorium- doted tungsten coating .

* barium . this is a coating usually applied in carbonate form ( barium-carbonate) whit some added reagent to make it harden .
these when these are out gassed the barium-carbonate reacts whit the oxygen and forms barium-oxide this is now most commonly used . because it wont kill your workers as fast as strontium does .

* barium -strontium this coating is applied in the same way as barium coating . but is has the advantage of GREATER emission but its not in use anymore because strontium is highly toxic . this was the coating of use for 30s and 40s radio tubes because they can have good emission at low temperatures and heater power .


* thorium- doted tungsten coating .
this is pure tungsten whit the radioactive thorium added . for strong emissions at 2400 degrees Celsius . these are used in high power transmitter tubes because they are resistant to high temperatures .
they also have much higher lifetime . some transmitter tubes reach in the 75K HOUR region .


Ways of applying the coating .

there are three ways of applying the coating .( for oxide coatings . )

* spraying . spray a alcohol solution containing the carbonates and a hardener directly on the filaments .

*pasting . applying the carbonates whit a hardener as a paste .

up to the late 40s they used :
* the azide-process . this is a tricky way of getting very high emissions out of very little heater power . in this process the filaments would be initially coated whit copper . and the plates held barium-azide as the plates where heated red hot the barium would vaporize and react whit the copper layer on the heater forming a very FINE layer of emissive material . the downside is the barium acts as a getter and therefore coats THE WHOLE INSIDE OF THE BULB . and it conducts electricity very well so it will short normal valves . so early valve had all their supports suspended in a glass bead . the coating is also net very evenly distributed so the emission readings will be all over the place . that is why some people find them hard to match

in the next post i will post some examples .

[v4lve lover]

some photo's of AD1's
Die AD1
notice that all tubes that do not have white fillaments (barium as you may remember ) have heavy blackening inside of the bulb some of those examples used the azide process.

a good site on the azide procces .
The Azide Process

wikipedia on hot cathodes
Hot cathode - Wikipedia, the free encyclopedia

any one have any suggestions , remarks . or have i done my homework ?

[jlsem]

The blacking inside the envelope is applied intentionally to absorb stray electrons.

John

[smoking-amp]

I think the intentional blackening applied to the inside of the bulb is some form of carbon coating. While V4lve is talking about the glass darkening after long operation hours from un-intended evaporated material.

There is a also new cathode process, in the last 10 years, that provides generous emission at just above room temperature using a thin film of Titanium oxide over nickel. It's not a point field emitter, the other scheme around, generally using carbon nano-rods.

interesting thought

[dave slagle]

Originally Posted by jlsem

The blacking inside the envelope is applied intentionally to absorb stray electrons.

John

Historically maybe, but today it sure does a nice job hiding the quality of assembly inside. Just think a little black and a reject with a crooked plate suddenly becomes a premium tube. :-)

(ps I'll have a box on it's way to you this week.)

dave

[v4lve lover]

that is wrong . the blackening in azide valves was not intentional . it was a by product of using the plates as a getter ( whit barium azide witch gives a black bulb coating)

here is a nice example of an early azide process valve .
PM2 @ The National Valve Museum notice that this blackening is not to absorb stray electrons . from somewhere like 1935 they used a black coating ...

PX4 @ The National Valve Museum this is another good example of azide valves . now where i was getting at is . that the azide process is terrible when it comes to even performance over time . but its the only way they could operate tubes whit a heater voltage of 2volts and 0.2A note that the PM series had that heater rating not the px4

by the way this is a blackened valve witch was done so on purpose .
KT66 @ The National Valve Museum

please feel free to leave a comment .

[jlsem]

The pictures of the AD1 type vacuum tubes you referenced in your link are coated with carbon on the inside of the envelope.

John

[jlsem]

Originally Posted by dave slagle

Historically maybe, but today it sure does a nice job hiding the quality of assembly inside. Just think a little black and a reject with a crooked plate suddenly becomes a premium tube. :-)

(ps I'll have a box on it's way to you this week.)

Dave

Can't wait, Dave.

John

[v4lve lover]

Originally Posted by jlsem

The pictures of the AD1 type vacuum tubes you referenced in your link are coated with carbon on the inside of the envelope.

John

i would like to correct that . they are coated whit barium azide .

allmost all the px25 made before ww2