[AMRadio] Screen Modulated 813


david knepper collinsradio at adelphia.net
Mon Dec 11 11:53:42 EST 2006


John, you better check your copy of  Eimac's classic on the Care and Feeding 
of Power Amplifier Tubes.

Of course, running a 6146 red hot is not acceptable.  I was referring to 
large transmitting tubes like the 4-400A's, etc.


Dave, W3ST
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----- Original Message ----- 
From: "John Lawson" <jpl15 at panix.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Monday, December 11, 2006 11:35 AM
Subject: Re: [AMRadio] Screen Modulated 813


>
>
> 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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