In practical electronics it is necessary to have a voltage source, which is adjustable in a wide range. We offer a stabilized voltage source built on vacuum tubes. The device has an unusual appearance and high overload resistance.
Many beginner radio amateurs believe, that the lamp is only suitable for audio equipment, but this opinion is absolutely wrong.
Advantages of the proposed lamp power supply
- short-circuit resistance, time-limited thermal strength of transformer windings.
- lamp life several thousand hours, which ensures trouble-free operation of the circuit for many years
- no problem, related to cooling and galvanic isolation of the circuit
- unusual, original appearance.
Unfortunately, there are also disadvantages, among which:
- decrease in efficiency due to the need to power incandescent lamps
- wear of the emission layer
The schematic diagram of the power supply is shown in the figure. 1.
Element, separating the outlet from the mains, is the transformer TR1. It is not permissible to bypass this element due to the risk of electric shock to the user..
The transformer supplies voltages to the secondary windings: 240 AT / 0,05 A i 33 AT / 0,35 BUT. first (high voltage) the secondary winding feeds the anodes of the lamps, and the second, with less voltage, nor.
A Gretz bridge made of diodes D1 is connected behind the transformer … D4. Elements C3-R1-C4 form a circuit, which filters the voltage, bridged. The role of resistor R1 – in addition to limiting the current pulse, arising after switching on, – increase the efficiency of filtering the variable component. It would be ideal to use a choke with an inductance of several hundred square meters here., but this decision was rejected, as the cost and dimensions of the device increased unnecessarily.
Capacitors C1 and C2 block any RFI. Divider R2 + R3 has multiple functions:
- discharges electrolytic capacitors C3 and C4 after system shutdown. The accumulated charge on them can cause shock with unpredictable consequences.
- raises the potential of the lamp filament to about 100 AT, thereby protecting from insulation breakdown on the way from the cathode to the fiber.
Diode D5 and resistor R5 form a circuit, which generates the reference voltage for the tube amplifier (L2).. This part of the circuit also discharges the filter capacitors., but only up to the voltage of the Zener diode D5 (39 AT).
Resistor R4 feeds the screening grid of lamps L1, a R7 – lamps L2. Resistor R8 and capacitor C5 form a passive low-pass filter, preventing accidental excitation of the system. resistors R9, R10 and potentiometer P1 – this is the output voltage divider, providing smooth regulation and stabilization. Appropriate selection of resistors R9 and R10 determines the adjustment range. Capacitors C6 and C7 reduce the internal impedance of the power supply at high frequencies, which is useful when checking built or repaired tube radios.
CON4 connects to a DC voltmeter with the appropriate range, a to CON5 time delay fuse, protecting the mains transformer from fire in case of prolonged overload. Silicon diode D6 protects the circuit, in particular voltmeter and capacitor C7, against overvoltages and voltages of reverse polarity. Resistor R6 can (not without exaggeration) to name “brain” this scheme. To understand the seriousness of the task at hand, the table will help 1. The reference point for lamp L2 is its cathode, which is under the potential of approximately 39 In relation to the ground, which is provided by diode D5 of the zener diode.
On the other hand, the principle of operation of the vacuum tube shows, that the lower the grid potential, the fewer electrons reach the anode and the less current passes through it. Let's take, For example, case with Uwyj = 300 AT. The voltage on its control grid will then be 35 B relative to mass, i.e -4 B relative to the cathode. The voltage drop across R6 is then about 120 AT. Ohm's law gives:
Where: Usas – filter capacitor voltage [AT], in theA-land – Voltage between L2 anode and system ground [AT], R6 – resistance R6, in this case 680 KB. In this case, its anode current is about 176 mA.
Calculations for the variant with Uwyj = 200V will show, what then the current will be about 360 mA. The voltage on the control grid is almost zero, so the current rises. By observing the ratio of the change in grid voltage to the anode voltage, we get another interesting parameter – the voltage gain of the L2 lamp in this system:
Where: ∆UA – change in anode voltage (measured between anode and cathode) [AT], ∆US1 – control grid voltage change (measured between grid and cathode) [AT] In this case, kin the≈30 and increases with input the area of positive voltage of the first grid.
Summarizing: R6 – this is a converter for changing the anode current of the L2 lamp into voltage, control lamp L1, which works as a cathode follower. Without it, no stabilization in this system would be possible.. Current gain is large, but we will not deal with it, since this parameter will not make any sense.
The circuit requires a minimum output voltage 115 AT. This is due to the impossibility of further lowering the potential of the driver grid L1 to the anode L2.
Assembly and launch
The printed circuit board of such a lamp power supply is shown in the figure 2.
The circuit is assembled on a single-sided printed circuit board with the size 144 mm × 80 mm. Installation should begin with soldering jumpers and sockets. The sockets are soldered to the side of the tracks, which allows you to expose lamps and cool them effectively.
The following nodes are assembled traditionally – from highest to lowest. Lamp L1, (PL504), designed to work at the final stages of horizontal scan TVs. Significant overvoltages occurred when working with the anode, calculated in kilovolts, which forced the designers to move the anode terminal away from the other terminals.
Ceramic plinths with inner diameter 6 mm for such lamps are commercially available. In this circuit, the anode has a potential of about 400 In relation to the ground, therefore, their use is even necessary for the safety of the user. To connect it, a soldering point is provided on the board, designated as ANODA.
Purchase a panel electromagnetic voltmeter with a range, required device on sale, complicated. This problem can be solved, using a voltmeter with a range 0 … 35 In or 0 … 40 B and connecting a resistor in series with its coil (or a set of resistors) with resistance, nine times the resistance of this coil. Voltmeter, used in model device, has a range 0 … 35 V DC and consumes 1 mA at a voltage 35 AT. this implies, that the resistance of its coil is 35 k, and the additional resistor must be rated 315 k (300 k + 15 k). At maximum voltage, it will give off as heat 315 mW.
The assembled circuit does not require additional settings. Lamps are inserted into sockets at the very end, just before launch. The measured glow current after one minute of operation must be in the range 285 … 315 mA. If it turns out to be too big, a resistor of the appropriate rating should be connected in series with the lamp filaments.
Too high cathode temperature will lead to premature lamp wear, too low – to impossibility to achieve full current efficiency. When starting up and using this power supply, be aware of the high voltages in it., including at the exit.
The ratings of the circuit parts are shown in the table. 2.
Author: Michal Kurzela