[AMRadio] Screen Modulated 813


John Lawson jpl15 at panix.com
Mon Dec 11 11:35:24 EST 2006



On Mon, 11 Dec 2006, Jim Wilhite wrote:

> Yes, but we in the amateur service do not even consider running them at the 
> max.  Red is ok if you take into consideration all parameters and follow 
> design considerations.

   Dave Knepper also writes:

>The reason that the tube is a bit cherry red is to continually remove 
>built-up gasses inside the tube.



    There is a wealth of practical, hands-on - and also theoretical, 
knowledge available on this List - a rarity among reflectors to be sure. 
Jim has provided myself and many others with constant 'good information' 
regarding AM transmitting gear.


   But.... but but but:  Speaking as a design engineer, lifelong tube-geek, 
and (moderately succesful) thermionic designer - I would like to make the 
point that plate incandescance may, or may not, be 'OK'....  depends on 
the device amd the regime it's run in.


    Unless designed for such dissipation, it is most certainly NOT OK to 
run many tubes at a blush.  The venerable 6146 is a case in point - 
running the plates red in a 6146 alters them irreversibly and generally 
kills 'em...  get a 'fresh' one, test it for Gm, run it good and red for a 
while, then re-test.  ooops....

   There are of course tubes designed to run red, even bright yellow - like 
the various radiation-cooled tubes - IIRC 4-65s, etc...  There are some 
tubes, like the 833, that can stand a little color and not get terribly 
upset.


   Running plates red does not, in most cases, re-adsorb gasses - if 
anything it facilitates thier release - depends entirely on the plate 
alloy, thickness, and any coating, element spacing, grid material and 
design, spacer materials, etc., etc..  You can't just make generic 
generalizations like this... the situation is way more complex. Red plates 
also has implications for permanent grid damage - not to mention that fact 
that we want that plate to collect electrons, not emit a bunch of them... 
;}


   And as for "running them to the max" - all tubes have a point of maximum 
'efficiency' where the power transfer function is optimal.  Is the plate 
red at that place under the curves?  If it's a radiation-cooled tube, you 
bet.  If it's a 6146 - it just died. And just who is the "we" you speak 
of? ;} I run my Valiant on the raggedy edge most of the time - and I've 
chewed up one brand new set of finals finding out just where that edge 
is... d'oh!  But now I know just how to get the max out of the transmitter 
and still keep the Output Devices happy - even if I do make 'em sweat good 
and hard...


   Anyway - the study of the theory and design of vacuum tubes is pretty 
damn fascinating to me - and if anyone is interested, I have a fairly 
comprehensive bibliography on the subject that I'd be happy to post, if 
there is any interest.   And seeing as how most of us are using power 
tubes, and building / operating devices using power tubes - might save 
some bucks in the long run.


    Just my 200 millidollar for a Monday Morning...



   Cheers and Best of the Season


John
KB6SCO
DM09fg




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