[Hammarlund] : Tube shields and heat

[Kenneth G. Gordon]

On 27 Sep 2014 at 17:24, Bill wrote:

> All this redesign of tube shields, make me wonder why the engineers that
> designed them didn't think of some of these tips...OR they designed them to
> work in the way they do. Bill W2CQ

I suspect that the reasons are several: first of all, I still believe the shields 
were added to prevent radiation, RF, either from an external source (a big 
transmitter in the same room) from effecting certain circuits in the receiver, or 
to prevent RF-radiation from certain circuits in the receiver from causing 
other outside effects.  

The heat problem was not considered, or if it was, it was considered to be 
minor, especially in a military situation in which (supposedly) maintenance 
was ongoing and organized, and tubes were bought by the millions.  

Secondly, much of the information and data on the effects of heat in tubes 
and how it shortens their lives came about AFTER the earlier types of 
shields were designed and implemented.  

As soon as the heat issues became better known, the military had 
heat-reducing shields manufactured and used: i.e., the so-called IERC tube
shields.

After all, there is a pretty constant improvement in knowledge and 
understanding of many things. Heat in tubes is probably one of those.

We come along much later and need some way to reduce the heat in tubes, 
thus increasing their life expectancy, since those tubes are becoming more
difficult to find, since many haven't been made in many years.

And, most of us cannot find, or if we do happen to find, cannot afford, IERC
tube shields.

Therefore, we do what we can to at least mitigate the problem.

I believe it was Eimac who first did some extensive testing of tube-life versus
heat (and filament voltage) and published their findings long after the earlier
tube shields were designed....which were designed for the sole purpose of
cutting down on radiation, RF, effects.

Anyway, that's the way I see it, and it is worth exactly what you paid for it.

Ken W7EKB