Classic 40
A classic CW/AM transmitter based on the emtx
10-14W on CW, 4-6W on AM

Above: schematic of the classic 40.
Below: Classic 40 tested on a piece of wood.



The design and the story behind it

Older ones may remember the era where transistors first came in. They were expensive and their performance was limited at the early days compared to the tubes. Many HAMs already building their own equipment, the so called "novice transmitters", using vacuum tubes. There are many such designs to look around, usually one tube transmitters that can be built easily compared to multi-stage ones. With the passing of the tubes and the rise of transistors, HAMs were gradually moving to the solid state transmitters. Solid state technology, allowed more stages to be implemented at lower cost and effort. Usually, more stages meant better performance or more features, so HAMs went for such designs. It seems that, with the rising of transistors, the art of minimalistic single-stage designs was lost. The only field where single stage transmitters continued, was the QRP homebrew equipment. However, most of these designs were and still are, low power transmitters, 1-2 watts the most, most of them being less than a watt. Whereas they work, they don't have enough power to get you DX most of the times, with the exception of when the propagation is very good, or if you have really good antennas, which many don't. Indeed, the tube still remains the way to get much output power on a single stage transmitter. However, tube-related components were once easy to find but today they are more rare and expensive. Even a single stage tube transmitter will cost you much more than you think, if you can find the components. And whereas you can build a multi-stage powerful solid state transmitters, it is still challenging to built a powerful one in one stage with solid state devices. I wanted to take this challenge, so I designed a classic transmitter to satisfy these requirements, the "Classic 40".

The classic 40 is based on an earlier design of mine the "emergency transmitter". In the design of that transmitter, simplicity, high power (in terms of QRP levels), multiple input voltage, "any NPN" and "any antenna", were the requirements. The classic 40, is optimized for maximum output power only, using more specific components and a single voltage source, usually a laptop SMPSU. However any CB-type power transistor can be used in the power oscillator and you can alter the design easily to suit your requirements.

My emergency transmitter was only CW. Then I thought, why not "give voice" to the design? CW is good, but it is half of the fun. If you could use your simple CW transmitter to sent your voice, this would be great. You could now chat comfortably on the nets and have much more fun. The simplest modulation you can apply to a transmitter is the AM modulation. And whereas this is an old modulation, mostly abandined by HAMs due to beeing inefficient, there are still AM nets on HF. AM can can also be heard by SSB receivers by zero-beating the receiver to the AM carrier. So you could still use your simple AM transmitter to chat with the SSB guys.

In the old days, the most common way to apply AM modulation was to modulate the high voltage to the plate of the tubes, using a transformer and a powerful audio amplifier. In low voltage solid state circuits, you can still do it using transformers, but you can also use series transistors instead of the transformer. All these things require many components and/or powerful AF amplifiers if one is to modulate higher power transmitters. This does not match the keep-it-simple philosophy of my Classic 40.

So I thought of a simple trick with the use of the extremely common LM317. The modulator I have tested uses just 6 common cheap components and it is able to achieve remarkably good modulation levels for it's parts-count, just from line audio input. It juices every bit of the internal circuicity of the LM317, just look at where the base current of the 2N2222 comes from.

The circuit is being used as a current booster, the current being the supply to the oscillator and dependent on the voltage it produces. The LM317 always tries to keep 1.25V between it's output pin and 'adj' pin but where you benefit here is the current at the 'adj' pin is very low so it is easier to apply audio to it. The error amplifier inside the voltage regulator is used as an additional amplifier stage. The output pin voltage varies according to the voltage on the 'adj' pin so if you use it to bias the transistor you get negative feedback which improves the modulation quality. More output voltage = more bias current = lower output voltage. It is very difficult for a circuit to match that kind of simplicity in balande to such performance.

