The Allied Knight Kit T-60 Transmitter

As a kid I really wanted one of these.  I had the schematic, and I felt I could construct one from scrap TV parts, rather than buying the "expensive" kit.  Look, the T-60 used a 6DQ6, a B&W TV horizontal output tube.  I could have went that route, but I gave up on learning Morse code.  I did get on the air, but with no one to talk to, as I "bootlegged" on the broadcast band, a story in itself.  Look for an installment in my "About Me" page.

This transmitter had a lot of novice and "poor boy" appeal.  The price was right.  Although Knight kits were "neat", in retrospect, Heathkit designed perhaps better, and definitely sturdier gear.  I think Knight had the edge in catalog appeal, at least to the youngsters.  The T-60 had a companion receiver kit, the R-55.

The T-60 Schematic

As you can see, the T-60 is a three stage transmitter, Oscillator, Multiplier, Final (Power Amp).  The 6DQ6 is commonly categorized a "sweep tube" in the CB and Ham community, but being an old TV repairman myself, I'll call it what it is, a horizontal output tube, for black and white TV.  It provided the power to sweep the beam from the left to the right of the picture tube, by creating current in the deflection coils.  It also had another important function, and that was to create a pulse during retrace time, or the time when the beam would fly back to the left side of the screen to start another trace.  The "flyback transformer", was in the plate circuit of the horizontal output tube, it's primary drove the deflection yoke coils, and it's high voltage, doughnut style secondary coil, created typically 20,000 volts for the picture tube second anode shell.  Stop me before I tell you anymore.

In fact, all of the tubes in this transmitter were TV types with the possible exception of the 12AX7 which was primarily used for audio.  The  6HF8 was used in sync, video, and sound circuits.  The 6DR7 modulator, was not too common, but was designed as a vertical output tube for small sets.  The triodes are unequal types, the left section designed as a vertical oscillator and the other section as a vertical amplifier.  The dissipation of the tube sections are 1 watt and 7 watts respectively.

As can be seen from the ad, this rig required three different crystal ranges to cover all of it's band capability.  In the  80 meter band position, the crystal, or vfo range of 3.5 to 3.99 MHz tuned up each stage to that fundamental frequency, or "straight through" as they used to say.  The T-60 used ganged bandswitching and just two tuning controls, drive and plate, so it was simple to operate.  Note that the oscillator is capacitively coupled to the driver / multiplier section, so there was no tuning to get you on the wrong harmonic there.

40 meters worked straight through also, using 7 meg crystals, but in the 20 meter position the second stage "doubled" that is it's plate circuit was tuned to the second harmonic, or 14 MHz.  (Forgive me if I'm boring you old timers, but the younger generation may have never heard this)  Likewise on the 15 meter band, the second stage, or multiplier, "tripled" , that is you tuned its plate circuit to the third crystal harmonic, 7 X 3 = 21 MHz.  Same for 10 meters, 7 X 4 = 28 MHz.  

The criteria for designing an RF multiplier stage is to make it hot, dirty, or distorted.  You actually want to "flat top" the output signal.  Let me take that back, not total flat top, you don't need a square wave, but you do need to bias into the region of grid current, any portion of the cycle that grid current flows causes a dip in the plate voltage waveform.  This creates harmonic distortion.  For this application it is not a bad thing, because it allows the final grid to be tuned to the second third and fourth harmonics using only 7 meg crystals.  Look at what they are doing to that poor little 6HF8.  Nearly full B+ on the plate and 160 volts on the screen doesn't make for a "linear" amplifier.

It may have dawned on you that there is a reason for the for the ham bands to be harmonically related.  It's easier to build rigs and antennas that way.  Isn't that sweet.  Someone was thinking.  Prior to television and VHF aircraft signals, hams had a 5 meter and 2.5 meter band, that were reallocated as the 6 and two meter bands.  The 1 1/4 meter band (220) is still there in part.  There were minor pitfalls to this arrangement.  Some rigs like the Johnson Viking transmitter, had a dozen knobs, two or more bandswitches, multiple stages each with a tuning control.  Knobs were labeled with loosely thrown around terminology, like buffer, coupling, drive, grid, multiplier, etc.  If you didn't know exactly how to tune one up, you could be putting out an equal amount of power on two bands simultaneously, and this happened, a lot.  It gave something for the OO's to do, but better to get a notice from an official observer than from the FCC.

You will notice this rig also has 6 meters on it.  They were trying to capture the technician class market, that would use the then popular 6 meter AM mode.  This really wasn't designed as a 6 meter rig.  The terrible thing was they doubled in the final stage.  All you have is the pi output network.  I don't know what the spectral purity requirements were at the time, but with no further filtering, it must have put out a pretty good signal in the 25 MHz range also.  I don't know what radio services are in that band, but maybe it didn't bother anything.  You used 8 meg crystals for 6 mtr.  so the rig worked as a tripler / doubler.  Do the math backwards, 50/2 = 25.   25/3 =8.333 MHz.

The AM modulator for the T-60 is almost identical to the T-150.  It used "controlled carrier modulation" and modulated the screen grid of the 6DQ6.  I'll talk about this in the next section on the Knight T-150.