[Hammarlund] purpose of tube shields

[Robert Sauvan]

Well,
This has been very interesting for sure, but I will have to say, and you 
all know quite well, that I am not an electronics guru by any means 
whatsoever. It seems to me, after reading the numerous replies, why 
would a shield be put over something that needs to dissipate heat 
outwards. That just doesnt really make a whole lot of sense to me. Take 
the HQ-180 I am working on right now. The case around all of the tubes 
is huge in cubic area compared to what is housed in it. Seems to me, the 
tubes have plenty of room to let out steam inside the box and then out 
the top which is completely filled with venting holes. I would think 
because those shields are very tight to most tubes, this would just get 
hotter because of the metal, instead of the tube just venting if you 
were to not have the shield on it. The black paint idea kinda helps the 
heat dissipation argument though. Hummm. I kinda have to lean more 
toward an issue of helping keep rf in check.I remember reading 
somewhere, and I think it was an article about an HX-50, the guy was 
having trouble with some self oscillation issue and someone else pointed 
out that he needed to be sure his tube shields were tight and making 
good contact. So, now that I know both sides of the story, I might just 
see what happens if I leave the tube shields off for a bit and maybe do 
some experimenting with temperatures just for the grins and see what 
happens. But, how can one doubt the technology that has been around for 
50 years and more. We are still playing with these old radios and they 
work purdy darn good I would say!! You guys are great and thanks for the 
info.

73-Bob-W0YBS



On 9/25/2014 1:51 PM, Arthur Delibert wrote:
> I think they were meant for RF shielding, like the metal-envelope tubes one sees on WW II era receivers. (Researchers discovered during the war that enemy forces could locate our forces by detecting low-power emissions from the internal oscillators of superhet receivers.)  I think it's common practice among boat-anchor hobbyists today to use black shields on oscillator tubes to limit their emissions, and to remove the shields from other tubes.
>
>> On Sep 25, 2014, at 2:06 PM, Robert Sauvan <rsauvan at beyondbb.com> wrote:
>>
>> Hi all,
>> Can someone please explain to me what exactly is the purpose of tube shields? Are they for rf shielding? It seems to me that a major complaint about old tube radios is that they run so hot. As I have been working on my various radios, lately of course my HQ-180, I have spent a lot of time trying to swap tubes to find the "best" one for the radio. Or, even one that works. Of course, you let it warm up for a while to see if it works any better than the last tube you had in there, but, man they are hot! When I leave a shield off for testing purposes, the tube cools off really fast. Seems to me if the tube ran cooler, it might last longer. Im just plain curious!
>>
>> Thanks-Bob-W0YBS
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