Following my detailed instructions, the classic 40 transmitter can be reproduced easily, within hours. The result is always success, this is one of the circuits that are not critical at all and a successfully working transmitter can be reproduced every time. I have built this transmitter several times, using similar components (even toroids) and it always worked. The transmitter meets the next expectations:

1. Output power (including harmonics): 10W to 14W on CW, 4W to 6W on AM (modulation level varies with power).
2. It can drive any antenna directly, 50 ohm or higher impedance, without external tuners.
3. Band of operation: 40m (out of band operation can be done but not allowed by regulation)

4. Modulation:
AM, ON/OFF (CW, Feld-Hell, TAP code).


Components selection

The classic 40 has been designed to use only readily available components. Most of them you may already have in your junk box. After all, there are very few parts in the whole transmitter design. The ones you don't have, can be taken out from various equipment, or bought even at a modest-stock electronics shop at your area for peanuts.

The transistor:
This transmitter has been designed so that it can operate with any NPN BJT in place. This includes small signal RF and audio transistors and high power RF transistors like the ones used on HF amplifiers and CB radios. However, the modulator has been optimized for usage with CB power transistors, such as the
cheap 2sc2078. Other CB power transistors are the 2sc2166, 2sc1971, 2sc3133, 2sc1969 and 2sc2312. There are many others. If you use another transistor or another input voltage (eg 12v), you may need to alter the values of the modulator resistors. That is the only change you need to do in such case, as the transmitter alone is capable of operating with any NPN transistor and any voltage from 1.2v to 30v.

The crystal:
This is the most uncommon part of the transmitter. You have to find the crystal for the frequency that you want to operate on. In such a single stage design, when you operate the transmitter at high powers and currents, you will notice crystal heating and chirp on CW at the frequency of the transmitter. This chirp is not that much. You can still work stations and it can still pass through the CW filters of the receivers. However, if this chirp annoys you, then you have to use these vintage bigger size crystals, that can handle more current through them. But these are evem nore uncommon. The approach I have used in my prototype, was to connect two or more modern crystals of the same frequency in parallel, so that the current is shared ammong them. This improved the CW chirp at high powers just if I was using a single vintage crystal. Again, this is optional, but if you want to minimize chirp (and crystal heating) without searching for rare vintage crystals, this is the way to go. On AM mode chirp is not a problem, since the transmitter operates continuously and chirp is unnoticeable on AM.

The incandescent bulb:
This bulb is used as an indicator of the state of the transmitter and the modulation level. This bulb will light on when the transmitter oscillates. It monitors the actual RF signal, so it's brightness changes according to the amount of RF power the transmitter produces.
This is just what you need to know in order to set the variable capacitor properly. On AM, the bulb will slightly change brightness according to the AM modulation level. Although not as accurate as a current meter, this monitors directly the output power of the Classig 40. Miniature incandescent bulbs may not be that easy to find nowadays. However, there is a good source of these, that almost anyone has in their houses. This source is the old Christmas lights. You do save old Christmas lights, don't you?

The meter:
In my "emergency transmitter" no current meter was used, due to cost, components count and simplicity reasons. However, in the Classic 40 I relaxed the rules, so now a nice 1 ampere panel meter can be used. Tune the 25pF variable capacitor for around 500-600mA of transmitter current. If you want more power you can push it to 700mA or even 800mA but don't go further than that, as chirp and crystal heating increases. Along with the incandescent bulb the meter will give you a nice indication of a correctly tuned transmitter.

The heatsink:
The transmitter transistor does not get very hot, even on AM, so a small heatsink should suffice. However, the LM317T regulator, being inefficient and operating all the time on AM, requires quite a large heatsink. If you build the classic 40 on an aluminium chassis, you could use the chassis as a heatsink, with the appropriate insulators and thermal paste for the
transistor and the regulator of course. The regulator package is the same as the power transistor one, so use the same type of insulators. I like mica insulators better, but use what you have available. The size of the heatsink I used in the prototype is ok for CW and AM talk, but it is quite small if one is to operate the transmitter on AM for prolonged periods of more than a few minutes. Of course you can always use a blower, but I prefer passive cooling for various reasons.


Transformer construction

Step 1:
Take a piece of 32mm external diameter PVC pipe from a plumber's shop. Alternatively, a suitable diameter pills box can be used, or any other suitable diameter plastic tube.

Step 2:
Cut a 4cm piece out of this tube. 4cm is the minimum length required.


