No.34
Amateur Radio
572B

The amplifier drawn is an issue from the author's wish to improve the hardware in his shack and to offer some hours of technical amusement.

Preface

The low propagation conditions prevailing during the end period of the last solar cycle have extended the installation and use of power amplifiers by many DXers. This fact has been (among others) an evident consequence, the signal level received by the operator of any DX -or just "interesting" station, is noticeably higher than in previous years, doing every so often more difficult to be heard in a pileup with the customary hundred watt from my rig at my city QTH. So, while deeply disappointed about such "watt war" policy, I set out to build a modest amplifier that allows me to raise my signals some decibels above the general rumble of the pileups, but without damaging the peaceful conviviality in a condominium. Considering all variables, I decided to build an amplifier with power tubes in the 300-400 Watt level. Some of the necessary components require a little bit of searching, but are not impossible to find. In the following considerations we explain some of the criteria applied to the project and some interesting details of its practical model.

Tube Selection

Of course, I did not choose power semiconductors but certainly not from any lack of wishing to. The project directives were simplicity, a reasonable cost and to employ as many parts from my limited junk boxes as possible. The solid state option would have meant exactly all the opposite! The first dilemma to solve is the triode or tetrode option. The triode advantages are less cost per tube and higher simplicity in its power and bias circuitry. But its inconvenience comes from a need for a matching input impedances network and possible slightly higher intermodulation products. The best known triodes and easiest to obtain are the 811A and 572B types. For an output power of 300 watts two 811A tubes in parallel are required, while a single 572B may suffice. The possibility to buy the Russian manufactured Svetlana tubes at very reasonable prices meant that we could get a pair of 811A for about 90 US$, while a single 572B costs around 117 US$, plus the cost of the socket(s). The modern ceramic tetrodes offer the advantages of higher gain, so less driving power is needed, along with very low intermodulation distortion and the possibility to build a purely resistive broadband input circuit. But its disadvantages are the greater complexity of the bias circuitry (specifically the screen grid) and a noticeably higher price. Between the appropriate tetrodes at this power level we found, among others, the metal-ceramic 4CX250B and 4CX350A. But, even with the less expensive Russian tubes, the cost of the tetrode option was remarkably higher -especially with the special socket needed. Considering the complexity of the screen grid bias, the final decision came down to one of the triodes and between the two, the Svetlana 572B (Photo 1) was the choice. This tube, with a maximum plate dissipation of 160 watts, accepts an input power of 600 W at 2,400 V of plate voltage, and may supply an output greater than 300 W. Another advantage was the short warm-up time due to its directly-heated cathode, which allows the amplifier to activate it only when it is needed. Circuitry Considerations

Class B Working and Load Line

Grounded grid with zero bias was the chosen configuration. The available plate voltage supply was limited to about 1,600 V under load as would be obtained by doubling the 410+410 Vac from a HiFi audio amplifier supply transformer and far from the 2,400 V mentioned by the manufacturer for class "B" ICAS service (Table 1). In this table a value of 45 mA as idle plate current is listed and a mean 250 mA for a single-tone signal.

Table 1 - Svetlana 572B RF Power Triode
Linear RF Power Amplifier, Class B Grounded Grid

The minimum plate voltage during the working cycle is established at 200 V, as to avoid an excessive grid current. This allows us to mark on the plate curves family (Figure 1) the point "A" (idle point -no signal) and the peak plate current (250-45) x 2 = 410 mA, at point "B". Between these points are traced the loading line, at which the tube will work during the negative half cycles of driving signal applied to the cathode. From the examination of these characteristic curves we find that the necessary RF driving voltage will be of around 82 V (negative peak, or 164 V peak-to-peak). It may appear, at first glance there could be a problem. The plate current curves with grid bias of zero and +10 V are, around the point "A", very close. That means that with pure zero bias, the plate current increase would not take place until the driving voltage exceeds 10 V, which is an important fraction from the total. This, however, does not appear to be much of an inconvenience in CW or RTTY/FSK (resembling class "C" operation), but in SSB that simply means low-end flattening and distortion. To reduce such distortion we may bias the grid with a little positive voltage (between +5 and +10 V) when operating in SSB mode. We will come back to this point later. During the positive peaks on the cathode (grid negative) the plate current is cut, while plate voltage swings at nearly twice the supply voltage due to the flywheel effect of the plate tank (point "C" in Fig. 1).

