[BC] Eimac Cavities

[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
Book: http://www.AbominableFirebug.com/

----- 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.