Radiophile valve transmitter

[ppppenguin]

That's an unusual looking technique, with the output transformer connected between the screen and anode of the EF80.

Using google translate:

realized this project in 2016. The project Hi-Fi AM Transmitter presented here by Joe Sousa had me jumping in with the 6AS6.

Before that I did not know the possibility of modulation via the G3 of a pentode in this form. I was fascinated by the possibility to achieve a 100% modulation with good linearity. Unfortunately I had no 6AS6 at hand when I started my project. That's why I studied tubes like EF80, EF85, EF89 and EF184 and measured their behavior using G3 modulation. Before I show the results of my μTracer, I would like to show you how this modulation principle works.


With a pentode, as is well known, the G3 (brake grid) is at cathode potential. Thus, it prevents slow electrons that are "beaten" from the anode as secondary emission by the fast electrons coming from direction G2 to fly back to the grid G2 and thus reduce the anode current. Without this measure, the anode current would decrease in spite of increasing anode voltage in a certain core line range of the UA / IA characteristic. This leads to the so-called "tetrode kink" in the UA / IA characteristic of a tetrode. With a Pentode, this kink does not occur at all, thanks to the G3.

However, for the faster electrons accelerated by G2 accelerated by G2, the G3 is usually "pervious". Now, if the G3 continues to be made negative, even electrons coming from the direction of the cathode are prevented from reaching the anode. They are then absorbed by the G2 and increase the G2 current. With very negative G3 and low anode voltage, the anode current can almost completely disappear in favor of a higher G2 current. The following picture shows an EF80 at 90V anode and G2 voltage. The potential of G3 (not G1!) Is shown on the X-axis. G1 is fixed at 0V in this measurement:

As the picture shows, the anode current decreases with G3 becoming more negative while the current of G2 increases proportionally. With the same scaling of the G2 and anode currents, in the area where the two characteristic curves cross, the anode and G2 currents are the same. In other words, their difference = 0.

My measurements with the μTracer showed that the effect at about 90V at G2 and anode can be used quite well, because here are larger linear areas to the left of the crossing point to find a linear modulation allow. My experiments have shown that the region in which the G2 current is higher than the anode current usually has better linearity. At voltages lower than 90V, the usable linear part of the curves decreases. A higher voltage results in flatter curves, which requires a higher drive voltage at the G3.

In the appendix I show some of my measurements with different tubes.

The effect can be used to 100% suppress carrier oscillation in AM modulation. It only needs to be formed by means of a transformer, the difference of the anode and the G2 current. Here is the basic circuit:

In the circuit I use an EF80 is operated at about -13V G3 voltage. Here, the G2 stream outweighs as can be seen above. At the same time, the NF modulates the G3. The G3 voltage varies by +/- 2-3V by the -13V mark thereby increasing the G2 current at the positive NF peaks and the negative G2 current being equal to the anode current. A carrier which is supplied to the G1 makes these fluctuations also with. If the anode and G2 currents are the same, the carrier in the transmitter is extinguished because then the difference of the currents = 0. A further increase in the modulating NF level results in a renewed increase in the carrier amplitude when passing through the zero point, since then the anode current is higher than the G2 current in the transformer. The following images show the modulation trapezoid for near 100% modulation and 110% overmodulation. Here you can clearly see the rising carrier amplitude on the right.

100% modulation of the EF80 circuit with difference G2 / anode

about 110% modulation under the same conditions.

Practical design, the circuit diagram is attached:

Since I have many P-tubes that are waiting for a sensible use, I have designed a circuit that uses a PCF80 as NF amplifier and oscillator in addition to the EF80, which represents the actual modulator. The modulation is indicated by a magical band PM84. I have solved with a PABC80 the necessary amplification of the NF signal and its rectification. Here is a circuit description:

The triode of the PCF80 V1A amplifies the audio signal to approx. 5Vss. This level is necessary to control the EF80 at the G3 enough. The 1k5 resistor in the cathode branch serves as a negative feedback to keep the harmonic distortion low. The pentode of the PCF80 V2B runs as an oscillator. Here I use again a ceramic as a frequency-determining element