[R-390] Wanted to Trade

[odyslim at comcast.net]

 Good Point Bob,

 Now I wonder which should have black shields. I have noticed
some tubes in the radio get hotter than others. I do know for a 
fact the 26Z5's need them. The 3TF7 needs one. I have touched
some in the IF that could cook an egg.

 I also know if tubes cool off to quickly they will fail. 


 Scott W3CV

-------------- Original message -------------- 

> Hi 
> 
> You could always liquid cool the tubes. That would keep the glass and 
> seals at a nice constant temperature. 
> 
> There is reason to believe that reducing the temperature of the glass 
> to metal seals at the base of the tube has an affect on the long term 
> performance of the seals. If you have tubes mainly failing due to gas 
> then this is something to be concerned about. The tube socket 
> probably heat sinks this end of the tube pretty well. 
> 
> At the temperatures you find in a receiving tube the temperature of 
> the glass envelope by it's self doesn't have much of an affect. If 
> you get the temperature up a lot higher the envelope will have some 
> problems. If we start to see tubes collapsing in use then this is one 
> to worry about. I have seen pictures of transmitting tubes that have 
> failed this way. 
> 
> The real question is what makes receiving tubes fail. Transmitting 
> tubes have been studied quite a bit. Audio power tubes and rectifiers 
> have been studied to a lesser extent. Low power / small signal tubes 
> have only been studied in a fairly cursory way. You can look at it as 
> tubes are tubes, but we don't stress them all the same way. Plates 
> and grids in receiving tubes simply do not heat up the same way they 
> do in power tubes. Filament power is the main source of heat in a 
> normal receiving tube. 
> 
> Cooling tubes down helps reduce the temperature of things like plates 
> and grids. Cooling also helps seals. I doubt that external cooling 
> has any big affect on the filament. Most receiving tubes I have seen 
> go bad die from filament related issues. Some simply do the light 
> bulb thing and stop glowing. Others get to the point that cathode 
> emission drops below the level needed to keep working. Certainly 
> things like shorted grids do occur, but they are not very common. 
> 
> If what I swap out here is any indication emission is the main (> 
> 80%) issue for dead receiving tubes. Open filaments make up almost 
> all of the rest. I can probably count the number of receiving tubes I 
> have seen fail for other issues without taking off my shoes. 
> 
> Black tube shields look cool. That alone is a good reason to use 
> them. They definitely cool down the tube (though the radio is just as 
> hot). Cooler is always better. They sometimes are easier to get on 
> and off - never a bad thing. Given the way we use the radios these 
> days I would not put them in the same category as the capacitors. 
> They are a nice thing to swap out, and the radio is better with black 
> shields. I don't think the radios are enough better with black 
> shields to run out and spend hundreds of dollars re-shielding all our 
> radios to make them work better. 
> 
> Take Care 
> 
> Bob Camp 
> KB8TQ 
> 
> 
> 
> 
> 
> On May 4, 2005, at 11:08 PM, Flowertime01 at wmconnect.com wrote: 
> 
> > Scott, 
> > 
> > You may not need that many shields. 
> > Recommendations were to shield the RF, 1st, 2nd, 3rd mixers, VFO, BFO. 
> > Just the RF mixers and Osc need shields. Recommendations were to 
> > uncover 
> > every thing else. 
> > 
> > We know the good shields will run cooler than the plain cans. 
> > I do not know if the good shields will actually get you a cooler 
> > glass bulb 
> > temperature than no shield with ventilation being the same. 
> > So it still may be best to run most of the tubes naked. 
> > 
> > Just because we used those crummy old shields is not reason to 
> > continue. 
> > We put in better caps today, so why not better shields. 
> > 
> > Round up as many as you can. One never has enough spare parts. 
> > 
> > Roger KC6TRU 
> > 
> > 
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