[AMRadio] Screen Modulated 813

Jim Wilhite w5jo at brightok.net
Mon Dec 11 12:24:09 EST 2006

You are most correct John, I was speaking of power tubes not 
the lesser cousins.  I didn't make that clear but in light 
of the subject, I plead not guilty.

----- 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 10:35 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
> DM09fg
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