[Hammarlund] : Tube shields and heat

[Bob Camp]

Hi

The impact of temperature on component reliability was well known in the 1930’s (and probably long before that). The same is true of things like filament voltage. The other thing that was well known is that the seal between the pins and the package is the weak point for most tubes. Magic shields don’t fix that. That (among other reasons) is why black shields were a novelty right up to the end of the tube era. 

Now, all of that is specific to small signal / low power tubes like you find in these receivers. Transmitting tubes are a different beast.

Bob


On Sep 27, 2014, at 6:19 PM, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:

> 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
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