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Re: [Amps] SB-220 update and gettering

Subject: Re: [Amps] SB-220 update and gettering
From: Lee <>
Date: Sat, 26 Nov 2016 18:15:48 -0600
List-post: <">>

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.


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