[Hammarlund] purpose of tube shields

[Bob Camp]

Hi

The only thing I would disagree with is the “tubes were cheap” part. On an inflation adjusted basis, tubes were quite expensive. If you trained as a circuit designer in the tube era, the first dictate was “minimize active stages”. Unless you were making a cost is no object design (think HP), you used as few tubes as possible. Yes, that was in part to reduce the transformer size, but next up was the cost of the tube. 

Back when these radios were new, I don’t remember any amateurs swapping out tubes as part of a PM process. You could not afford it. Even on stuff like tube computers, the standard drill was to run them all past a tester and swap what read as marginal.  There are a lot of people who used to tell stories about that being their first “computer job”. Possibly in remote site gear where access was difficult and failure was a big deal, swap outs were an option. Tube repeaters would fit that bill, I don’t remember any of the shops I worked for doing PM swap outs on them though. I *know* who would have been the lucky one to go do it (the new kid). Maybe they just didn’t have the high priced clients ...

Bob

On Sep 29, 2014, at 11:11 AM, Todd, KA1KAQ <ka1kaq at gmail.com> wrote:

> On Fri, Sep 26, 2014 at 12:16 PM, Les Locklear <leslocklear at hotmail.com>
> wrote:
> 
>> Removing the tube shields completely cools even better. I have IERC
>> shields on a couple of receivers,
>> the others have none. Unless you are transmitting close by and don't want
>> to spring for the pricey IERC shields, simply remove them.
>> 
> 
> Les is right - removing the shields except in 'shield-critical'
> applications is the best bang for the buck, for sure. And as Pete says, in
> sporadic amateur use it's not an issue anyway. Even line voltage shouldn't
> be a problem when you consider the +/- spec built into most gear. Though
> reducing higher line voltage is a good thing, especially considering old
> components like transformers.
> 
> As others have stated, more often than not in later gear (particularly
> military), the locking shields were used more to keep tubes in place. In
> early radio gear proper shielding was a much bigger issue than decades
> later.
> 
> The late Bill Kleronomos/KD0HG did a pretty extensive review of heat
> dissipation for tubes with and without shields of different types. IIRC, it
> was published in Electric Radio Oct '94 and shows that the IERC shields
> actually work better than a bare bulb. No doubt due to their heatsink
> design. No other tube shields do. He tested all types, including the Elco
> and Collins types as I recall along with shiny and black painted. My
> recollection is that painting them black is only a slight improvement over
> a bare shield.
> 
> We need to keep in mind that tubes were cheap and plentiful then,
> everything used them. So if their life expectancy was cut by 25% or more,
> so what? Swapping out tubes before failure as part of a PM plan was
> commonplace. Many a ham has benefited from broadcast transmitter pulls.
> 
> If you look around online you can likely find Bill's article and the
> columns of data are clear.
> 
> 
> ~ Todd,  KA1KAQ/4
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