Driving Circuit

With a driving voltage of 82 V, and the plate voltage at its lower limit of 200 V, the peak grid current will be 100 mA. From this it may be derived that the cathode-grid impedance during the driving peak will be of 82/0.1 = 820 Ohms.

In an intermediate zone, this impedance will be slightly greater, approximately 1,000 Ohms. From this, several consequences are deduced. First, the load value seen from the exciter is not linear, and this load diminishes as the driving power increases. Second, some compensating device of input impedance will be needed to match it at the customary value of around 50 Ohms needed by most transceivers. And third, if just a single resistor in parallel is chosen for this matching, the dissipated power through it will be quite high. The peak RF voltage needed (164 Vpp) corresponds to an effective power value of 67.6 W, from which only about 4.5 W will be used into the cathode-grid space, while the rest were dissipated in the compensating resistor; this means an inadmissible low efficiency. This matching can be done, i.e., with a "pi" circuit, but this implies the need to switch components for each band, with a consequence of troublesome circuitry. So, we'll try to design a suitable wide band transformer. In a grounded grid amplifier with directly heated tubes it is necessary to isolate the heater from the chassis to inject it in the driving RF. This makes it a must to apply the heating current across an RF choke that is capable of conducting the full ac current of the 572B, a value of 4 A. This relatively low value allows us to consider combining, on a single component, the functions of choke and matching transformer, designing a suitable toroidal transformer in which the secondary windings, wired in series in phase, will conduct the heating current. This 1:2 ratio RF transformer is composed of a five wire winding, forming a tight-coupled primary at two secondaries of double number of turns on an iron powder toroidal core. The ac heating current does not affect the core magnetizing flux, as it runs in opposite directions on each half secondary canceling itself. Doubling the number of turns of the secondary means to divide the reflected impedance on the primary by four, which turns out to be of around 250 Ohms. This will make it possible to add in parallel a compensating resistor of some 75 Ohms to get a good matching with a transmitter. This loading resistor was easily assembled joining 40 carbon resistors of 1.500 Ohm, 2 W in parallel between two pieces of p.c. board. The driving voltage to apply at the input will now be half (29.1 Veff) and the theoretical input power will now be 16.9 W. In reality we may expect that a little more power will be needed to compensate the matching losses, but these are already adequate values. Furthermore, the compensating resistors' presence linearizes the load, reducing the distortion at the exciter. The broad band transformer was built by grouping in parallel 5 pieces of 1 mm diameter (No. 18), enameled copper wire about 1 meter long (3.3 feet), winding nine flat turns on an iron powder core. The core is 37 mm O.D., 20 mm I.D. and 15 mm in height (an equivalent Amidon would be the FT-140-43). The dual secondary halves were wired in series with short leads and the whole connecting set (including the coax end) was fixed and protected with thermal resin (Photo 2).

Bias Circuit

As said before, with zero bias (both the grid and cathode grounded for dc current) the tube works well in CW, but in SSB its intermodulation products index would not be the most desirable. To avoid this, the grid is not connected directly to the chassis, but is RF bypassed with a couple of ceramic capacitors with very different values (this reduces the parasitic lead inductance and self-resonating effects). Via a small RF choke, the grid is connected to the chassis (CW) or to a positive bias (SSB) of 6.8 V, obtained by a zener diode with a biasing current of around 100 mA (maximum expected value for the grid current). Naturally, this positive bias is totally removed during receiving periods, grounding the grid lead via an auxiliary section of T/R relay RL3b. Plate Circuit

