Copyright 2020 - John F. Schneider
& Associates, LLC
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The KPO 50 kW transmitter site
in Belmont, California
KPO's 50 kW GE transmitter
Inspecting a Tube for the Big 50 KW KPO Transmitter
KPO's transmitter building, under construction 1932
Motor Generator Room, 1933
The KPO transmitter's cooling pond
What was it like to work at KNBC
in the 1950's? 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 didn't 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, here goes.
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 Angeles.
KNBC TransmitterThe 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
Today we can only speculate on why that old GE transmitter was not replaced. Possible reasons were:
- 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 available.
- Television and FM broadcasting were expected to
replace AM in a few years so it didn't make sense to replace a
transmitter for a short time.
By the 1950's 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
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 1950's 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 1940's 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 good question.
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 problems.
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
KNBC Transmitter EngineersTwo
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
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 1960's 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 employed.
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 1950's 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
announcer's 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.