A NEW 23-TUBE HIGH-FIDELITY ALL-WAVE RECEIVER
By E. H. Scott
President of E.H. Scott Radio Labs, Inc.
Reproduced from the June 1935 issue of Radio Craft
The receiver described here is the first true high-fidelity set designed by E.H. Scott with truly then-modern features. The previous model had two tuning dials, plug-in coils for band-switching, and other less-than-desirable features. Here is the Scott receiver that truly began the Scott high-fidelity era. With minor modifications it was produced until the introduction of the Philharmonic series a couple of years later.
The all-wave receiver to be described has been designed primarily for the radio fan who is looking for an instrument which will give the best possible reception of distant stations in every part of the world, combined with the finest tone quality.
Laboratory tests prove it has twice the frequency range of even the finest of the "high fidelity" receivers available today, with a practically flat response from 25 to 16,000 cycles. The selectivity is continuously variable from as sharp as 10 kc. at 5,000 times field strength, to a band wide enough to allow frequencies up to 16,000 cycles to pass without attenuation. Such results have never before been attained in any excepting a few extremely costly receivers built especially for laboratory use.
The new Scott Imperial All-Wave receiver is a very highly developed superheterodyne, with a wavelength range from 13 to 556 meters. The 23 tubes incorporated in its design are used as follows: R.F. stage using one type 6D6 tube; oscillator using one type 76 tube; oscillator voltage regulator using two high-conductance gaseous tubes; converter stage using one type 6A7 tube; first, second, and third I.F. using three 39/44 tubes; fourth I.F. stage using one type 6D6 tube; detector, I.F., and A.V.C. system using one type 76 tube; first audio stage using one type 6C6 tube; second audio stage using two type 6C6 tubes in push-pull; third audio stage using four type 2A3 tubes operating as parallel push-pull pure class A power output tubes; rectifiers using one type 83-V heavy duty tube and one type 5Z3 tube; noise suppressor using one type 76 tube; beat frequency oscillator using one type 76 tube; R.F. and converter A.V.C. amp., 6B7; and, a 76 as tuning meter amplifier.
It will be noticed that although twenty-three tubes are used, a careful study of the specifications will show that every single tube incorporated in the set has a very definite function to perform.
FULL-RANGE HIGH FIDELITY
If a radio receiver is to give perfectly natural reproduction on all notes or sounds, it must have a frequency range that covers the entire tonal range of the human ear, or from 25 to 16,000 cycles. (Some musical instruments are richer in harmonics or overtones-the higher frequencies that enable one instrument to be distinguished from another-than others; for example, the harmonics of the cello go up to 8,500 cycles; the bass clarinet to 10,000 cycles, and the violin to 14,000 cycles.) It can clearly be seen, therefore, that if you are to secure absolute naturalness and lifelike fidelity of musical instruments or voice, a radio receiver must be capable of reproduction on all frequencies without attenuation from as low as 25 cycles up to 16,000.
At the present time, the highest frequency reproduced by sets in each "class" is as follows: popular-price sets, up to 3,500 cycles; the medium-price receivers, up to 5,000 cycles; higher-price models up to 6,000 cycles; and a few of the high-price "high fidelity" receivers, up to 7,500 cycles. The new "Imperial" has twice the frequency range of the latter receiver, thus establishing a new standard in home radio reception.
Schematic of the tuner chassis of the Scott Imperial
(Click on schematic for larger view)
Amplifier and Power Supply of the Scott Imperial receiver
(Click on schematic for larger view)
NEW POWER AMPLIFIER PREVENTS OVERLOADING
High fidelity is a function not only of frequency but also volume. To secure the most perfect reproduction at all times, it is necessary that the receiver be capable of handling every loud passage or "peak" that comes in without overloading or distorting. Most of the time the audio power level does not exceed 6 watts, but there are often dozens of passages in the course of a single program where "peaks" or loud passages may rise for short periods to power levels as high as 30 or 40 watts, and it is necessary that we have a reserve power of about five times above the normal level if we are to eliminate distortion during these loud passages or sudden "peaks" in the reproduction of speech and music.
