[Hammarlund] purpose of tube shields

[Kenneth G. Gordon]

On 25 Sep 2014 at 12:06, Robert Sauvan wrote:

> Hi all,
> Can someone please explain to me what exactly is the purpose of tube 
> shields? Are they for rf shielding?

Primarily so, as I understand it.

> It seems to me that a major 
> complaint about old tube radios is that they run so hot.

Yes. There are several factors at work there, the shiny tube shields being 
only one of those. 

Another being (IMHO) unnecessarily-high operating voltages. Again, in my 
opinion, no receiver needs 250 VDC operating voltages except in the audio 
output stages. 

I run my ARC-5 receivers which normally used 250 VDC as operating 
voltages, at 170 VDC. Heat is noticably less, and operating parameters do 
not appear to change. Some users of the ARC-5 receivers routinely use 
voltages even lower than I use, down to as low as 24 VDC, with the only 
effect being much reduced audio output power, and the receivers take longer 
to warm up. They do have to adjust the screen voltages upwards, however.

> 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!

Oh,yes.

> 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!

A year or more ago, I ran an experiment with those solid, shiny, common 
tube shields in a Heathkit SB-401 transmitter, since I surmised that the shiny 
surfaces were reflecting the heat BACK into the tube.

I had just purchased a DVM which came with a temperature probe.

I first measured the temperature of the tube envelopes by putting the sensor 
down between the tube and the shield so that the sensor was held hard 
against the glass of the tube, and recorded the temperature for several 
tubes.

Then I painted both the insides and the outsides of the respective tube 
shields with FLAT black paint, making sure that the necessary electrical 
contact areas at the bottoms of the shields were bare of paint to ensure 
electrical contact was maintained.

The tubes were, on average, 17% cooler with the black paint applied than 
without it.

When I cannot find, or afford, those special IERC heat-mitigating tube 
shields developed by the military to address this problem, I routinely paint 
any reflective surface near a tube, including the flat shiny aluminum shields 
around final amplifier tubes, with flat black paint.

The tubes and the rigs run very noticeably cooler.

Ken W7EKB