The anode voltage (HV) is applied to the plate across a choke coil that is not resonant on any amateur band (CH1). This choke is not difficult to build, but requires some attention. A series parasitic resonance on a given band would cause it to heat up. Figure 3 shows a drawing described by Doug DeMaw, W1FB, in 1979. The different windings in series are placed on a Teflon rod 110 mm long and 20 mm in diameter. A pair of threaded holes at the flat ends hold a terminal lug at the top and a fixing screw at the bottom, while the other lug is on the side near the low end, fixed with a short self-tapping screw. The wire turns were fixed with clear nail polish (a gift from my XYL). Once the amplifier has been assembled, it is convenient to temporarily short the choke with a piece of solid wire or copper braid and with a grid-dip meter check that there are no parasitic series resonances on any band. Inserted between the plate cap and the loading tank is a L-R network intended to avoid parasitic VHF oscillations, formed by a self-supported coil (1 x 1 inch) of four turns No.10 silver-plated cooper wire, shunted by a 39 Ohm / 2 W carbon resistor placed outside the coil. The plate voltage is applied across resistor R21 which limits the short-circuit current in case of arcing during the tuning process. The dc-blocking capacitor C22 may be a somewhat difficult component to obtain. Luckily, I had a military surplus mica one, 5,000 pF at 5,000 V in good condition. If a similar one cannot be found, a series-parallel set of ceramic high-voltage capacitors can be used until you reach the adequate capacity and voltage rating. I must point out that a failure in this component may have dangerous consequences to the rig or the operator when applying the high voltage to the antenna terminal. The choke CH4 -600 µH, (from the same military equipment) in parallel with the antenna output is included to ground the HV in case C22 is shorted. In such a case the HV doubler fuse F2 will blow. The plate tank is a "pi" for a nominal output impedance of 50 Ohms. With the design center-value plate voltage and current (2,000 V / 260 mA) the plate loading impedance of the 572B is around 3,500 Ohms. Using the computer programPI7-CMIN.EXE included with ARRL Handbook 1996, page 13.6, with this impedance value we obtain the optimum values for capacity and inductance for the classic bands. The values for the WARC bands are easily calculated by interpolation, and the whole set is depicted in Table 2, where a tank "Q" value of 12 and a minimum plate capacity of 30 pF are assumed.

Table 2. Plate tank values
(Q=12; Rp=3,500 Ohm; Rl=50 Ohm)

Table 3. Taps in the plate coil and capacitors scale.
(Turns count from plate end)

The capacitor C23, the coil L3 and the switch I2 were removed from a piece of surplus military equipment. C24 is a receiving three-section 400 pF variable air capacitor with two sections permanently in parallel, the third section is added while in 80 meter band, allowing greater impedance matching on this band. The maximum capacity available in the plate capacitor added to the output tube and parasitic capacitance only reaches 130 pF, completely ruling out the 160 meter band. On 80 meters we may use a L/C ratio slightly different from those recommended. Because the progressive-shorting band switch available had only 6 positions it was necessary to choose between only six possible bands. This decision fell on the 80, 40, 30, 20, 17 and 15 meter bands, where the "pushing" needs are needed more. (We hope that in a few months a piece of wire on the roof and 10 W will suffice for 12 and 10 meters! HI!). The coil L3 contains 27 turns of silver-plated copper wire, No.14, wound on a ceramic form 52 mm in diameter, and occupying 78 mm. Its 17 µH provides enough inductance for the 80 meter band. Once assembled the taps for the other bands were experimentally determined to the final positions by means of a grid dip meter, while connecting in parallel series-parallel capacitors until reaching the values seen in Table 2 (i.e. 33+3.3 pF in series with 180+18 pF for the 20 meter band, etc. while the output was loaded with a 56 Ohm carbon resistor. Finally, the taps and tuning positions of the variable capacitors were established as shown in Table 3 (linear scale 0-100). Note: On output "pi" capacitor, the minimum capacity (max Load) corresponds to a higher figure on the dial.

