When I was a boy (now I’m 47), father some time said me that “se puede obtener buena calidad de un AM” (It is possible to get good audio quality from an AM receiver).
This tuner has been designed having in mind this idea, and an animus or spirit of to experiment some own ideas that I previously had in mind, and never before tested (by me) in a real circuit. Also, I used in it, materials recovered from the box of old stuff, most of them has been kept unused for more than a decade.
The first thing that I wanted to test, is an AM receiver with a RF amplifier stage, using a remote cutoff pentode, with the ferrite rod antenna (aerial) directly wired to the signal grid. Looking into my stock of tubes, I found an EF89 for the stage.
The gang condenser (G in the schematic) is triple equal sections made with brass sheets, at both stator and rotor plates, rescued from any Philips radio receiver.
The antenna (L1), the RF inter stage transformer and the oscillator coil has been designed following what is suggested in the Langford-Smith’s (LS) "Radiotron" book. The antenna I used is a rod saved from a discarded transistor radio, but rewind completely for this purpose using Litz wire, the last recovered from the inductor in wasted electronic ballast for fluorescent lamps. As I lack of characteristics or data sheets of the ferrites employed, a test coil has been made in each of them, and tested with a RLC digital meter.
RF transformer (L2) and oscillator coils (L3) use the same wire, but for them, I recycled the bobbin, the core and the magnetic shield from PC monitor horizontal width coil control.
The choused converter tube is an ECH81, the signal grid of the hexode is tied to the RF tuned transformer, and the injection grid wired together with the oscillator, which has the form of a tuned-grid tickler-coil (or Armstrong) oscillator, with series feed in both windings.
The RF and antenna coils have been calculated as is suggested in chapter 23 of the LS's book. Oscillator coil, padder and trimmers make use of the formulae at chapters 24 and 25, mainly from Section 3 of the last, in which the tracking between RF and oscillator method is described.
IF section follows those described in Chapter 26, but making use of standard double-tuned mutual-inductance-coupled transformers. The tube selected is a remote cutoff pentode type EBF89, which also includes a dual diode intended for the audio detection.
At the detector stage, start to flow my own ideas. First, the AGC line isn't derived simply filtering out the audio signal at the detector, in fact it has its own transformer (IFT3), rectifier and filter. More over, the filter isn't classic at all.
Audio signal is detected using both diodes, connected in a non standard manner. The AM signal is rectified, and no capacitors are found in the signal path to ground, because it generates audio distortion as is demonstrated by LS, and by S. Seely in his book entitled "Electron-Tube Circuits" (Pages 351 and the followings). The second diode cooperates to the signal detection, improving the rectification, acting as a shunt diode (Idea taken from the Seely's book, page 125, figure 7.4 and associated text). This way I gain more than 6dB of recovered audio respect to single diode detection with no extra components added.
Filtering out the RF signal is made in a similar way of those inductor input rectifiers, but at the cathode of a follower, then at lower impedances, I can use sane values of inductances and capacitors. L4, L5 and C15 forms a 3 pole
T filter, designed to a crossover frequency of about 10KHz. C14 bootstraps the grid circuit of the follower, and thanks to it, more than 4MΩ is viewed from the grid, so a small AC load to the detector circuit is offered, also improving audio recovered quality. The two 36mH coils are extracted from wasted cassette recorders. A 2KΩ potentiometer in the cathode circuit is the DC and AC cathode load and also acts as a volume control, and sound take off. The triode cathode follower is one section of an ECC82 envelope. An additional advantage is that audio output is made at low impedance.
The AGC section also includes ideas from other sources. A tap in the primary of the second IF transformer is coupled to a third tuned coil, in which normal rectification is done, thanks to one of the diodes inside the EABC80 triple diode-triode tube. The amount of signal transferred from one circuit to other is dependent of the value of capacitor C11 linking them, this way the AGC signal level can be adjusted independently of the audio one.
The DC from the AM signal recovered is filtered at the higher FI frequency by C19, but audio signals are still present in the load resistor R20, whose other end returns to a fixed voltage of -2.5V provided by a (provisional) NiCd battery, also performing other tasks in the circuit. Then, the minimum bias at the AGC line is -2.5V, sufficient to the tubes selected, whose cathodes are earthed directly.
Audio signals in the load resistor are removed in the next filter section that includes a double time constant circuit R19-C17 and R18-C18 (See the behavior of this network in the simulation below). The purpose of it is to let the AGC line some gain at frequencies between 1 and 10Hz, just that may occur when rocking the dial between strong and weak adjacent station’s signal; or fading. The 100KΩ and the 3.3µF mesh doesn't account for those quick variations, giving fast response to sudden variations of incoming signals. When the tuned station signal is stable, very low frequencies are filter out by this network as ordinarily. The voltage developed at the AGC line is directly feed to the IF and converter stages by means of a low time constant decoupling networks to the bottom of the grid coils (R1-C1, R3-C5, R6-C9). Relatively high capacitor values at these points ensure easy RF return from the grid to the cathode of each tube.
RF AGC has another difference from standard circuits. Minimum negative bias is injected to the grid of the RF amplifier, via the isolated diode in V4, also contained in the EABC80 envelope. But at its anode, also is added a positive current derived from the screen grid stabilized voltage, and whose value is selected via a preset, P3 and the 10MΩ resistor R21. Diode clamps this voltage at this point to the -2.5V level. When signal strength is sufficiently strong, then the negative current flowing trough R17 from the ACG voltage developed, overcomes the positive current, and then bias to the RF stage is correspondingly increased, as D4 goes cutoff (Plate goes negative respect to its cathode). This way, RF amplification is held at a maximum for lower signal levels, making better the signal/noise ratio of the stage. This idea is called "sinking diode" borrowed from the LS book, at page 1117, figure 27.38B and the explaining text at page 1108, and adapted to my circuit. This sketch also have been used in tube TV sets, I saw some schematics using this topology.
The triode inside the EABC80, the second one in the ECC82, the full voltage of the NiCd battery and the associated circuitry performs an adjustable and stabilized 105V supply to be used by the oscillator’s plate and the screen grids of the RF amplifiers, via a decoupling resistor and capacitor in each one of them. This way the voltage is kept stable at +105V and independent of the AGC and line voltage, improving AGC action.
Plates are powered from a +275V source, via a 1KΩ resistor R12, which limits current in case of a short circuit or overload when manipulating the board (still under development), and putting across it a voltmeter, the current drawn in milliamperes is easily read. C21 decouples this bus to ground. Similar resistor is inserted in the anode of the voltage regulator (R13), for the same reasons and C22 keeps this wire to earth potential for RF signal. Decoupling between stages is made by 2.5mH coils from PC monitors and .22µF caps to the earth bus.
A 1mA FS 42Ω permanent-magnet moving-coil milliammeter is used as signal strength meter (S-meter), and a VTVM is constructed around a second ECC82 V6 tube. The –11.5V via a zero pot is injected to one of the triodes, while the full AGC is feed to the other. The difference between them is converted to a current who deflects the needle of this instrument. It has been designed by me to ensure full scale with about -10V at the AGC bus, sufficient for receive a local powerful station without overloading the meter.
The full spread sheet is available upon request, in Spanish and in a txt
CP437 format, made with the edit.com from MS-DOS 7.10, this file also includes some ideas (some of them not tested or implemented yet), calculus, formulas, and sundry data collected from books, the Internet and from my own imagination.