Kit a Knight
in Shining Armor
by Jack Cheese
(reprinted from Radio World, April 1, 1987)
Back in 1964, when it was still profitable to
operate a local AM daytimer, KHGL signed on the air in Pasadena, CA.
The station operated on 860 kHz and covered the city of license
well using an end-fed long wire antenna.
But even in the heyday of AM radio, dollars were
tight, and the station's construction budget had to be watched
For this and various technical reasons, the
transmitter we chose for the new KHGL was a model manufactured by
Knight Electronics. The Knight transmitter (or Knight-Kit, to be
accurate) was ideal for our application.
The transmitter was compact, taking up only l/2
square foot of floor space, and could be powered by 115 volts AC or
DC, single phase. There was no need to install three-phase AC service.
It used only three tubes and didn't have
any unusual cooling requirements. In addition, the Knight
transmitter included a built-in turntable and microphone preamplifier,
modulation level control and an audio output for monitoring program
modulation with a conventional loudspeaker.
Even with a relatively low output power of 100 mW,
the Knight AM transmitter was rather cost-effective with a price tag
of under $12 (plus shipping via UPS).
There was only one catch: As it's name implied, it
was a transmitter kit; the buyer had to build it.
The Knight unit was assembled using point-to-point
wiring, 1964 was too early for PC board technology.
Assembling the Knight-Kit transmitter was
straightforward, thanks to a well written and illustrated manual. The
process took about two days.
The transmitter design was conventional, employing
three tubes: two type 50C5 beam power pentodes and one 12AX7A dual
low-noise triode. One of the 50C5 tubes was the oscillator/RF power
The carrier was generated using a free-running
oscillator, the frequency of which was determined by a variable
capacitor in the “tank” circuit. The operating frequency was
adjustable over a range of 530 to 1610 kHz.
The RF output was taken from the plate circuit of
this same tube, and coupled to the antenna with a broadband output
circuit. There was no need for plate tuning or loading adjustments;
the output section was broad enough to permit adequate efficiency on
the entire AM band.
The RF oscillator/PA tube was plate modulated by
the other 50C5, the modulator. The modulator circuit was also
conventional, except that the modulation transformer primary was wired
to the plate of the PA, and its secondary was therefore available for
connection to an 8 ohm speaker. This provided a convenient means
of monitoring the modulating signal and eliminated the need for a
separate mod monitor.
The most unique aspect of the Knight Kit
transmitter was the inclusion of an RlAA-equalized magnetic turntable
Never since have I seen any transmitter that
actually had an RCA jack on it labeled "mag-phono input. The
12AX7A tube was the phono preamp, and would provide more than adequate
modulation level when used with the recommended GE VRII cartridge.
A ceramic microphone was also provided, and would
work when plugged into the "phono" input, though the RIAA EQ
created somewhat exaggerated bass response.
When the Knight transmitter was first powered up,
there was an unusually bright momentary flash from the filaments of
the 12AX7A tube. We determined this was because the 12AX7A did
not have an 11 second controlled warm-up as did the 50C5's, and this
was normal. (The tubes' filaments were powered directly
from the 115 volt AC line.)
When all tubes reached their operating condition,
full RF output was realized. The transmitter was operating perfectly,
though off frequency. The tuning capacitor was adjusted (with
full RF output) until the correct frequency was obtained, as noted on
a nearby RCA Victor AM receiver.
Before regular programming could begin it was
necessary to run a Proof of Performance. Frequency response was tested
using an audio generator, and confirmed expected response from 100 Hz
to 8 kHz, being down 10 dB at 50 Hz and 11.2 kHz. Distortion was
also checked ... it averaged about 5% throughout the pass band, rising
to 10% at 100 Hz.
The lack of low-frequency response and excessive
LF distortion was evidently caused by the limitations of the minute
Noise performance was a bit disappointing. The
SN ratio was only 30 dB at best, referenced to 100% modulation. Most
of the noise was low frequency hum; reversing the AC line cord in the
socket helped only a few decibels. Even shorting the audio input
had little effect. I suspected an AC ground loop in the chassis ground
Since the Knight transmitter would operate from AC
or DC, we actually connected 110 V worth of batteries to the unit and
powered it from pure DC. The hum remained. I could only assume
that there was RF pickup somewhere in the audio circuitry causing the
problem. Other than that, the audio performance was respectable.
Modulation was adjusted via a front panel knob
(violet knob with white dot) to a maximum of about 85%.
Connecting a speaker to the audio monitor output
lowered this to 70%, evidently due to the limited power output
capability of the 50C5 modulator tube
After the performance tests were complete, KGHL's
regular programming began in the summer of 1964. It was very hot, yet
the Knight-Kit transmitter performed flawlessly even with no cooling.
Frequency stability was good, with less than 50 kHz of drift after a
30 minute warm up period.
Only after three years of constant use did a
problem develop: the selenium rectifier stack failed, causing a loss
of plate HV, and producing an overwhelming odor in the control room.
Repairs were made in a few hours, and the
Knight-Kit transmitter has been on the air ever since. A few rust
spots have appeared on its once-gleaming blue chassis, but the
transmitter has been reliable for over 20 years.
Fly by Knight
Unfortunately, Knight Electronics has been out of
business for several years, probably due to stiff competition from
"the big boys."
There were several hundred Knight AM broadcast
transmitters made in the '60s, some of which are still on the air
today. They are an excellent choice for many AM daytimers,
especially those with low-power pre-sunrise operating authority.
Though a used Knight transmitter will command a
high price, usually well over five dollars, checking RW's used
equipment listing will be worth the effort if you find one of these
fine works of engineering expertise.
At KHGL, we wouldn't have anything else. As the
saying goes ... "'It' keep station profits high as a kite, you
must be on the air, Knight after Knight!"
Jack Cheese is CE of KCHZ Powercheese Pario
(formerly KHGL) and surfaces once a year on April Fools Day. His alter
ego, Hank Landsberg is president of Henry Engineering and can be
reached at http://www.henryeng.com/
and does not take kindly to the label "fool."