Scott Phantom Deluxe
Scott Phantom Deluxe radio (1940)Here is an exceptional custom-made all-wave receiver, combining amplitude and frequency modulation receiving systems, with a power output of up to 40 watts and a frequency range of 30 to 15,000 Hz! Its sound system therefore takes full advantage of F.M.'s hi-fi characteristic; the A.F. amplifier is particularly suitable for phonograph reproduction and includes a needle scratch suppressor system. Other circuit innovations are described.
The receiver described here employs 28 tubes and is designed for the reception of amplitude-modulated signals in the frequency range of 540 kHz. to 23 MHz. (about 13 to 550 meters); and frequency modulated signals of the wide-band type in the frequency range of 41 to 50 MHz.
Chrome chassis of the power amplifierThe amplitude and frequency modulation sections are separate superheterodyne receiving systems up to the input circuit of the 1st stage of the audio-frequency amplifier, which is capable of handling either type of program with full fidelity and volume range as transmitted.
Eight of the tubes function solely for the benefit of frequency modulation; 9 tubes for amplitude modulation; 8 tubes in the A.F. amplifier; 2 in the power supply and 1 tube as a common voltage regulator. Eight controls on the front of the chassis, including the tuning knob and its associated vernier, permit adjustment of the performance characteristics to meet varying conditions of reception.
THE R.F. SYSTEMThe R.F. and I.F. systems and the 1st 2 audio-frequency stages are assembled on this base; with the remainder of the A.F. amplifier and the power supply on a separate chassis, which is usually mounted with the speaker system in the lower section of the cabinet. The variable condenser associated with the F.M. section is assembled on a common shaft with the standard tuning condenser, and both are housed under the long, rectangular shield toward the left-center of the chassis. The R.F. and mixer tubes of this section are located near this shield. The F.M. intermediate-frequency section is located on the extreme right-hand side of the chassis with the audio output feeding through a shielded cable to the band switch, where it is connected to the A.M. amplifier input circuit when the F.M. range is selected.
Separate antenna connections for the A.M. and F.M. are provided in the rear of the chassis, so that a double doublet system may be used for the shortwave- and broadcast-band signals, with a separate dipole of the proper length for the F.M. signals. This arrangement insures maximum efficieney on all bands and is quite practical where the listener is desirous of obtaining the best results. The F.M. antenna transformer is designed to match a low-impedance line and dipole as an added precaution against noise pick-up in the lead-in, which should be of the close-spaced type for this band. Electrostatic shield rings are employed between the primary and secondary windings of all of the A.M. antenna transformers to insure a minimum of noise pickup from the lead-in.
THE F.M. SYSTEMThe F.M. uses the early 41 to 50 MHz.
In the F.M. section the high sensitivity level possible with this tube line-up results in limiting action on signals having a strength as low as 1 microvolt-per-meter. The screen-grid and plate potentials of the limiters have been adjusted so that the overall action is absolutely flat after the knee of the output curve is passed. Therefore, no distortion from any artificially created amplitude modulation is possible once the limiters have commenced to operate. The I.F. selectivity is adjusted at the laboratory so that it is sufficiently broad to prevent phase distortion which can result when the selectivity is too sharp or unsymmetrical.
An intermediate frequency of 5.25 MHz is used to obtain high image and spurious signal rejection ratios, and the temperature-compensated oscillator operates on the low-frequency side of the incoming signal to avoid image signals from the television bands and to insure maximum stability.
The F.M. tuning indicator, which is located on the right-hand side of the chassis above the dial, is operated by the D.C. voltage across the discriminator load resistors and, therefore, it is a true zero center meter. The discriminator circuits are properly loaded to eliminate distortion and are tuned by air dielectric condensers to prevent any drift in that characteristic. The peaks of this curve are separated by 250 kHz to allow a sufficient margin of linearity beyond the total wide-band modulation swing of 150 kHz.
The adjacent-channel selectivity of the I.F. amplifler is such that any portions of the detector characteristic outside of the 200 kHz channel receive a minimum amount of voltage from any adjacent-channel signals.
THE A.F. SYSTEMDegenerative feedback is used to keep the distortion level at a minimum. A maximum output of 40 watts, with approximately 35 watts of undistorted power, is available.
In the electrical fidelity of the audio amplifier as it operates in both the A.M. and F.M. bands is shown. The high- frequency peak around 6,500 Hz compensates for the attenuation of the 455 kHz I.F. system used in all A.M. bands. This peak is eliminated in the F.M. position by contacts on the wave-band switch which selects all of the circuits required for that type of reception. Treble and bass response controls on the receiver provide a certain degree of quality adjustment.
To obtain a high degree of electrical fidelity a special loudspeaker system, consisting of 2 high-frequency tweeter units and one 12" Iow-frequency unit has been developed and has been found to provide a sound pressure response which is effective up to 15,000 Hz as measured on the axis of the high-frequency units. The high-frequency output energy is separated from the low-frequency energy by a constant resistance filter before it enters the high-frequency speakers. The speaker units mounted on a baffle together with the special filter network.
PHONO NEEDLE SCRATCH SUPPRESSORAn interesting feature in the audio amplifier is the phonograph needle scratch suppressor system, which is mentioned below in connection with the tube complement. It functions by reflecting an appreciable shunt capacity across the input terminals of the audio amplifier when weak audio voltages are applied. As soon as the audio signal level rises above the threshold level the shunting capacity is automatically removed and the fidelity is restored. The result is that relatively weak needle scratch and surface noise between the musical passages of a phonograph record are effectively silenced; but as soon as a musical passage appears, the audio voltage becomes sufficient to release the bypassing action. This circuit is also capable of increasing the intelligibility value of some weak A.M. signals in the shortwave band.
TUBE COMPLEMENTThe tube complement of this receiver is divided in the following manner:
This article has been prepared from data supplied by courtesy of Scott Radio Laboratories, Inc.
From "Radio-Craft" for September 1940