Power Supply

For the power supply an excellent transformer (Roselson AL-60) was found, which was originally designed to power a 100 W Hi-Fi tube amplifier. This transformer has a high voltage center-tap secondary providing 820 V, with a current capacity exceeding 280 mA, continuous service. Moreover, it has two low voltage secondaries, one 6.3 V center tap at 5 A and other of 5 V at 4 Amp (for the original rectifier tube). All the power requirements were satisfied on a single transformer. The high voltage is obtained by a doubler full-wave rectifier from the 820 V ac secondary, composed of two branches with diodes and capacitors series-connected. All the diodes are shunted with high voltage surge-arresting ceramic capacitors. The maximum voltage attainable, during rest periods may reach 2,300 V. To endure such voltage a pair of good oil-filled capacitors of at least 40 µF at 1,500 V were required. These are expensive components and difficult to find, so we decided to use standard electrolytic capacitors, knowing that it will be necessary to install trimming resistors in parallel with them in order to equalize the respective voltages. Also, is not easy to find electrolytic capacitors formed for higher voltages locally. While it is possible to manufacture them for voltages up to 550 V, almost the only available ones found are the mass consumer types in the 375-400 V range. With the expected maximum voltage we'll need eight capacitors in series (C5 to C12) to sustain it safely. So we chose cheap capacitors of 150 µF / 385 V, connecting in parallel with each one resistors of 20 kOhm / 4 W (2 x 10 kOhm / 2 W) metal film type, to assure the equilibrium of the total voltage along the series. This series provides an overall capacity of 18.75 µF, that keeps a time-constant high enough for SSB mode, and the 160 kOhm of the equilibrium resistors shunted over the high voltage source provide a 14 mA bleeding current that discharges the battery of capacitors in a short time after switching off the equipment.

The filament voltage for the 572B is taken directly from the 6.3 V ac center-tap secondary. This circuit is wired with thick cable to keep the voltage drop at a minimum, while the other 5 V ac secondary will be used to generate (via other doubling rectifier) the auxiliary +12 V needed for the control circuits and grid bias mentioned earlier.

As a safety precaution, fuses are included in the primary and HV secondary windings of the supply transformer. R1 limits the inrush loading current on the filter capacitors when the relay RL1 closes. This relay is subjected to high voltages and, consequently, it is not possible to use a small cheap one, as there is a real risk of arcing when opening, possibly causing a fire. A solution was the use of a 250 Vac 10 A tri-phased industrial contactor, wiring the three set of contacts in series. An auxiliary voltage is taken from a suitable primary tapping for the small surplus fan, which provides the necessary cooling.

Measuring Circuits

My experience in using power amplifiers allowed me to advise the simultaneous and permanent control of both plate and grid currents and also the output power. While the RF voltage figure on the antenna line is a useful indication, the output power -direct and reflected- is preferably measured by a external wattmeter. So then, the interesting currents and voltages are measured by means of two moving coil instruments. One of them (M1) is a 1 milliamp end-of-scale, shunted with a 500X multiplier resistor, continuously showing the plate current and, for safety reasons, it is connected to the negative return of the power supply. The other instrument (M2) is a 50 µA microammeter in multi-measure arrangement, joined to a function switch that reads either the grid current, the RF voltage at the antenna terminal or the HV voltage. The resistor R12-R16 has an overall value of 50 MOhms (5 x 10MOhm) that will provide a full-scale deflection at 2,500 V, so the instrument scale (50) may be multiplied by 50. Note: at the lower end of the resistor chain is an additional high value resistor in respect to those of the instrument; that keeps high voltage from appearing at the switch when opening the circuit without distorting the measured voltage.

The grid current is measured over the return path between center tap of filament winding and chassis closing the grid dc circuit, while the plate supply return (negative) is done over this point. The value of the multiplying resistor (R25) for grid current measurement will depend on the internal resistance of the instrument used as M2. In this case it has a value of 1317 Ohm; to multiply its scale by a factor of 2,000 (0.1 Amp end-of-scale), the value of shunt resistor needed is, without great error, 1317/2000 = 0.66 Ohm (Four resistors of 2.7 Ohm in parallel). The current value in mA will be the scale figure multiplied by two.

Control Circuits

For the T/R switching of the amplifier, two relays are used, activated by two separated circuits. One of them (RL2) switches the RF signal, while RL3 cuts the driving current to the HV contactor and grounds the tube grid. This arrangement allows easy adjustment of the open-close timing, in a way that assures the amplifier would not be activated without loading. This reduces the risk of arcing on the tuning components or in the antenna relay. The relay control circuit contains four transistors, and it is wired on a silvered-copper-isles perfboard (150 x 90 mm). Transistors Q101 and Q102 are conducting while the PTT terminal is not grounded, cutting Q103 and Q104. Once the PTT/VOX circuit is grounded, Q103 quickly closes the RF relay RL2; a short time later Q104 activates the HV relay. The time delay between both relays is determined by the time-constant R103/C105. During the pass to receiving, when the PTT circuit opens, the C104 charging time delays the Q101 conduction, while Q102 conducts immediately, discharging C105. This means that the HV relay opens first and the RF one opens later. The I5 panel switch (STBY/WAIT) operates or inhibits the control circuits completely. A bicolor LED in front panel shows the operator the three possible conditions; "Standby" (no light), "Wait" (Green) or "Run" (Red) when the PTT circuit is activated. The control circuit uses the auxiliary 12 V obtained from the 5 Vac secondary winding. The relays and circuit do not allow the QSK mode in CW.

