[BC] Eimac Cavities

[John Lyles]

At work we had a number of the Eimac CV2226 cavities that ran 44-55 MHz with the 4CX3500A tetrode. There were used in a now extinct particle accelerator. I grabbed a couple to play with before the rest got scrapped few years ago. 
As designer of the first commercial broadcast cavity amplifier using this tetrode (after Eimac) at BE, I had a personal interest in seeing what Eimac made. I think Mr. Sutherland probably had his hands in the design. Its not a bad amplifier
but as Richard suggested, it had a few pieces that weren't really ready for 24/7 CW operation for years. But a good starting point for a manufacturer to test a tube in. 

Corrections:
Thompson is now Thales and remains in France. They make some very good cavity amplifiers, which are used in many industries including broadcast. I use one now, a very sturdy design. RCA/Thompson is a different company, they got the RCA name 
and made VCRs and other consumer stuff in the midwest. RCA tubes is not "long gone". I believe I have seen their booth (small) at NAB a few years ago, as Burle Industries. The Lancaster plant still makes many of the same tubes and also builds 
cavity amplifiers of old style or custom new designs. They just made a new one for high power military work. I am running 5 of their Y1068C cavities at 150 kW peak at 200 MHz. They use the 4616 tetrode, one of the neatest tetrodes made. 
It is run common cathode, and has extremely high gain without neutralization. It was developed in 1958 from their 6448 used in early RCA UHF television amplifiers. Tom Yingst, who has been writing the Quincy series of articles on Barry's new site, 
was at RCA at the time of this tube development. About 20 years ago the company was sold to Mr. Burlefinger and Carl Rintz, former RCA executives there, who ran it for over a decade. Now it has been bought by Photonis of France but it is still called Burle Industries. 

Question: 
How can a plate blocker dielectric be failing from RF current. Dielectric heating would occur in an intense E field but not a magnetic field. <= =>If the capacitor were not made with enough C, so that it had appreciable reactance at the fundamental frequency, 
then I could imagine a voltage appearing across it. Is this what you mean? This would imply that it was underdesigned however. 

John Lyles

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QUOTED MESSAGES:
Message: 14
From: RichardBJohnson at comcast.net

Both RCA (long gone, now Thompson) and Eimac made "evaluation cavities" which for the most part were used to show the capabilities of their vacuum tubes. Some experimental setups at colleges used these cavities to produce RF power. 
Princeton Plasma Physics Lab (PPPL) comes to mind. They are often not quite suitable for broadcast transmitter use because many are constructed in very expensive ways with components that were not designed to be replaced.
When I worked at McMartin, Industries, RCA came out with a high-gain tetrode that would produce 70 kilowatts of RF in the FM band. This allowed for the design of a single tube 70-kilowatt transmitter, something that the marketing
 department thought would be a good seller. I got one of those cavities and built a transmitter around it.

The major problem with its design was that the plate-blocker capacitor, the thing that isolates DC from RF was at the "cold" (for RF) end of the plate line. This meant that there was an extremely high RF current flowing through it. 
There were no dielectrics that could take that RF current for long. We tried everything available from ruby mica to FEP Teflon. The only solution was a redesign that moved the plate-blocker to the tube-end of the plate line like everyone 
else who used foreshortened tuned lines (not everyone).

Anyway, we did Type-Accept and sell the transmitter. I expect that product died because the single expensive tube was soon unavailable as RCA closed their Lancaster, PA, plant.

Cheers,
Richard B. Johnson

----- Original Message -----
From: "Bob Barnswatts" <AMFAN at collins21e.com>

Speaking of Eimac cavities, I've seen these and read specs on them for
years and years but never ever saw one in a transmitter, homebuilt or
commercial. <= =>Where do these cavities end up? If I recall a 10kw cavity
from Eimac cost more than an Entire 10kW transmitter from Harris.