[Amps] re: baking out tubes

[R.Measures]

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.
>
> 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.  
The normal flow of electrons is away from the cathode.

> The direction of electron flow is determined by the electric field 
> direction. And secondly why does it emit electrons at all?

Because that is what BaO does when heated, which is why it's used on 
the cathode.

>  If gold poisons the cathode and makes its emission drop why does  the 
> barium on the gold emit electrons?

Because the BaO is on the surface and the gold not.  However, in a gold 
sputtered tube, the emission poisoning gold is on the surface of the 
BaO.

> 1. does not gold poison electron emitters?

Only if it's on the surface.

> 2. The temperature of the gold grid should be much less than the 
> cathode.

Agreed.

> Also, the amount of surface area on the grid that has been covered by 
> the cathode material is very, very, mall compared to that of the 
> cathode.

Quite true, however, in g-g, with an old tube that has been operated 
with high heater V, one can still see a steady decrease in anode 
current for around 10-seconds with maximum-signal NØN emission.

cheers, Bill
>
> I have to admit I have not used 8877s and prefer big glass tubes. I 
> love to see the anodes glow. But I have had lots of 4-125, 4-250, 
> 4-400 and 4-1000 tubes and found few that were actually gassy. The 
> only exception has been some new Triton tubes and  Penta Lab tubes. 
> Even the 1960s GE, RCA and EIMAC 4-1000 tubes that I have had have not 
> been gassy. And the place to get 4-1000 and 3-1000 tubes is from 
> France now. The AMPEREX (Covimag) ones are great. But you may want to 
> allow longer blower cool down time due to the fact that the massive 
> graphite anode takes a while to cool down.
>
> 73
> Bill wa4lav
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org