CW transmitter for 40m

 

This project shows, that design of a well performing transmitter could be very simple. Using only one valve, it is possible to build a transmitter for telegraphy purposes. The design is very convential. Many similar one-tube transmitters were built during the '50s and '60s of previous century. As transmit tube I have selected the QE5/40 (the equivalent of the RCA 6146B. The complete datasheet of the 6146B can be retrieved via the www: Datenblatt 6146B). The max allowed anode voltage is over 600 VDC. I have connected only a 350 VDC, which will certainly increase the tube's lifetime. At this supply voltage, the transmitter is capable of delivering about 25 Watt RF power. Plenty to make fine business CW QSOs. The choice of the tube was rather random. A 807, PL509 or PL519 will definitely work well here. The cut-off frequency of the QE5/40 and 6146B is rather high at 70 MHz. It might be necessary to take counter measures against hi-frequency oscillation at a different frequency than the X-tal is rated on. I had no problems with this phenomenon.

Circuit diagram 1 The 40m X-tal controlled CW transmitter

In order to have the transmitter delivered a stable output signal, an X-tal of 7.000 MHz takes care of the control to the grid. In series with an RF choke of 10 m H and a small adjustable capacitor (trimmer) of 4..30 pF, the frequency range is adjustable over 7.001 - 7.009 MHz. This is the begin segment of the 40 m CW band, the area where the DX traffic and contests take place. Make sure the trimmer in parallel over coil L1 is able to withstand higher voltages (up to 400V). A 24 Volts relay enables keying in the cathode pathway. This relay is controlled via the PC on which the program CWType is running. The capacitor over the 56W resistor rejects the 'keying click' effect. From the reports I received, I was told some clicking was detected (TONE was 8 instead of a perfect 9). A better click filter will sure do no harm here.

The power supply circuitry is not shown on this page, but it isn't that surprising. (Transformer 2 x 290 VAC, 4 x 1N5408 diode (1N4007 also allowed), choking coil, capacitor 50 + 100 m F). Make sure the transformer is not too small. At 6.3 VAC, the filament draws a 1.2 Amps current.

 By the way, keep awake while working with high voltages. Make sure you fix a cap on top of the anode of the tube

The 40m transmitter being built. L1 wound onto a PVC core comes good into view on this picture. A p filter has been added to the transmitter to fullfil the 40dB harmonic supression requirement (In particular the 3rd harmonic in the 15 m band (21 MHz) had to be dealt with). A solid SO239 connector makes sure there is a proper link with the antenne.

The transmitter is controlled by the PC on which CWType is running. While using the inverted V- dipole, with this TX I have been able to realize some good CW QSOs (all within Europe and during daytime) for example with DL5KUD and SM7Q from Sweden