One of the many unusual features of the power amplifier of this new set is the 35 watts of undistorted output with strict class A operation and from 35 to its full 50 watts, class A prime. These results have been attained in our new amplifier by the use of a constant fixed bias; practically ideal plate voltage regulation, having an exceptionally low resistance; the use of a total filtering capacity exceeding 100 mf.; a first A.F. stage using a 6C6 as a triode; a second A.F. stage using two type 6C6 triodes in triode push-pull; and a third audio stage using four 2A3 triode tubes operating as parallel push-pull pure class A output tubes.
SELECTIVITY CONTINUOUSLY VARIABLE
The degree of selectivity possessed by a receiver determines its ability to tune through powerful local stations and bring in weak distant signals. To secure the maximum degree of selectivity, and at the same time a high degree of fidelity, a new selectivity-fidelity control is incorporated, which is continuously variable. In the most selective position, adjacent channel discrimination of approximately 5,000 to 1 is obtained, while at the maximum fidelity position, reproduction up to the limit of the human ear, or the limit of the highest frequency being broadcast by the station selected is obtained. This enables the listener not only to reach out and bring in weak distant stations which would ordinarily be blanketed by interference from powerful nearby stations on adjacent channels, but also to listen to programs with the most perfect tone ever heard from a radio receiver.
Four stages are used in the I.F. amplifier system which represents the ultimate in high gain, combined with absolute quietness of operation and exceptional selectivity. By using four I.F. stages, less gain is required per stage. This permits use of the type 39/44, the high plate impedance of which more nearly matches into the tuned primary of the I.F. transformer.
DOUBLE A.V.C. SYSTEM
Everyone who has tuned for weak, distant foreign stations knows from experience how reception is frequently spoiled by the constant fading in and out of the signal. To obtain the best possible reception from stations in all parts of the world, this receiver incorporates not merely the regular single type of A.V.C., but two distinct A.V.C. systems, each designed to provide the most efficient A.V.C. action, and keep the signal practically constant at any desired volume level, irrespective of variations in signal strength.
For the finest noise-free reception of very weak to moderately strong signals, the R.F. stage should be operated at all times with maximum gain-that is, with no A.V.C. in it. However, due to the great field strength of local and super-powerful broadcast stations, the R.F. tube would be overloaded when tuned to these powerful stations if it were worked without A.V.C. This problem has been solved by providing a separate R.F. stage, A.V.C. system, which allows the R.F. circuits to operate at maximum efficiency, but prevents overloading of the R.F. and converter tube when tuned to a very powerful local station.
A second section of the A.V.C. system controls the converter and I.F. tubes. The major part of maintaining the volume level constant in reception of all signals from the weakest to the strongest stations, is done by this part of the system, which is adjusted so that its action extends completely down to the noise level of the quietest possible location.
BETWEEN-STATION NOISE ELIMINATED
If a receiver is to bring in station programs from distant parts of the world, it must have a very high degree of usable sensitivity. If it is to hold the signal from a distant station at a constant level, it must have a very efficient A.V.C. system. But a highly sensitive receiver. with a very efficient A.V.C. system, means that when you tune between stations your A.V.C. opens up the full sensitivity of the receiver, in which case, unless you are in a location that is extremely quiet and free from all forms of electrical interference, you can't help bringing in a large amount of noise when tuning from one station to another.
The idea of the noise suppressor between stations is not new. However, in all systems so far introduced, the principal fault has been to destroy the effectiveness of the A.V.C. system when the noise suppressor was in operation, and also to cause considerable distortion on medium weak signals. All of these defects have been eliminated in the new noise suppressor incorporated in the receiver described here.
In this short article it has been possible to touch on only a few of the outstanding features of this new receiver. In addition to those mentioned, it also has incorporated a perfected bass control, which allows the bass response to be adjusted at five separate cutoff points between 25 and 150 cycles; an attenuation equalizer at 10,000 cycles, which eliminates the high-pitched whistle from adjacent channels when using the receiver for reception of programs above 10,000 cycles; a visual indicator making it easy to tune in every station perfectly; an improved beat-frequency oscillator which makes it just as simple to tune in stations on the short waves as on the regular broadcast band; a self-stabilized oscillator with voltage regulation which keeps the plate voltage on the oscillator constant at all times.