The AM Forum

AMfone's Online AM Handbook => Transmitters => Topic started by: Steve - K4HX on July 12, 2015, 01:42:26 PM

Title: HB TX from OE3WHB
Post by: Steve - K4HX on July 12, 2015, 01:42:26 PM
Willi sent me the details on his homebrew transmitter. It's a 12BY7 modulated by a EL84/6BQ5 and uses RF-derived negative feedback to lower audio distortion.

AM 6m-Transmitter with 4W

This transmitter is a crystal controlled 2 stage device with a 12BY7
in the PA. It is AM modulated with a Heising-circuit driven by a tube
EL84. The Mike-amplifier is based on Transistors and OP amplifiers.
The input stage of the audio amplifier is automatic gain controlled and
has an output voltage of approx. 0.28Vrms. The next 2 stages are build
by OP amplifiers.

The first OP amplifier has a summing node to mix the mike signal with
a negative feedback signal (= Gegenkopplung, GK, in German) which stems
from a RF-detector on the transmitter output port (M). The second OP
amplifier builds up the driving level for the EL84. This feature
improves significantly the modulation distortion you will find without
negative feedback.

The mike amplifier puts out a constant level no matter how loud you talk
into the microphone. Depending on the sensitivity of the mike it can be
useful to reduce the gain of the second transistor. This can be done
by inserting a resistor of 0 to 1k in the emitter circuit. The final
value has to be tested in a QSO and to some degree it is also a question
of personal taste.

Proper adjustment of the negative feedback:

1) the feedback loop has to be opened (Rgk removed) and the gain of the
second OP amplifier is adjusted with Rv for 95% modulation at full
output of the audio preamplifier.

2) Rv has to be changed to a 10 to 15 times greater value and the
feedback loop must be closed by connecting a variable resistor of 500k
from the RF-detector M to the GK-input (Rgk). Now adjust Rgk for 95%
modulation degree. Thats all.

You will notice a significant improvement of the shape of the RF envelope
when you feed in a 1kHz sine signal to the mike input. Please notice,
the RF detector (M) works with a load resistor of 12k and a carging C of
2.7nF. It is important that the time constant of these elements will NOT
exceed 33Ás otherwise the cap will not discharge fully during the negative
half-period at the higher audio frequencies and some distortion arises.

Title: Re: HB TX from OE3WHB
Post by: Steve - K4HX on July 12, 2015, 01:51:07 PM
Scope modulation envelope displays with and without feedback.

Title: Re: HB TX from OE3WHB
Post by: Opcom on July 12, 2015, 07:13:15 PM
10 to 15 times, so it is in a range of 10+dB for NFB possibly? Very nice.

Title: Re: HB TX from OE3WHB
Post by: W7TFO on July 13, 2015, 03:20:03 AM
Nice design/execution.

Western Electric used that feedback topology in their Doherty final type broadcast transmitters from the late 30's.

It was amazing to pull the RF rectifier tube and hear the program distortion rise.

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands