[AMRadio] Screen Modulated 813

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Mon Dec 11 12:24:19 EST 2006

Most 813's had graphite plates, and I never remember
them showing color, I always thought that would be way beyond
what they were good for, or was good for them...if they were glowing!

811/812 can show some color, but I don't think its GOOD
for them to do so.
6146, 6l6, 4D32, and others of their ilk don't normally show color,
4-65/125/250/400/1000 can, and its ok.
So can the TH and TL series, they can run almost white hot plates.

The harder you run them the shorter they last, and have less room
for mistakes (we don't make THOSE here).

While I don't often run tubes at the maximum plate
dissipation, I do greatly exceed the plate voltage ratings,
I don't know where they came up with the ratings, but you
seem to be able to double it without problems on most tubes.
I have been running a pair of 4D32's at 1200 volts under modulation,
without problems, they are rated for 600 volts in 
plate modulated AM service...

> 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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