>There is a family of three ceramic CERMALOX tetrode tubes made by RCA and
>EIMAC and nowadays by BURLE. They are identical inside but cooling is
>different: The 8072 has no cooler. The 8122 is cooled in the same way as the
>4CX250 or a 8874 and needs a squirrel cage blower and mechanical work in
>a MLA 2500. All three tubes have the same socket as the original 8875
>tubes. So no mechanical rework there. The third tube is called the 8121 and
>is cooled in the same way as the 8875s and are very difficult to find.
>However, they may be just right for the application. My idea was to do some
>mechanical work to convert a 8072 into a 8121 and run it in either high mu or
>super cathode grounded grid. Much simpler than using the very cheap Russian
>4CX400 tetrode tubes with their screen grid complications.
>
>How these tubes were manufactured was described in an old CQ magazine issue:
>The grids were cut out by electronic rays in cups immersed in a liquid (
>water?). The G1 and G2 cups were placed on top of each other during the
>carving process and you thus got perfect alignment of the grids. Why do I
>mention the manufacturing process? Well, these ex cups grids can take more
>current than ordinary wire grids and RCA says 100 mA maximum G1 current but
>say nothing about the max G2 current. But as the G2 cup was bigger and the
>G2 is connected via a ring connector that can be attached directly to your
>chassie, one can assume the G2 should be able to withstand even more than
>100 MA grid current.
>
>My idea was to run these tubes in high mu with the grids tied to ground.
>But, in an article using two 8072 ( was it in 73 magazine ?) the author
>mentioned that the G1 has to be connected down from the cathode in order to
>get equal currents in the grids. He used a tuned circuit beetween cathode
>and ground with a tap for the grid. For a multiband amp my idea was that you
>make a new bifilar cathode choke and put the tap there.
>
>As you perhaps know, there are many amps, including MLA 2500s, that are not
>used because of new tube prices. Very high nowadays. This idea of mine, not
>tried yet in MLA 2500s, could eventually put them on the air again. As
>nothing has been published on distorsion tests in this application, the
>field is now open for some experimenting.
>
>The 8122 and 8072s are getting hard to find nowadays I have been told. A
>source of some 50 8072s are avaiable in Scandinavia.
>
>Would be nice to hear if someone is willing to experiment along my ideas.
>There is at least one dead MLA 2500 in Sweden. If the owner is willing to
>sell it to me, I could do the experiment myself.
>
>Nuff said and would be nice if someone takes up my idea.
>
>73s de Hans SM5KI
>
>
>
>
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