A Visit to the KNBC Belmont
AM Transmitter in the 1950s
By Fred Krock
What was it like to work at KNBC in the
1950s? All the people who could tell us are gone now.
The story must be second hand. As an eager young broadcaster I would visit the NBC Belmont transmitter
occasionally to listen to the old timers talk about the good old days. I think we all realized even then that
the golden days of radio were behind us.
I worked at a broadcast transmitter about fifteen minutes drive from the KNBC transmitter.
Sometimes when I didnít feel like going home after signing off the station at midnight, I would go hang
out at the KNBC transmitter for a while. I knew almost all of the San Francisco NBC engineers. We were
members of the same union, NABET. We knew each other from union meetings and from walking the
picket line together.
This is an oral history, with all the shortcomings of human memory. These are my recollections of
conversations which happened over forty years ago. I never dreamed that today I would be writing about
those events. I made no notes. I never checked the accuracy of what I was told. So, with that disclaimer,
The KNBC Transmitter
The original call letters of KNBC were KPO. They were changed to KNBC in 1947. Later they
were changed to KNBR when NBC wanted to use the KNBC call letters for a television station in Los
The 50 kilowatt 680 kHz AM transmitter in Belmont was built for NBC by General Electric
in 1932. It was installed in a building custom-built for this transmitter.
Later NBC made a rule that all equipment used by the company must be made by RCA. To
comply, the KNBC GE transmitter had been converted into an RCA transmitter. It was painted RCA
umber gray (actually a brown color). RCA obtained FCC type approval for this transmitter. RCA supplied
a new name plate, giving an RCA model number which replaced the GE name plate. All traces of the GE
name were obliterated.
That old GE transmitter used a lot of electricity. The 5 kW driver and the 50 kW final stage were
linear amplifiers which were very inefficient. An engineering study in 1952 showed that a new transmitter
would pay for itself in under two years from savings on power bills alone. NBC continued to use the old
transmitter in spite of this showing.
Today we can only speculate on why that old GE transmitter was not replaced. Possible reasons
- NBC was spending almost its entire capital budget on television.
- The ampliphase transmitter was under development by RCA, so NBC was waiting until it was
Television and FM broadcasting were expected to replace AM in a few years so it didnít make
sense to replace a transmitter for a short time.
By the 1950s the GE transmitter final amplifier tubes were used only by NBC owned stations.
Replacements were hand-made by the RCA transmitting tube factory at great expense. These tubes were
reported to last for a long time. I was told that typical tube life was over five years, which was not unusual
for bright tungsten filament tubes.
The water cooled final tubes were so big and heavy that a small crane on wheels was needed to
remove them from their sockets in the transmitter.
KNBC engineers kept accurate records of all tubes in use, even receiving-type tubes used in
audio amplifiers. They calculated the average life for every tube used at the Belmont transmitter. After an
average life figure was determined, tubes were replaced automatically before they reached that age even
though they showed no signs of failure. High power transmitter tubes were run until they showed reduced
emission before they were replaced. These tubes very rarely had sudden total failures.
The transmitter literally was built into the building. The plate transformer, switch gear, water
pumps, and motor generator sets for filament and bias voltages were on the ground floor. A well equipped
machine shop and parking garage also were on the ground floor.
From the front door you walked up a grand staircase to the second floor where the transmitter
was located. The transmitter was arranged in a " U" shape with a control console in the center of the
floor. I never saw anyone sitting there. Operators usually were in an adjoining room with racks of audio
equipment, monitoring equipment, a desk, and log-keeping typewriters. I believe the transmitter also
could be controlled from this desk.
Access to the rear of the transmitter was through interlocked doors on both sides of the
transmitter. Many of the transmitter racks had no rear walls or doors. Once you walked through an access
door, you were inside the transmitter.
Transmitting tubes used at KNBC required DC voltage on the filaments to reduce hum. Motor
generators were the best way to get low-voltage high-current DC for the filaments. I never understood why
bias voltage also came from motor generators because the voltage was not very high and the current was
almost zero. I believe the bias motor generators were replaced with conventional tube rectifiers in the
1950s to reduce maintenance expense of mechanical devices.
The transmitter had two independent sets of motor generators. One motor generator could be out
of service for repairs while the other one was on the air. Even after the bias generators no longer were
used, they were kept in serviceable condition in case of an emergency.
A pond of cooling water was out back. My understanding was that the cooling pond was not
used. The only source of water was a well that gave very brackish water. Distilled water cooled the tube
anodes. Then it went through a heat exchanger or radiator to cool it. The well water was so hard that it
gave serious problems with mineral deposits in the heat exchangers so the air-cooled radiators were used.
Bottled water was supplied for drinking and making coffee.
The original antenna was replaced in the late 1940ís with a Franklin antenna. It lowered the
vertical angle of radiation. This antenna increased ground wave radiation at the expense of skywave
radiation. I was told that with the old antenna KPO could be heard in Hawaii and on the West Coast like a
local station at night. The new antenna reduced distant listening while it improved the nighttime signal in
the far suburbs of the Bay Area.
The "Secret" GE Limiter
KNBC always sounded good on the air. The modulation monitor showed almost no negative
carrier shift with modulation. Stations were limited to 100% positive peak modulation by the FCC in
those days. How that GE transmitter would have performed after the FCC relaxed positive peak limits is a
A new GE audio peak limiter was used on the air. It, too, was repainted RCA umber gray. The
original GE silk-screen lettering had been reproduced painstakingly with decals. A small metal RCA
emblem was screwed to the front panel.
