I saw this happen in an old Gates 1 KW MW transmitter, in which the 807
driver stage was direct coupled to the 833 final, and the driver cathode
current in turn, set up the final current (or it may have been in the
modulator stage, my memory fails here). It lost bias for the 807 (or maybe
that tube quit). No interlock protected the 833 triode, so it lit up like
an incandescent lamp and shortly sucked a hole into the glass and went up
to air. It looked like what you described. Melted. It was from a MAJOR
over dissipation in the plate of that jug.
John
K5PRO
>From: "Doug Person" <w4dxv@ibm.net>
>Well, I opened up the AL-80A to see what had happened. I was stunned. I
>expected to find a fried plate RF choke as Vic suggested. Instead, when I
>removed the 3-500z, I first noticed the top inside surface of the glass had
>a powdery substance on it in pattern not unlike the fins. Then I discovery
>the really interesting new characteristic of the tube. There was a hole
>clear through the glass as though it had been shot by a high-powered laser.
>There are fine cracks around the hole and the glass had actually flowed
>inward almost half an inch. Wow. Watch out for 17 meters on this baby. I
>guess I'll replace the choke before I put the new tube in. I tried to order
>a Taylor 3-500zg from RF Parts. They were back-ordered, so I accepted their
>RFP Special which I suppose is their Chinese version.
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