SOME DETAILS OF THE LF SET

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The picture at left shows the complete set for 137 Kc, RTX, Preamp., 200 W P.A. and the new 400 W P.A., while the layout at right shows the connections between the various items; the Wattmeter at the output of the amplifier and the Ammeter at the cold output of the matching box are not shown.

The RTX (see the layout below) is a single conversion set with an intermediate frequency of only 3 Kc. The band-pass filter, 100 Hz bandwidth, uses 4 high Q coils, 40 mH each, arranged as double stage parallel tuned coupled circuits. The image problems have been solved with an RF preamplifier equipped with a suitable filter. The local oscillator uses two VXOs in the range of 14 Mc, mixed and filtered opportunely. It serves also as carrier generator for the TX circuitry, that consists only in an adjustable gain amplifier giving an output of about 100 mV , in order to agree with the input level of the TX preamplifier(see below). Recently a notch filter has been inserted (see the layout at left and the circuit below), wich generate a very narrow deep( only 10 or 15 Hz wide) very useful to attenuate a strong incoming signal avoiding or at least limiting the AGC action and so permitting a comfortable reception of a weak contemporaneous signal. The circuit used is the well known 'gyrator', that allows an easy regulation of the deep value and may be tuned on the wanted audio frequency. Being the gyrator a relatively low level device, I've lowered the gain of the IF amplifier and improved the gain of the AF amplifier. Return to 472 Kc page

The PREAMPLIFIER uses an AF integrated circuit, the LM380, followed by a TIP35 operating near the class B. The output power is around 12 watts, and the input may be switched to an SSB phasing type generator that uses two old SN76514 mixers and an RC phasing network, optimized around 800 Hz AF input; that allows operation with Hell or Jason, using the AF tones generated by the available programs (see the circuit below).




The 200 W P. A. is a single stage using 4 transistors TIP36 in parallel, working almost in class D ( the wave shape at the collectors is nearly a square wave). The choke L1 is a 7 uH air cored coil, 21 turns, 33 mm winding diameter, 2 mm dia silver plated copper wire 2.54 mm spaced. The Tank coil L2 is also a 7 uH air cored coil, 14 turns, 60 mm winding diameter, 3 mm dia silver plated copper wire 5.08 mm spaced. This coil, togheter with the capacitor C7, act as a very effective L filter, lowering the armonics more than 40 dB. Output power is 200 W with an efficiency over 60 %. The circuit has been a bit modified recently to improve the safety of transistors (see the circuit below)


The Supply unit is a 24V brute force design, roughly regulated by a choke-input filter with swinging choke, 76000 uF capacitor and 30 ohm bleder resistor (see picture above).


The new 400 watts P.A., recently built, uses a very old tube, the TC2/250, it was realized as well to increase the power, but, above all, as a revival for this 80 years old direct heating triode. Being the height of the tube about 40 centimeters, the case results a bit bulky as appears in the picture at top of this page.

The circuit at right shows some complexity in the power supply, having used components alredy at my disposal but not utterly adequate for this purpose, particularly I was compelled to use a voltage doubler to obtein the requested 2000 volts, from a trasformer with only 800 volts output. At bottom, on the right, is visible a safety circuit that switches off the H.V. in case of lack of the grid bias voltage. SW1 is the main switch, SW2 switches on the H.V. trasformer and SW3 switches over the primary of this trasformer to obtein a reduced voltage for tuning purpose. F1 is a fan that forces the air to flow from the pierced base to the top of tube so securing a suitable cooling, F2 is a small fan inserted in the vicinity of the output components, to limit the heating due to the losses in the tank coil and tuning capacitors.The construction of these last, marked with an asterisk, has been the greatest problem because the heating due to the losses in dielectric, at last, having tried with various materials, the problem has been resolved using TEFLON as dielectric and copper foil for plates. The resistor RI is actually a group of resistors whose value has been adjusted to minimize the unmatching with the 50 ohm output of the 200 watts P.A. used as input device with its power limited to about 50 watts. T1 is a toroidal transformer, with 12 turns primary winding and 60 turns secondary winding, carefully insulated, being the primary referred to ground and the secondary referred to -1200 volts. L1 is a trap, 8 turns, 1 mm. dia silver plated copper wire, wound over the 51 ohm resistor. The tank coil L2 is wound over a PVC tubing with 70 turns,1 mm. dia enamelled copper wire, 85 mm. winding diameter, L3 is the output link, 12 turns of the same wire, wound over a PVC ring movable up and down along the tank coil to perform a variable coupling. L4 is a 4 H inductor, performing, togheter with the 300 F capacitor, an effective choke-input filter, The value of the resistors RA and RV depends obviously from the caracteristic of the meters used. Return to 472 Kc page

The picture at left shows the amplifier with the back cover removed, the picture at right shows the inside of the base, where are arranged the components of the supply circuits.(Click over to enlarge) The TC2/250, although rated 500 watts output at 2000 volts class C telegraphy, works actually with 1750 volts anode voltage, 360 mA. anode current, 630 watts d.c. input, 400 watts output power, efficiency about 63 %.






Cesare Tagliabue I 5 TGC
Updated: October 2009


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