QRP 50C5 valve CW transmitter

A good design for low parts-count valve xtal controlled QRP transmitter (CW) can be seen at:
50C5 Euro QRP transmitter Mk 2 from Mike WU2D
Based on a design originally featured in American, Electronics Illustrated Mag' March 1968



The circuit is "elegantly simple" and acheives the maximum performance with very few components.
No mains transformer is used and all operating voltages are derived directly from the 230V AC mains supply.
A single 50C5 valve is used; as a xtal controlled, self oscillating power amplifier.

In the USA the 50C5 was used in the mass-produced All American 5 (AA5) superhet bcs't receivers.
These popular, economical sets used a series string filament AC/DC type of circuit. (click for details)
Because Australian made radio's generally avoided the cheap transformerless type design,
for safety considerations (i.e. electric shock) because of the hazard of a live chassis
if active and neutral mains connections were (accidently) transposed,
the (50 volt filament) 50C5 valve is somewhat uncommon in Australia (I think?)
I did find an Ebay vendor (Taiwan) to source some cheap 50C5 examples (see graphic below)

An alternative to using a voltage dropping resistance in order to supply the 50 volt tube filament,
is to use a capacitive/resistive voltage divider as shown/explained below.
While both methods will work, the capacitor uses no power (heat),
and also, a 2 uF (motor start/run) cap' may be easier to source than high wattage resistors.
The Capacitor must be non-polarised and at least 250 Volt AC rating.
Motor start capacitors would be suitable, however I decided to try 630V rated
CBB type polypropylene film capacitors, which are readily obtainable.

The capacitive divider also acts as a "soft-start" for the valve filament
with initial voltage being 25 volt and rising gradually to rated 50 volt as the
(+ve temp-coeff) filament resistance increases with temperature.
The 50C5 filament resistance measured: cold 46Ω, hot >240Ω.

The (key-down) DC input power to the 50C5 Anode circuit was measured as 32 mA @ 132 volt = 4.2 watt (P=V x I)





I managed to fit my "construct" into a small 100 x 60mm plastic box which has a snap-in lid.
This is insulated construction, prudent because of the hazard of a hard mains connected circuit, and any electric shock possibility!
The 2uF filament "voltage-dropping" capacitor is made with two 105 (1uF) 630V CBB polypropylene capacitors in parallel.
The crystal plugs into a configured 8 pin octal valve socket; which will accept a FT243 style xtal, in any orientation (see schematic)
The anode "tank" circuit uses a fixed 47pF silver mica capacitor, with a variable carbonyl slug tuned inductor,
thus eliminating the need for a 'bulky' tuning capacitor.



Additional improvements to the circuit (due to Mike WU2D)
Parasitic stopper (~0.5uH) choke at anode connection.
82 Volt zener stabilizing of screen voltage, less chirpy CW 'note'
Adding 2 capacitors and 1mH RFC to grid/cathode circuit,
which then changes from a simple xtal, to a colpitts type oscillator
and is less strain on the quartz xtal resonator; (i.e. less of a "rock-crusher")
220K bleed resistor across B+ supply - improve safety.
NE2 type neon indicator bulb as a tuning peak indicator.

The 50B5 valve can also, be used in this circuit, however its base "pinout" is different.
See: 50B5 Valve details

One baud keyer circuit
I have used a simple R-C relay oscillator (approx 1Hz with values shown) to 'key' the transmitter.
The transmitter can then be used as a simple "radio-beacon" producing a continuous series of dits'

At switch on : 2 x 2200uF Capacitor initially discharged.
Relay-2 picks up, capacitor begins to charge thru 100 Ω resistor.
When capacitor is charged sufficiently, Relay-1 will pick up, NC contact opens so Relay-2 drops out.
Capacitor then discharges back thru Relay-1 winding resistance and after time will drop out.
The cycle then continues ad infinitum (i.e. oscillation)
Time constant = 4400uF x 100 Ω = 0.44 sec
or 0.44 sec per half cycle which is approx 1 Hz.



Test bench: 7MHz QRP transmitter into 12V lamp/RF dummy load,
keyed by battery powered 1Hz R-C relay oscillator. Press 'play'



Nostalgia
In April 1970, the first ham rig I used on-air was similar to the above design !
Constructed from re-cycled componentry, the handbuilt transmitter used a
single tube 6V6 beam-power tetrode running approx 12 watts DC input.
A mains transformer and 5Y3 rectifier were however needed to supply the various operating voltages.

Typical design from the era, click to enlarge


My logbook entries show good strength contacts were made up to distances around 800Kms!



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