|[AMRadio] Screen Modulated 813|
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
> 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
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