[Hammarlund] Tube shields and heat

[Bob Camp]

Hi

Chemical rate law phenomenon are one of many things that cause degradation in a device. The rate law is true once you reach an activation energy level. For many of the materials in a tube that occurs in the > 1,000 C range. Yes a lot depends on exactly which reaction you are tracking on each material. 

Cooling the outside of the tube does little for things like the filament. It’s going to be at “glow” temperature regardless of what you do on the outer envelope. The grid to cathode spacing for a tube to work is such that the control grid will pretty much follow the cathode, regardless of what you do to the plate. The only thing you may be cooling with a black shield is the plate and to a lesser degree the suppressor grid (if it’s a pentode).  I have never seen a tube fail for glass envelope degradation due to temperature. Dropping them does not count in this case. Doubling the life of the glass envelope, when it never falls regardless does little to “improve” the tube. Glass to metal seal temperates are a different issue, they are little impacted by a tube shield. 

Since you depend on heat to activate the getter material, and it’s on the tube shell, cooling the shell may actually degrade the reliability if it’s taken to far.

———————————————

Why do most tubes fail?

1) The filament / cathode system runs out of coating / electron radiation capability. It stops glowing, at least in terms of electrons.

2) The envelope fills with gas, but that does not poison the filament. This happens when a glass to metal seal fails. 

3) A weld breaks and the tube just goes dead. Normally this is due to cycling the tube. Welds are a “high activation energy” item. 

4) A grid slumps and it shorts to another element in the tube

At least of the few 10’s of thousands of tubes I’ve seen, number one wins out by far. That one is not impacted by cooling at all. Numbers 2 and 3 depend a lot on what sort of tube it is. They happen more often than number 4 by far. I have yet to see a receiving tube with a hole burned in the plate. Maybe it happens, if it does, it’s rare (so it’s not on the list). 

Cooling the tube isn’t going to impact 1 or 2 (the big ones) at all. It may impact 3 depending on the location of the weld. It certainly would impact 4 on the suppressor (but not the cathode or control grid). Holes burning in the plate are the big one that cooling would help. Since you aren’t addressing the main wear out mechanisms, cooling isn’t worth it. 

Bob

On Oct 4, 2014, at 8:00 PM, Robert Moses <rhmoses at earthlink.net> wrote:

> The rate at which chemical reactions such as those involved in the
> degradation of tubes takes place increases 2.3 times for each 10 degree
> C increase in temperature. Cooler is always better as long as it is warm
> enough to work.
> 
> On 09/27/2014 12:54 PM, Kenneth G. Gordon wrote:
>> On 27 Sep 2014 at 13:31, Guido Santacana wrote:
>> 
>>> As for tube shields, if they are not IERC shields, paint will not do much. 
>> Guido: on the basis of experiment, FLAT black painted tube shields reduces 
>> the heat in the tube by 17%, so yes, it does do something worthwhile. For 
>> instance, reducing the heat of 300 degrees to 249 degrees, or -51 degrees. 
>> 
>> You may not think this is worthwhile, but I do.
>> 
>> According to my reading on the subject, reducing tube heat by even 1 degree 
>> contributes significantly to longer tube life.
>> 
>>> Better to leave the tubes uncovered and use the fan. 
>> True.
>> 
>> Ken W7EKB
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