John,
OK I understand what you are saying. But, how does a graphite 3-500Z
tube get gettered after sitting around? You say heating the anode to
glow does not getter the tube. I realize the graphite tube was gettered
when manufactured. Since glass seals leak, can a graphite 3-500Z be
gettered again down the road or was it a one time deal at the factory?
Lee, w0vt
On 11/26/2016 5:37 PM, John Lyles wrote:
Hello Rob,
I believe that you misunderstand that I am only talking about graphite
anode tubes. Sheet metal anodes with vertical fins, made with
Tantalum, which is what Eimac and some knock-offs used, act as a
getter themselves. The graphite anode 3-500Z originally was the
Amperex/Covimag version. Covimag shut down as a factory just a few
years ago. It appears that China is now copying or licensed to the
same design with horizontal 'fins' to improve radiation area.
I stand by my statement that graphite anodes do not act as a getter.
They have a lot of other desirable properties, as discussed in the
1935 paper I sent. Further discussions are found in chapters
discussing anode materials in the RCA Tube Design bible from 1940 and
the RCA Red Book electron tube bible (1962) which was originally only
issued internally but is now found on the web photocopied. In
addition, the text books by Kohl "Materials and Techniques for
Electron Tubes" 1960 by General Telephone and Electronics discuss
these aspects of tube manufacture. I understand that you do not have
time to read these reference books and reports, but these describe the
standard techniques that all tube manufacturers still follow, as no
real breakthoughs have been discovered in the past 2 decades. The last
big changes made in the 1990s are in the very high power tube arena,
where pyrolytic graphite was developed for grids and multiphase
liquid/vapor cooling was implemented.
There is nothing wrong with running graphite anodes with color, but
vacuum improvement is not the result, only extra output from the
device as well as the extra dissipation internally. There is some
amazement at the antics that some hams will do to enhance their tubes
by running periodic overheat to try and reduce vacuum pressure. With
sheet metal anodes, it is very common to have orange or even red color
in the center of the anode. With graphite, it is merely running them
harder. Many tubes also have tab getters near the filament that are
activated with filament heat, which leads to the specification on
datasheets to warm up the tube for 15-30 minutes before first HV is
applied.
As an amplifier designer professionally, since 1981, for broadcast FM,
industrial RF generators, and particle accelerators, I am somewhat
familiar with tube use and misuse. I will agree with you about the use
of chimneys and lots of air, water, to properly remove the heat from a
tube.
73
John
K5PRO
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