Archive for January, 2015

More on 6V6 and 6L6 Vacuum Tubes in Transmitters

January 26, 2015

We previously discussed how vacuum tubes are used differently in code (CW) circuits than in audio circuits. In simple code circuits, such as the Pastime Projects “Remember When” reproduction “breadboard” transmitter kit, or the new 6V6 MC  kit, the tube can be thought of as a switching amplifier/oscillator. The tube is biased and set so that it either operates at optimum capability or it is silent (off). On or off.  It’s that simple.

In a crystal controlled oscillator circuit, there are many other factors.  We quickly learn that crystals vary in activity, current carrying capabilities, stability, temperature drift and other factors.  In putting the cw tube type transmitter on the air, we are concerned in transmitter the best stable and chirp free signal we can.  The best signal may not occur at the peak output point, and it is important to sacrifice a bit of output for best signal.  Maximum power out with best possible signal quality is our goal.

While it may not be so obvious today, the “RST” code (Readability-Strength-tone) was derived to grade a signal on its quality at a distance station. The “T” portion of this report dealt with tone.  While this is not a consideration much today, back in the days of crystal control, especially in the novice days after World War Two, the tone gave us a good idea of how bad (or good) our surplus crystals were working in our homebrew transmitters.

We have been testing each of our tubes in a transmitter circuit before stocking for our kits. The 6L6 tubes are a heavier duty tube than the 6V6 tubes. We have found either work very well in our transmitters, with the 6L6 giving a little more output than the 6V6 with the same power voltages.  (The current draw is a little heavier with the 6L6, but our power supply can handle the difference with ease.)

Please consider our 6V6/6L6 tubes next time you are in the market.  Of course, one will be included in your new Pastime Projects 6V6 type transmitter kit.

We have a few new “Coke-bottle” style 6L6 tubes (which are no longer available) left in stock. They are great performers and they add to the vintage look of our “Remember When”  6V6 Slat Board transmitters.  Contact us for more information.

http://www.pastimeprojects.com or  pastimeprojects (at)yahoo(dot)com.   73  Glenn W8JZI

Don’t forget the Vacuum Tube

January 10, 2015

My XYL (Ham operator’s term for wife – “Ex Young Lady”) who is also an amateur radio licensed operator, was asking tonight about the local club’s growing interest in CW operation.  That is “Continuous Wave” or “Code” operation.  That is where the operator uses code operation to communicate with other hams.  Using a key (switch) the person sends a series of short on-off pulses which represent the letters of the alphabet.  CW code is a language that, once learned, allows one to communicate with other operators around the world.

Even newly licensed technicians have certain areas of the amateur radio spectrum where they are authorized to transmit and receive code. There is no microphone required. There is no speech amplifier, compressor, modulator, or phase shift networks required.

Now, if we consider that we only have to switch a circuit on or off,  we can consider the vacuum tube.  There is much being said about the power tubes in audio amplifiers as used on guitar amplifiers, HI Fi amplifiers, low distortion, and so on.  In  CW work, we are operation our vacuum tubes as switches.  The tube must simply transmit when we hit the key, and not transmit when the key is off.  If we keep this in mind, the tube characteristics (and somehow the resulting price) are not so important to the CW operator..

Long story short,  if your CW transmitter uses a 6V6 or 6L6 tube for the ‘final’,  don’t hesitate to purchase on at the local hamfest and give it a try in your rig.  You may find the power output is as good or better than that more expensive tube advertised for its great audio characteristics.

The 12K5 “LookMa” CW  transmitter kit sold by Pastime Projects http://www.pastimeprojects.com is an under thirty dollar kit which will produce a signal in the 40 meter ham radio  band. Its enough of a signal to be heard across your ham shack in a nearby receiver. You can use it for code practice.  It does not have enough power to be heard on the air at any distance, so don’t expect to work DX with it. All you need is a CW key, a 12 volt power supply,  and a 50 ohm resistor for a dummy load, and you can practice code to your hearts delight using your 40 meter receiver. It comes with a crystal! And a great manual. Most often, we even wind the coil for you to save you a little time.    73 Glenn  pastimeprojects(at)yahoo(dot)com