I had lot of trouble with the Eimac 3-500 in the mid 1980’s. I was operating a
commercial ship to shore coastal station using a Scientific Radio 500KHz
transmitter which used them.
As Rob described, the plate structure would tilt under heat and short to the
grid. Eimac replaced them several times under warranty but I gave up on them
and went to Amperex carbon plate tubes. I suspect that these failures were part
of the reason why Eimac stopped making glass tubes.
Now with declining sales, Eimac has raised prices so much that their vacuum
tube business must be spiraling downward.
Their extremely high prices are hastening the end. The final broadcast holdouts
using vacuum tubes in their transmitters have dumped the older units and gone
solid state in the last decade. There are one or two transmitter makers still
offering an FM transmitter with a tube final such as the 3CX5000 and perhaps
the 4CX35,000 in 50Kw units but I doubt they sell many units.
One of the first makers of all solid state broadcast transmitters was Nautel of
Canada. I was in awe of their 50KW output all solid state AM broadcast
transmitter introduced around 1978. It was the beginning of the death knell for
empty state transmitting devices.
Doug/WA1TUT
> On Jan 24, 2022, at 10:28 AM, Rob Atkinson <ranchorobbo@gmail.com> wrote:
>
>
>>
>> My last suggestion is to buy an Eimac tube. They are far superior to the
>> Chinese tubes in my humble opinion.
>
> Eimac blew it a couple times I know of. The Utah plant made 3-500s
> that had graphite anodes spot welded to a cone that attached to the
> vertical rod passing through the glass seal. Not sure what the cone
> metal was, but those spot welds would fail if the anode got hot enough
> which resulted in the anode hanging down and shorting to the grid.
> Another problem I heard about but am vague on the details, had to do
> with the gas used in Utah to deposit carbon on anodes. The tube
> performance was inferior to those produced in California and at first
> no one knew why. I don't remember what it was but eventually they
> determined the gas in Utah had some property that was undesirable. I
> don't know about the external anode tubes but most tube manufacturers
> have committed mistakes on occasion. A good tube is a really hard
> thing to make.
>
> 73
> Rob
> K5UJ
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