SM6EHY on Amplifiers

SM6EHY on RF-AMPLIFIERS

In order for a radiostation setup to be a good performer it has to be equally capable in both receive and transmit. It is no point in beeing capable of to transmit further away than You can hear, considering that the station in the other end also uses high power.

If You are fortunate enough to have a quiet surrounding, you will often realise that you can not get in touch with other stations You hear loud and clear. I beleive that You sooner or later decide to do something about it. Either insert an attenuator in Your receive path o r increase Your transmit power.

Output configuration

All equipment in a radio transmitter has to be clean of distortion products. Most commonly this is true due to the close by neighbours home electronics equipment, of questionable quality.
The part of a transmitter, located closest to the antenna is the tuner and output coupler. In order for the harmonics (overtones) to be as low in level as possible the output circuitry must be arranged in a special manner.
The key to success is to use the "PI - L"-configuration.

The tubes dynamic impedance is here 3500 ohms (a common value). The inductances are made up by 2 roolers, here tuned to 80% of their full value; 10 µH. The descrete 470 pF-capacitors can by means of a switch be changed for different bands.
With the values used as indicated above, we have 0.09 dB attenuation at 3.6 MHz, and in the stopband 32.4 dB at 7.2 MHz and 44.5 dB at 10.8 MHz etc.
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TUNING CONTROLS

Tuning capacitors for high voltages are very bulky if You want a large dynamic tuning range. In order to reduce miscontacts that commonly occur in the tuning capacitors, we amateurs usually get hold of, use discrete capacitors and do the tuning by means of a rooler inductor. A method of obtaining an even contact force to the inductors axles is described here:

Rooler contact discs The axle "A1" is the main rooler inductor which is made contact with 2 metal discs attached to the "A1" by means of a spring between them. They are themselves connected to the axle "A2" on a slided sleeve. In this way the contact can be made to the roolers periphery.

Always try to tune the amplifier on a GRID current monitor. A peak in this current is very sharp and much easier to observe than the usual dip in the plate current. For a tretode and pentode amplifier this means the SCREEN-grid.

GENERAL CONSIDERATIONS

An amplifier is always intended to switch a rather high voltage and wattage powersupply in one way or the other, depending upon wether it is designed for linear or non-linear operation. Amplification classes for linear och quasi-linear operation include A, AB1, AB2 and B.

We also see on occations the amplification classes of D, E and F. These classes include a modulated power supply. This means that the supply delivers some AC-power to the amplifier, no DC. The most efficient waveform of such a supply is a SQUARE-wave. The amplifier is then just acting like a simple switch. It has though to be fast enough for the input signal to be switched. Somewhat ordinary types of switching power mosfets can be used up to some 10 MHz. Efficiency can be up to some 95 % ! The tuning network "PI-L", maybe with an additional LowPass-filter, assures that the switching transient level is low enough for on the air operation.

In order for the amplifier to function properly, it has to be switched in and out in the very right sequence. This means that You have to measure the switching with an oscilloscope. If You hot-switch the RF- current of some 5 Amperes, You will most probably burn up the relay ! The amplifier can this way turn out to be a good RF-welding machine... One new approach is to use large type of PIN-diode(s), that does switch hundreds of times faster than any mechanical relay. Such types can be the formerly Unitrode UM4000-series (TX-side) and the UM9400-series on the RX side. If the only concern is Shortwave, one can try to use the very cheap 1N4007-type. This one is the only in it's family that is built up like a PIN-diode. For the TX-side, use some 10 in parallell, and an additional stack of 10 in series, if the isolation in off state is vital. Each diode can be seen like a capacitor in it's isolation state, connecting them in series reduce the circuit capacitance 50 %. In the isolation state it is vital to apply a DC-blocking voltage that is higher than the peak value of the signal to block. For example 1000 W in a 50 ohm system gives a peak of the sqr-root of 1000 * 50 = 50000. This value is 223.6 Volts; 2000 W gives 447.2 Volts etc under MATCHED CONDITIONS. If You have an SWR in Your line, the voltage at the coupler can be either higher or lower. Recently we have seen some 2kV or even 3 kV-diodes, which can withstand 75 to 100 W dissipation. With these types we can switch most amlpifiers with a safety margin reliably. The true CW-operating mode, QSK, is a reality !