The GE peak limiter was by far the best on the market. It was the only new equipment that the
tightwad owner of the station where I worked ever bought. It cost about three times as much as other
brands. (The RCA sold for about $600.)
Most peak limiters sampled audio level at the output stage, thus
sudden peaks could escape before the level could be turned down. The GE limiter sampled audio level at
the input. Then it put the audio through a delay line before it reached a variable gain stage. Since the
variable gain was adjusted before a peak got there, absolutely no peaks escaped to cause overmodulation. Stations using this limiter sounded louder and cleaner than stations using any other
I never found out how this GE limiter came to be used at KNBC. I heard rumors that all the NBC
owned and operated radio stations were using GE limiters but I never verified this. The disguise was so
well done that it took a broadcast engineer familiar with the device to recognize a GE product. No station
manager or non-technical manager would have been aware of the deception.
Programs were fed from the studio to the transmitter on an 8 kHz leased telephone line. It rarely
gave any trouble. An FM receiver allowed rebroadcasting signals from KNBC-FM in case of phone line
KNBC had a Morse code line between the Belmont transmitter and master control in San
Francisco Radio City. Almost all the transmitter engineers could send and receive code. This Morse line
was very cheap to lease from the telephone company. The reason, I was told, was that AT&T had no
desire to have to go into the telegraph business even though the second "T" in its name stood for
"Telegraph". Thus, AT&T subsidized Western Union to help keep it in business by leasing Morse lines at
very low rates.
At most radio stations, announcers kept the official FCC program log. At KNBC, one of the
transmitter engineers kept the program log. Announcers worked from a program schedule. The official
log was typed by one of the engineers on an old office typewriter. A copy of the program schedule was
available for reference. Any late schedule changes were sent on the Morse line.
If it sounded like a commercial, the transmitter engineer entered it into the log even though it
might not be on the schedule. This helped keep payola and plugola off the air. Announcers had a lot of
explaining to do if what sounded like an unscheduled commercial turned up on a log of their shift.
The Engineering Staff
Two operators were on duty at Belmont twenty-four hours a day, seven days a week. FCC rules
did not allow remote control of a 50 kW AM transmitter at that time.
Work shifts were 8 AM to 4 PM, 4 PM to midnight, and midnight to 8 AM. Operators worked
shifts which rotated weekly. An operator would work five days on the 8 AM shift, then after two days off
he would work on the 4 PM shift and then the following week on the midnight shift. The KNBC-FM
transmitter on San Bruno Mountain had only one operator on duty and only when it was on the air.
Two operators were on duty to minimize down time in case of a transmitter failure. Union rules
and common sense prevented an operator from working inside an interlocked compartment without
another experienced person present. Many repairs on that big old transmitter physically required two
people. In case of equipment failure, repairs could start at once without having to wait for another person
to arrive at the transmitter.
In those days few radio stations had auxiliary or backup transmitters. Operators were expected to
get the station back on the air quickly in case of equipment failure. KNBC did not get a backup
transmitter until some time in the 1960ís when Civil Defense authorities bought a ten kilowatt AM
transmitter for the purpose.
Over half of the Belmont transmitter operators had worked as shipboard radio operators at one
time or another. They had gotten tired of going to sea, so they got jobs at broadcast transmitters. A few of
them would take a leave of absence occasionally and ship out for a trip or two. Most of the rest had
amateur radio operator licenses. One of the operators was a licensed dentist. He preferred working
at a transmitter to looking in mouths, so he had given up his practice.
The transmitter and the building always were immaculate. Floors were polished. The inside of
the transmitter looked as if it had just come from the factory. This may have come from the seagoing
heritage of many of the operators.
Old timers did not refer to their employer as NBC. It was always "THE NBC". They would say
something like, "I have worked for THE NBC eighteen years now."
Most of the transmitter operators had high seniority, so were not affected by the NBC radio
engineering layoffs which began in 1952. Until the NBC Pacific Network was shut down in 1952,
fifty-two men were employed in San Francisco by NBC Radio as engineers. After that, it was all downhill.
When the station moved out of San Francisco Radio City in 1967, only twenty-three engineers were
Some low seniority San Francisco NBC engineers were allowed to transfer to Los Angeles where
NBC was hiring engineers for television. Others transferred to the NBC international short-wave station
at Dixon, California, about seventy miles northeast of San Francisco, where they bumped even lower
seniority men. The Dixon station was on the same seniority list as San Francisco. Dixon engineers always
complained about having to drive to San Francisco for union meetings.
The short-wave station was leased to the Voice of America, but operated by NBC. Later it was
sold to VOA, and the former NBC engineers there became civil servants.
In the 1950s KNBC broadcast a one kHz tone at about 30% modulation level as part of the Conelrad Civil Defense alerting system between about 2 AM and 5 AM, while no regular programs were
broadcast. Every half hour, a transmitter operator would play a transcribed station identification. Special
dispensation had been obtained from AFTRA, the announcerís union, to allow the station to broadcast
without a staff announcer on duty.
Today, radio transmitters are operated by remote control from the studio by announcers, or are
operated automatically. Studio engineers are unknown. One or two engineers are responsible for
maintaining as many as four or five different radio stations, all owned by the same company. The
broadcast industry is very different from the days when two men were on duty twenty-four hours a day at
the KNBC transmitter in Belmont.
© 1998 Fred Krock. All rights reserved. Reprinted with permission.