Below a 4cm PVC tube has been cut in size.


Step 3:
Wind 16 turns of 1mm diameter enameled wire onto the PVC pipe and secure the winding in place as shown in the picture below. Notice the winding direction of the wire. This is the primary of the transformer, the one that is connected to the two capacitors. Notice that this winding is wound a bit offset to the right of the pipe.


Step 4:
Wrap the winding with 3 turns of PTFE tape. It can be bought at any plumber's shop, just like the PVC pipe. The PTFE tape will help in keeping the second layer turns in place and it will provide extra insulation.


Step 5:
Wind 2 turns of 1mm diameter enameled wire on top of the primary winding and secure the winding in place as shown in the picture below. Notice the winding direction of the wire, as well as it's position relative to the primary winding. This is the feedback of the transformer, the one that is connected to the collector of the transistor.


Step 6:
Wind 14 turns of 1mm diameter enameled wire on top of the primary winding, starting from just next to the 2 turns one and secure this winding in place as shown in the picture below. Notice the winding direction of the wire, as well as it's position relative to the primary and the 2 turns windings. This is the secondary (output) of the transformer, the one that is connected to the antenna. At this point do not worry about the taps yet.


Notice in the picture below, the way the windings are secured in place onto the pipe. The wire ends are passed through the pipe using small holes and then bent towards the ends of the pipe and once more to the surface of the pipe, where the connections will be made.


Step 7:
Wind 1 turn of 1mm diameter enameled wire onto the pipe and secure the winding in place as shown in the picture below. Notice the winding position relative to the other windings. This 1 turn winding is placed about 1cm away from the other windings. This is the RF pick up winding, the one that is connected to the incandescent bulb.


Step 8:
Use a sharp cutter (knife) and carefully scrap the enamel of all the windings ends. Do not worry if you cannot scrap the enamel at the bottom side of the wire ends (that touches to the pipe). We just want enough copper exposed to make the connection.


Step 9:
Tin the scrapped wire ends, taking care not to overheat them much.


Step 10:
Now it's time to make the taps on the secondary winding. Use a sharp cutter (knife) and very carefully scrap the enamel of the wire at the tap points (number of turns). Take much care not to scrap the enamel of the previous and the next turn from each tap point. Do not worry if you just scrap the enamel at the top of the wire (external area). We just want enough copper exposed to make the connection.


Make each tap, a bit offset from the near by taps, like shown in the pictures. This will avoid any short circuits (especially at the 4, 5 and 6 taps) and it will allow for easier connections, especially if alligator clips are used to connect to the taps.


Step 11:
Tin all the tap points, taking care not to overheat them.


Step 12:
This step is optional and it depends on how you decide to do the connections to the taps. You may solder wires directly to the tap points, but in my case I wanted to use alligator clips, so I did the next: I took a piece of a component lead and soldered it's one end to each tap point. Then I bent the component lead to U-shape and cut it accordingly. This created nice and rigid tap points for the alligator clip.


Step 13:
This step is optional and it depends on how you decide to mount the transformer to your enclosure. In my case, I wanted to create three small legs for the mounting. I cut three pieces of aluminum straps and made holes at both their ends. I made three small holes onto the transformer pipe end and mounted the
aluminum straps using screws. After mounting them, I shaped the straps to L-shape. Then I used three more screws to mount the transformer to the enclosure.


The completed transformer is shown in the pictures above and below. The 6 connection points at the bottom of the pipe, are the low voltage points, whereas the 2 points at the top of the pipe, are the high voltage points.


If you have built the transformer as described, the bottom connections are as follows (from left to right):

Wire end 1, connected to the incandescent bulb
Wire end 2, connected to the incandescent bulb
Wire end 3, connected to the VCC (power supply positive voltage)
Wire end 4, connected to the VCC (power supply positive voltage)
Wire end 5, connected to the GND (ground)
Wire end 6, connected to the transistor collector

The top connections are as follows (from left to right):

Wire end 1, connected to the 25pF variable capacitor and the 47pF fixed.
Wire end 2, is the 14th secondary tap and it is left unconnected, or tapped to the appropriate impedance antenna.


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