[Amps] re: baking out tubes

[R.Measures]

On Oct 27, 2004, at 2:10 AM, Steve Thompson wrote:

> On Tuesday 26 October 2004 21:30, R. Measures wrote:
>> On Oct 26, 2004, at 7:26 AM, Bill Fuqua wrote:
>>> At 03:41 AM 10/26/2004 -0700, R.Measures wrote:
>>>> Thus, when a higher-mileage grid becomes warm from the flow of 
>>>> normal
>>>> grid current, the barium coating on it begins emitting electrons in
>>>> the Wrong stinkin' direction - and the anode-current starts
>>>> decreasing.  The barium that sticks to a gold-plated grid of an 8877
>>>> can apparently be partly dislodged by vertically and firmly  tapping
>>>> the top of the tube with a 4oz hammer -- which is the same procedure
>>>> that is used to dislodge gold meltballs from the cathode and from 
>>>> the
>>>> anode insulator.
>
> My thought was that the cathode material is more likely to be very 
> fine films,
> maybe even closer to atomic level deposits,

Directly-heated cathodes (thoriated tungsten filaments) have an atomic 
level (crystal lattice) epitaxially-bonded surface layer of ditungsten 
carbide (I was wrong about it being called "tungsten dicarbide") which 
does c. 99.5% of the electron-emitting.  This layer will stay bonded to 
the cathode with anode PS potentials of up to c. 22,000V.   .   
Indirectly-heated (metal oxide) cathodes have a Ni cylinder that is 
initially painted with barium carbonate and strontium carbonate.  When 
heated, the carbonates lose their carbon atoms and change to BaO and 
SrO.  The result is a cathode which can emit more electrons per watt of 
cathode heater/filament power, but the trade-off is that the anode 
potential limit is c. 25% as what it is for thoriated tungsten 
filaments.  For example, an 8877's cathode consumes c. 50w of heater 
power and it will produce c. 1.2A of average emission before it begins 
to suffer from BaO/SrO flaking off.  However, to get the same level of 
emission from a thoriated tungsten cathode, roughly 200w of cathode 
heat is required.   On the other hand, an 8877's cathode can withstand 
c. 5000V before it gets onto thin ice, but thoriated tungsten can 
withstand 22,000V, however, 22kV is X-radiation territory, which most  
definitely isn't nice.

> formed from low level long term
> evaporation rather than significant particles. In what you describe, I 
> reckon
> the cathode material will have been evaporated by whatever caused the 
> metal
> to melt, and it will have condensed into the metal balls as they 
> formed.

According to the engineers that originally developed the 8877, gold 
evaporation occurs only from the surface of the gold plating on the 
grid, and it happens during an "oscillation condition" (probably UHF). 
- - Ref:
http://www.somis.org/FooteL.GIF
http://www.somis.org/8877-gs2.JPEG
Although both maladies tend to reduce emission, gold evaporation from 
the surface of the grid is not related to BaO evaporation from the 
cathode.  The way to tell one from the other is that BaO on the grid 
causes a slow decrease in emission during the first 10 or so seconds of 
an NØN test, and loss of emission from gold migration to the cathode 
does not.

> a
>>>
>>> I am confused, how does the barium emit current in the wrong 
>>> direction?
>>
>> The side of the grid closest to the cathode receives most of the 
>> barium
>> oxide (BaO).   When heated, BaO emits electrons toward the cathode.
>
> Is this likely to overcome the force of any applied voltage?

Beats me, Steve.  The bottom-line is that it's probably not good 
engineering practice to run an 8877 heater at 6 or so volts -- like the 
Dentron DTR-2000 does.

cheerz
>
> Steve
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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org