[BC] AM HD power levels

[RichardBJohnson at comcast.net]

Calling some major segment of the broadcast transmitter
industry dumb will not win an argument nor obtain any
brownie-points.

The 833 rigs required about 170 watts of drive. In addition,
the driver needed to be modulated. Furthermore, the
modulator needed a low impedance driver (read high
power) as well. The result was that the overall efficiency
of the 833 rigs, Raytheon, RCA, and Gates, was about
40 percent. The newer designs using 4-400A and later
4-500A tubes had an overall efficiency of about 60
percent. I was involved in the design of those rigs and the
there were two criteria deemed most important:

(1) Fidelity.
(2) Overall efficiency.

The vacuum tubes were not chosen in a vacuum, either. The
basic compliment of the 4-400A modulators, with 4-400A
RF tubes in parallel trace its roots to the Johnson Associates
RBJ/1-C, the first type-accepted transmitter to use these
tubes. The engineer who wrote the book, "Care and feeding
of power-grid tubes," Bill Orr, was involved with the design
from the start. Early tube life was exceptional.

The fact that Eimac "lost the recipe" and was unable to
continue to supply reliable glass tubes 20 years after their
design, does not reflect upon the viability of the design,
only the failure of Eimac.


--
Cheers,
Richard B. Johnson
Read about my book
http://www.LymanSchool.org


 -------------- Original message ----------------------
From: "W2XJ" <rameuser at tmo.blackberry.net>
> Gates/Harris was smart enough to keep the 833 design for 1 KW not the 
> 4-400 crap others dumbly adopted.
> 
>