Schematics QBL 5/3500 PA + power supplies
Low voltage power supply, (small) high voltage PS and QBL PA
The various low voltages are made in a straight forward way.
Both the Ug1 as well as the Ug2 stabilization are built with a "hybrid" 200V high power zener diode, based on the schematic below.
The Ug2 stabilization uses 4 identical 200V zener sections in series. The proper 800V Ug2 voltage is obtained by exchanging some1N5378 zeners by 1N5377 where needed.
The Ug1 stabilization has a stack of various zeners in order to allow proper setting of the Ug1 voltage (to obtain the correct Ia idling current)
With the jumper the zener voltage can be set.
The low voltage cabinet with "hybrid zeners" under test
The schematics below give the "old" situation, before the power zeners were installed.
Old Ug 1 voltage regulator :
A shunt regulator is used to obtain a regulated voltage of approx. -110 V. An EL34 is used as stabilization device. Under test the voltage remained stable while being loaded by 85 mA. During receive the G1 voltage drops to -380V, effectively preventing any electron from leaving the cathode and causing noise while receiving.
Old Ug1 stabilized power supply
Old Ug 2 voltage regulator :
A shunt regulator is used here as well. For security reasons a tube with high voltage capabilities was chosen, a 4-125 (QB 3/300). The current through the resistor of 330 Ohm is 150mA yielding a stable G2 voltage up to a loading of 100mA. The voltage ripple is approx. 10V at 100mA loading, so a regulation of almost 99% is obtained. The OPA 445 is a high voltage OpAmp from Burr-Brown (now supplied by Texas Instruments).
Old Ug2 stabilized power supply
Low voltage cabinet in the old situation, with tube stabilizers
This cabinet contains the 12V, G1 and G2 stabilized power supplies and the protection circuitry.
The wire wound 30 Ohm / 100W resistor in the Ig2 line can be seen in the lower left corner.
The PC board on the right hand side contains all the protection circuitry.
The Ug1 circuit and stabilizer tube can be seen in the right corner aft.
The 4-125 (QB3/300) takes quite some space.
Three phase power supply
The "small" three phase power supply is capable of delivering 3.5 kV at 1 A. The 380 V AC is connected via a start-up circuit, limiting the initial current. It is a temporary power supply, used until the final one is ready.
Excessive anode current (or flash-over) are monitored by means of a voltage drop across a 1 Ohm resistor. The voltage is limited by a zener circuit and transferred to the protection circuits by means of a 10 kV opto-coupler.
As soon as excessive current is measured (among others) the 380 V AC primary will be disconnected automatically.
A high voltage sample is sent to the protection circuits as well, preventing any Ug2 voltage and Tx-state in case no Ua is present.
QBL 5/3500 PA
The PA itself is quite simple as far as the schematic is
concerned. The rf-decoupling of the heater has been taken care of in a special
way, using various capacitors in parallel combined with a quarter wave stub of
RG-58 coax.
The inrush current of the filament is reduced by inserting a resistor in the primary of the filament transformer.
A little 12V blower cools the cathode / grid part of the tube.
The RC combination of 4k7 and 5nF is needed to avoid keying clicks when running CW and to keep the gain down at low power levels .
The calculation of the anode plate circuit can be found at design details.