(DE9EBA, N1VBZ, DN1EBA)
Hans-Peter Dohmen
Laurenz-Pelzer-Strasse 5, D-47229 Duisburg-Rheinhausen, Germany
e-mail: dohmen-hape@arcor.de
English-text version of the 5 Watt single-valve transmitter
I found the description
of
this small Transmitter in a German Book about German Spy Equipment in
WW2
[1]
I was interested in how
it works.
The circuit is
standard,
using a pentode type AF-amplifier tube. The crystal is connected
between
the grid and ground, in series with a variable capacitor to adjust the
frequency by up to about 1.5Kc from the nominal frequency.
The keying is in the
cathode
circuit, and RF is blocked by the combination
of a 1mH choke with 10nF
and 200pF capacitors. The 200 ohm resistor limits the tube current.
The plate circuit contains
an adjustable L-C tank.
The coil is in parallel
with a variable capacitance with a range of about 20...150pF (max.
250pF).
This allows operation on
frequencies from 3...7.2MHz when the switch contacts are open. If
they`re
closed, then tuning is from 5.5...11MHz. One side of the tank circuit
is
grounded. The coil is around 32 turns of 0.5mm diam. insulated
copper wire wound on a
28...32mm
diam. plastic insulated tube.
The RF output is coupled
at about 1/6 of the maximum coil turns.
The plate gets its voltage
via an RF choke of about 2mH. The voltage is about
260 volts at a max.
current
of 50mA.
Between the plate and
tank
circuit it is necessary to use a coupling capacitor with a
high DC isolation rating.
Use a new 600 or 1000 volt ceramic or styroflex type, with a value of
about
10...50nF. Use 1/2 watt rated resistors. All other capacitors should be
350V ceramic types. The value of the parts can vary by up to +/- 50%.
Different types of
tubes
can also be used. Most of the AF-amp types will work
e.g. EBL 21/71 - EL84/6BQ5
- EL86/6CW5 - EL95/6DL6 - 6L6- 6V6
Between the transmitter
and
my longwire antenna I use an antenna tuner for good
adjustment and harmonic
suppression.
For receive I´m
using
commercially available types e.g. Telefunken, RFT, Collins. They have
narrow
IF-Filters and good preselection. This is particularly necessary during
the evenings on the 40m band.
I have worked a lot of
European
stations with 2-way QRP operation and good signal strength on both
sides.
So, good "CW-ing" and
think
about the High-Voltage in your transmitter!!!
For more on this theme also look at: Hans Summers, G0UPL