In order to avoid the generation of splatter by overdriving and to insure tube life it is necessary to take some measures to limit the driving power. This is accomplished by two circuits; the first one generates an Automatic Level Control (ALC) voltage that will be applied to the suitable input of the transceiver. A fraction of the applied input RF is rectified by a diode (D107) and added to a threshold dc voltage, set by a front panel potentiometer P2. The resulting negative voltage thus obtained (if the RF input exceeds the set level) is applied to the transceiver in order to limit its power at modulation peaks in SSB (or carrier peaking in CW). The other protective circuit cuts off the amplifier if a specified grid current for the 572B is exceeded for a given time. This trip grid current is fixed at 100 mA. The grid current flowing across R25 + R25 generates a voltage that, over an adjustable level (determined by P3) switches on the latch circuit formed by Q107, Q108 via the diodes D107 and D108 switches off the control transistors of the T/R relays. A bright red LED on the front panel shows this condition; the normally-closed switch I6 (on the front panel) allows for resetting the circuit. This idea was taken from The ARRL Handbook, originally using a single transistor and a latching relay, but changing the relay by a PNP transistor and a couple of resistors, that are much less expensive!

Practical Layout

An aluminum cabinet, recovered from an old signal generator, was available, and its size (19"W x 7.5"H x 10"D) easily accommodated the diverse components. The tube was placed in a vertical position, with its socket mounted below the chassis and held with suitable spacers, so only the glass envelope protrudes. This position is preferable to the horizontal one (adapted by several manufacturers), as this minimizes the possibility of grid-filament shorts. As an additional shielding measure between input and output, a piece of aluminum tubing, 50 mm in diameter and 40 mm long, was fixed over the chassis around the bottom side of the tube. The fan is mounted on the rear cover, in an extracting manner and aligned with the tube. On top of the chassis are mounted all the output circuit components as well as the power transformer, laid on its side due its size, and fixed to the chassis with four brackets (photo 4). The input RF transformer, the doubler-rectifier and filter module and the related tri-phased contactor, as well as the measuring and control circuits board were placed under the chassis (photo 5). For shielding and protection the bottom is covered with a punched plate on which are attached four rubber feet to facilitate table-top operation. The front panel holds the meters, the dials and knob controls of the variable load (Antenna) and plate capacitors (this one with an excellent original General Radio planetary demultiplier) and all the controls accessible to the operator (Photo 6). Between the capacitor shafts and the knobs are mounted flexible couplings to eliminate possible mechanical tension. The ALC and RF voltmeter adjusting potentiometers are mounted on the front panel to cover a couple of holes from the original generator layout, but this showed later to be a good feature. Along the lower row are: (left to right) the Standby-Wait switch and green/red LED, the main fuse and filament lamp, the CW/SSB switch, the grid trip LED with its reset switch and the main power switch. The labeling of various controls and knobs was done with adhesive letters protected with a thin layer of transparent lacquer. The use of good quality knobs and gray paint on the rear and top covers produced a good finish to the equipment.

Safety and RFI measures

Voltages higher than a hundred volts are potentially dangerous and it inspires within me a deep respect (the possible 2,300 V at the plate circuit are lethal). Because of this a few safety measures have been incorporated in the equipment against any excess of confidence from the operator. The main power switch effectively cuts both poles of the power line, and the main fuse is placed after this switch as a guarantee that no voltage is present on it when the amp is switched off. All connections with the primary voltage are protected with pieces of thermo rubber. The secondary High Voltage fuse is placed under the chassis, all faston plugs in the HV circuit are protected with plastic isolating covers and the rectifier and the battery of capacitors are hidden under a plastic sheet. In series with the coil circuit of the HV is a microswitch activated by the top cover that cuts the HV if this cover is removed.

