AM re-broadcaster design #3

AM re-broadcaster design #3

AM re-broadcaster (aka phono-oscillator) using Russian (Reflector brand)
6I1P-EV / ECH81 / 6AJ8 Triode-Heptode Tube.
Compared to other circuits; only 1 tube is needed, triode section as AF amp/modulator
and Heptode section as LC oscillator/mixer.
Design derived from an M G Scroggie app' note for frequency changer circuit.
Calculation returns 125uH as the required oscillator coil inductance
75 turns provides this inductance, which I then confirmed using an LC meter.
G₂ + G₄ (feedback) winding is 33% (25 turns) of main winding.
Winding polarity must be observed to guarantee oscillation (red dots)
If it doesn't oscillate 'first-time' try interchanging one windings termination.
Oscillator coil is wound on small salvaged ferrite drum bobbin.
A 390pF variable capacitor gives tuning over the frequency band (0.5 ~ 1.5MHz)
The modulation pot' is advanced so the volume is similar as other AM bcst stations.

Bench prototype construction.
Most componentry is recycled, but the 6I1P-EV / ECH81 / 6AJ8 tube
sourced from Russian Ebay valve seller (@ US$1 ea)

From a choice of 3 similar triode - heptode tubes: the 6AJ8/ECH81 is the only
one suitable, as it doesn't have an internal triode grid- heptode grid
cross connection. See Diagram;

Links:
AM re-broadcaster design #1 Uses 6BE6 pentagrid valve.
AM re-broadcaster design #2 Uses minature Russian 1.2V wire ended valves.

� 3Q 2018