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6-WATT 2-TUBE CW TRANSMITTER FOR 40 METERS

Cost estimate: $60


Here we go beyond the QRP upper limit - but not by much - with an economical, relatively powerful (six watts RF output) and dependable crystal-controlled transmitter using tubes! It's fun to build, easy to operate, and very inexpensive considering its performance. Larry Lisle's original early 90s design is modified by featuring either a meter or a lamp - or both - for tuning, a power resistor instead of a more expensive third tube for ballast in the filament circuit, and relay-keying for assured safety. A cigar box provides the chassis and front panel, and all parts are readily available.

Design: A glance at the schematic below reveals this is an oscillator using two tubes in parallel to obtain the RF output. The tubes' filaments are powered directly from the AC source. Two back-to-back common power transformers safely feed AC into a bridge rectifier. The resultant DC ("B+") voltage is filtered and goes to the twin-tube oscillator, which is cathode-keyed. Since high voltage (up to 160VDC) can be a hazard if a key is used directly, a keying relay circuit is shown here. Direct keying is of course fine, but if used it is recommended that the key terminals are safely isolated from the operator's hand! The meter shown is optional, as is the lamp; either scheme (or both) can be used to tune the transmitter.
R1: 100K/1W resistor
R2: 27K/2W resistor
R3: 100-ohm/5w power resistor
C1: 365pF variable capacitor
C2: .047uF/630v capacitor
C3: 22uF/350v electrolytic capicitor
C4: .01uF/500v capacitor
D1: 1N4004 rectifier diode
BR1: 1 AMP/250 PIV bridge rectifier
S1: SPST switch
L1: 15 turns hookup wire on a 7/8" form (wood dowel, plastic pill bottle, etc.)
L2: 7 turns of the same wire wound over L1
M1: 100ma panel meter
T1,T2: 117vac/12.6vac power transformers (center-tap models OK, center-taps not used)
V1,V2: 50C5 beam power amplifier tubes
RELAY: 12v SPST relay (a 9v relay could also be used)
I1 6-volt bulb (a #44 is perfect)
XTAL: 40 meter band crystal
ALSO: two 7-pin miniature tube sockets, two jacks (for key-in and antenna out), a crystal socket or holder, nuts and bolts, hookup wire, a socket for bulb I1, two 4-lug terminal strips, a knob for C1, and an AC cord and plug.


Construction: All parts are available from most electronics supply houses, except the tubes, which are from Antique Electronics Supply (other sources are certainly out there too). Use power transformers rated at least 1 amp. Transformers with lower current ratings may suffice, but erring on the side of caution is a good idea with any tube project. The 50C5 tubes use 7-pin miniature sockets. Attach the panel (cigarbox top) to the chassis (the upside-down box). Drill the holes and create the openings for the chassis and panel components and mount them; the major components "on top" of the chassis and the support wiring below. Be sure to include tie points for the subsequent wiring (two 4-lug terminal strips should do). Proceed with adding the remaining components and wiring. The builder may choose to use either the lamp and/or the meter for tuning. Both methods work well; the lamp-with-switch method has the advantages of less cost and ability to decrease output power to just under 5 watts if desired. It is recommended to finish by taping another cigarbox-top to the bottom for safety and convenience.

Operation: Attach an antenna or dummy load, insert a crystal, and depress the code key. The keyed input can be provided with any DC source appropriate for the relay, with the key connected to the positive (+) line. A filtered keying circuit can be be used if the direct-from-DC source doesn't cut it. Rotate the variable capacitor until the meter reads minimum (60-70ma) and/or the lamp reaches maximum brilliance. If necessary adjust the variable capacitor for cleanest tone. If using a lamp, when it is switched "in", it will light brightly when tuned - and reduce the output by about a watt. If the lamp is switched "out" it will light dimly when tuned, without sacrificing output. That's it! Different crystals may have better or worse performance, and some "remanufactured" crystals that are perfectly fine for transistor QRP rigs may be impossible to tune, but odds are this glowing wonder will thrill and delight.

Note: A homebrew Bare Essentials Transmitter from 1968 for 80 or 40 meters using a single 50C5 tube and no transformers has recently enjoyed renewed popularity. It can be built for well under $20, but it requires a ground connection for safety and puts out less power than the one presented here. If budget is a major issue, by all means investigate it - there are many happy users. 

Sources:
Transmitter:
     POPULAR ELECTRONICS, August 1992, p.45
Relay:
     QST, November 2003, pp.29-30 (modified)

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