The ground lead of the power socket at rear panel (removed from a computer power source) includes a choke coil L1, (5 turns wound on a small toroidal core) to avoid RF dissemination across the ground lead of the power line. A toroidal filter (L2) is also put on the power leads to break up the RF path. The choke CH4, in parallel with the antenna output, is nothing more than a hindrance in electrical terms, but in case C22 shorts, it will ground the high voltage, blowing-up the secondary fuse and keeping HV from appearing on the antenna circuit. The chassis is provided with a solid independent ground connection, that may be joined with all other equipment chassises to eliminate any ac potential difference between them. Let me point out that it is very useful to use a grounding bar, placed behind the work bench, and attach the ground leads of all chassises.

Warning

The High Voltage measurement if desired requires disabling ALL the safety measures of the amplifier, and is a risky maneuver. Be very careful! Be sure that your tester is able to measure up to 4,000 Vdc and that you are using the correct plugging point and scale. Do not disable the upper cover safety microswitch! Plug the equipment into the mains, close the power switch and ground the PTT lead. Attach the negative tester lead to the chassis and using ONLY ONE hand measure the high voltage at the positive terminal of the doubler-filter board. Do not ever touch the tube anode cap!

Getting Started and Initial Tests

After the assembly is completed and it has been determined that the tapings on the tank coil are correct as explained, and the entire circuit has been thoroughly examined, remove the HV secondary fuse. With the tube installed, but without the upper or lower covers, check the filament voltage on the socket terminals with a good ac voltmeter. Deviations higher than 10 % may possibly be fixed by modification of primary taps. Place the "standby- wait" switch to "Wait" and momentarily ground the PTT lead at rear socket and check that the dual-color LED lights red and the relays close and open in the correct sequence. Temporarily set the grid trip potentiometer P3 at mid-range and momentarily connect a 56 Ohm / 2 W resistor between points "D" (+12 V) and Gt on the control module. This simulates a grid current of some 130 mA. Ground the PTT terminal and slowly turn P3 counterclockwise until the limiter circuit trips. Disconect the 56 Ohm resistor and turn P3 back a bit; then push I6 to restore the function. P3 doesn't need any more adjustment.

Unplug the equipment from the AC power, replace the HV fuse, install the covers, connect the RF output to a dummy load across a suitable wattmeter, link the RF input to the output of a HF transceiver, attach the PTT and ALC cables and start the "live" testing. Apply power to primary and let the filament warm up a few moments. The normal color of the filament is bright yellow. With the "Standby-Wait" switch in "Standby" (green/red LED off) and the CW/SSB in CW position, load the transceiver in CW on the 80 meter band, adjusting the output to about 25 W. With the meter switch in RF measuring, the meter will measure the RF level at antenna terminal, adjust the Vrf potentiometer for a reading of about 10% full scale. Then reduce the transceiver output to zero and switch off the transmission. Set the "Band" switch to 80 meter, the "Plate" capacitor to about the 85% of its capacity and the load ("Antenna") to 50%. Switch the meter to measure Ig (grid current). Switch to "Wait" and push the PTT (or "SEND") key of the transceiver; the red LED may light. The idle plate current may reach about 40 mA. Increase drive noting a plate current increase when plate current reaches 100 mA, turn the "Plate" knob slowly finding the resonance point, noted by a dip in plate current. Increase the drive until the current reaches about 225 mA and then slightly move the plate and load capacitor knobs to obtain maximum output power. The correct point is such that the plate current is 260 mA, the current dip is barely noticeable; this point coincides with a maximum output power. During the tuning process, watch for a maximum grid current of about 50 mA (half meter scale). Attention! At full dissipation (160 W) the tube plate shows a uniform red color. Do not let it turn to orange! Increase drive until the grid current exceeds 100 mA and check that the electronic trip protection works, switching off the amplifier automatically. Reduce drive again and push I6 on the front panel to restore operation. Note the setting points of the plate and load capacitors. Change the meter switch to HV and check the high voltage reading. Then switch it to Vrf and set the Vrf potentiometer for about a 70% reading at full output power.

