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
Publisher of the Collins Journal
Secretary to the Collins Radio Association
www.collinsra.com - the CRA Website
Now with PayPal
CRA Nets: 3.805 Mhz every Monday at 8 PM EST
and 14.253 Mhz every Saturday at 12 Noon EST
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----- Original Message -----
From: "John Lawson" <[EMAIL PROTECTED]>
To: "Discussion of AM Radio in the Amateur Service"
<amradio@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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