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 carefully.
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 amplifier.
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 pre-amplifier.
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 modulation transformer.
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 connections.
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, usual]y 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!"