Switch off transmission, switch the transceiver to SSB and set the amplifier CW/SSB lever to SSB. Push the PTT on the transceiver and, while talking normally in the front of the microphone, increase Mic Gain from a low value (without any audio processor) until the ALC reading on the transceiver reaches mid scale. The plate current, which without modulation may be about 50 mA, will swing to approximately 250 mA. Alternately adjust the ALC potentiometer on the amplifier and the Mic Gain on the transceiver until both the ALC reading just reaches the end of the ALC scale and 260 mA plate peaks are attained without activating the grid trip circuit. Note: The greater control action of the ALC ("braking") occurs when the potentiometer center arm goes closer to positive (normally counterclockwise).

Do not be surprised if when talking normally in SSB it is difficult to surpass an average plate current value of 160 mA. This is perfectly normal and is due to the inertia of the meter system that does not allow us to measure peak values, showing only the lower average ones. Check the functioning of the amplifier protection circuit (and for neighboring frequencies!) by activating the audio processing in the transceiver and speaking into the microphone with Mic Gain overly increased.

Remember it is favorable to operate the amplifier with higher loading values even though it slightly reduces the maximum available power. By turning it counterclockwise the loading increases while the capacity decreases in the "pi" output capacitor.

Repeat the process on the other bands and note the relative positions of the plate and antenna knobs. Check the amplifier stability, disconnecting its input and activating the PTT. The RF voltmeter and/output wattmeter should not show any reading at any condition of tuning and loading. Attach the amplifier to a suitable antenna system and ask for reports from expert colleagues at medium distances. The insertion of the amplifier may provide an increase in signal level of about 6 dB (between 1.5 and 2 "S" units).

Epilog

The project and assembly of the amplifier was the result of my longtime wish to improve my installation and to get some hours of technical amusement at a reasonable cost. Both wishes were widely accomplished. I look forward to hearing from you in the next pile-up!

This translation was finished the October 28th, 1997. All rights reserved.

Xavier Paradell S.*, EA3ALV
* Industria 337-SA2 E-08027 Barcelona, Spain

Click on photo for larger version

Photo 2Photo 3Photo 4Photo 5Photo 6
Photo 7

Figure 1 - Figure 2 - Figure 3 - Figure 4 - Figure 5 - Figure 6

Components List

Capacitors (values less than 3000 in µF, except otherwise specified):

C1,2,3,4 Ceramic disk, high voltage, 4700 pF, 2000 V, 20 %
C5,6,7,8,9,10,11,12 Electrolytic, 150, 385 V
C13,14,17, 25,26,27,101,102,106,107,108,110 Ceramic disk, .01, 100 V, 20 %
C15,16 Electrolytic, 2200, 16 V
C18,103,111 Ceramic disk, .1, 100 V, 20%
C19 Ceramic or mica, .005, 3,000 V, 20 %
C20,21 Ceramic disk, .0047, 100 V, 20 %
C22 Mica .005, 5,000 V, 20 %
C23 Air, variable 15-120 pF, 4,000 Vdc
C24 Air, variable, 3 x 350 pF, receiver type
C28 Ceramic disk, 22 pF, 250 V, 10 %
C29 Ceramic disk, 68 pF, 250 V, 10 %
C104 Electrolytic, 10 µF, 25 V
C105,109 Electrolytic, 47 µF, 16 V

Coils

L1 5 turns insulated wire 2.5 mm dia. wound on a toroidal iron powder core
L2 2 x 5 turns wound on a toroidal iron powder core
L3 27 turns, 52 mm dia. 78 mm long. Silvered copper wire 1.5 mm dia on a ceramic form (see text).
CH1 Choke coil, 100 µH, 100 mA on ferrite rod 6 x 30 mm
CH2 4 turns, self supported coil 1" x 1", 1.5 mm diameter wire.
CH3 Plate choke (see details on text)
CH4 Choke coil, 600 µH / 250 mA, air core
CH5 Choke coil, miniature, 56 µH
CH6 Choke coil, miniature, 68 µH

Resistors

R1 Wirewound 50 Ohm, 15 W, 5 %
R2..9 Metal film 20 kOhm, 4 W, 5 % (2 x 10 KOhm in series)
R10 Wirewound, 0.22 Ohm, 2 W, 5 %
R12..16 Carbon, 10 MOhm, 1 W, 5%
R17 Carbon, 1.5 MOhm. 1 W, 10 %
R18 Carbon, 47 Ohm, 2 W, 5 %
R19 Carbon, 75 Ohm, 40 W, 5 % (40 x 1.500 Ohm, 2 W in parallel)
R20 Carbon, 30 Ohm, 2 W, 10 %
R21 Wirewound, 15 Ohm, 5 W, 5%
R22 Carbon, 47 KOhm, 1 W, 5 %
R23 Carbon, 4.7 KOhm, 1/4 W, 5 %
R24 Carbon, 22 Ohm, 2 W, 5 %
R25 Carbon, 0.66 Ohm, 1 W, 5 % (4 x 2.7 Ohm 1/4 W in parallel)
R101,102 Carbon, 15 KOhm, 1/4 W, 5 %
R103,104,105,106,116 Carbon, 4.7 KOhm, 1/4 W, 5 %
R107,108 Carbon, 10 kOhm, 1/4 W, 5 %
R109,110 Carbon, 560 Ohm, 1/2 W, 5 %
R111,117 Carbon, 47 kOhm, 1/4 W, 5 %
R112,115, Carbon, 1 kOhm, 1/4W, 5 %
R113 Carbon, 2.2 kOhm, 1/4 W, 5 %
R114 Carbon, 1 MOhm, 1/4 W, 5 %
R118 Carbon, 22 KOhm, 1/4 W, 5 %
R119 Carbon, 1 kOhm, 1/2 W, 5 %
R120 Carbon, 5.6 KOhm, 1/4 W, 5 %
P1 Carbon potentiometer, panel type, 25 KOhm, linear
P2 Carbon potentiometer, panel type, 47 KOhm, linear
P3 Carbon potentiometre, pcb vertical adjust, 4.7 KOhm

Diodes

D1,2,3,4 Silicon diode 800 V / 1 A (1N4007 or equ.)
D5,6 Silicon diode 250 V / 1 A (1N4004 or equ.)
D7,101,102,103,104,105,106,107,108 Silicon diode 50 V / 50 mA (1N4149 or equ.)
D8 Zener diode 6.8 V / 1 W D109 Red LED diode, Hi-brite
D110-111 Bicolor Red/Green LED diode

Transistors

Q101,102,106,107 Silicon NPN 30 V / 100 mA hfe>80 (BC548 or equivalents)
Q103,104 Silicon NPN 45 V / 500 mA hfe>50 (BC337 or equiv.)
Q105,108 Silicon PNP 25 V / 100 mA hfe>50 /BC558 or equiv.)

Transformers

T1 Power supply; primary 210/220/230 V. Secondaries: 820 V @ 280 mA; 3.15+3.15 V @ 5 A; 5V @ 4 A. (Roselson AL60W or equiv.)

T2 Toroidal wideband RF transformer, twin secondary, ratio 1:2 (See text).

Miscellaneous

F1 Glass fuse, 5 x 20 mm 250 V/6 A, slow
F2 Glass fuse, 6 x 30 mm 250 V/1 A, medium
I1 D.P.S.T. panel switch (250 V / 10 A)
I2 RF Switch, ceramic wafer, 6 position 1 circuit.
I3 Panel rotary switch, 3 positions 1 circuit
I4 S.P.D.T. panel switch (120 V / 3 A)
I5 S.P.S.T. panel switch (120 V / 3 A)
I6 Panel pushbutton switch, 1 circuit (normally closed)
J1 Universal 2-pole+ground AC power socket, male
J2,3 SO-239 panel sockets
J4,5 Fono RCA panel sockets
M1 Moving coil milliammeter, 1 mA / 100 Ohm
M2 Moving coil microameter, 50 µA /1325 Ohm
P1 Panel pilot lamp, 6 V / 0.2 A
RL1 Tri-phased contactor 250 V/10 Amp, coil 220 Vca
RL2 Relay 2 circuit 2 positions, coil 12 V / 270 Ohm
RL3 RF relay, 2 circuit 2 positions, coil 12 V /170 ohm
V Axial fan, 85 mm dia. 20 cfm. 200 Vca (Tobishi 3951 or equiv.)
µI Microswitch, 250 V / 1 A 1/4" throw
Cabinet, knobs, dials, fuse holder, tube socket...

**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.

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