SQUIRES-SANDERS, INC. SS-1R/701 RECEIVER - LIGHTS AND SHADOWS - SS-1R/701

  TO HOME PAGE 

 

 

The SS-1R (701) receiver was praised by many and criticized by others.

Over the years there has been a lot of talk about this receiver but maybe not always

things have been said and described as really are. In this article it has been tried to

clarify some aspects by highlighting its merits and also its flaws.

 

 

 

 

LIGHTS and SHADOWS on the



SQUIRES-SANDERS, INC. SS-1R/701 Series Receiver


by Vincent Italia

 

 

 

 

INDEX


ACRONYMS and ABBREVIATIONS

TECHNICAL SPECIFICATIONS

BLOCK DIAGRAM

SCHEMATIC DIAGRAM

TABULATION OF XLO CRYSTALS

 

2-AESTHETICAL and MECHANICAL DESIGN

The SS-1R receiver was introduced in 1963 and the first thing that hit was its elegant and out of the ordinary line. In the aesthetical design everything was guessed (especially in the later version than that released in November 1963), starting from the unusual cabinet, to the backlit S Meter, to the black slide rule tuning dial and the rotating white drum with red and black frequency markers, to the various knobs with aluminum trim and finally to the two black buttons with chrome bezel, all this gave to the receiver that special look. The escutcheon shaped logo* with a green background and the two white interlaced S, which are the initials of the two company founders Squires and Sanders, gave it a touch of class (Fig.1). The three white dots above the first S and below the second S show this letter in Morse code. In the U.S. there was in the habit of comparing the amateur radio equipment of a certain class to important cars, for example Collins receivers were compared to the Cadillac; someone compared the Squires-Sanders SS-1R to the Rolls-Royce!
The SS-1R/701 is an apparatus that is fairly small in size; about 41.30 cm. x 19.70 cm. x 33 cm. and not very heavy, less than 12 kilograms. The U shaped cabinet that, from one side to the other including the front, wraps around the chassis of the receiver, is made from a very robust extruded aluminum profile and is externally sandblasted which gives to it a light grey color (Fig.2a and Fig.2b). Two frontal prominent strips with mirror finish, one in the upper side and the other in the lower side of the U shaped cabinet and part of the extruded, are all along the profile. The lower and the rear panels are a single L piece where the upper one is hinged; together they form a single U element (Fig.3a and Fig.3b). The top panel being hinged can be lifted to provide access to the components installed on the chassis. These panels are made of dark grey painted perforated aluminum sheets.  Other equipment, which I remember, with the casings of similar U shape are those from General Radio test instruments of the fifties and sixties. The chassis that supports the various components is made of gold color anodized 21/2 mm. thick aluminum. The most cumbersome part, installed in the chassis, is the VLO with the tuning dial assembly (Fig.4a and Fig.4b). The dial was modified, in one of the various evolutions of the receiver, to make it more simple, robust and reliable. To take out the chassis, after removing; the U element (4 screws), the knobs, the two screws securing the chassis itself, the two S Meter wires and unplug the two under chassis connectors (the older version of the receiver had 2 terminal strips with soldered wires instead of the connectors - Fig.5 and Fig.6), just slide it in the two inner side rails that are part of the profile. Even without sliding the chassis all the parts and components, that are in the top and in the bottom, are easily accessible (just by removing the U element) thanks to this system of "boxing", which is simple, robust, functional and elegant, you could say that it was an innovation.

* The Trademark and Logo were filed on June 6, 1963 and registered on April 14, 1964 (N° 768,129)


3-DESIGN PHILOSOPHY and ELECTRICAL DESIGN
(With reference to the block diagram and to the electrical diagram)
See Figures A, B and C

As can be seen, in the block diagram, the SS-1R/701 is apparently a normal single/double conversion receiver à la Collins, where 12 tubes are used of which 4 are Noval type and 8 are Miniature type. Born as a receiver for the amateur bands but really can cover almost the entire short-wave band (see Technical Specifications and Tabulation of XLO crystals), from 3.5 MHz to 30 MHz (5.5--7MHz excluded), by adding or replacing crystals (3). The variable first intermediate frequency, which covers from 5000kHz to 5500kHz (this band of frequencies can be received by setting a switch inside of the receiver - Fig.7), heterodyne, in the 2nd balanced mixer, with the signal generated by the variable local oscillator (VLO), which extends from 6000kHz to 6500kHz, and generates a fixed second IF of 1000kHz where selectivity and gain are concentrated. The variable IF is employed to receive almost all the amateur bands plus two auxiliary General bands GA and GB (excluding the 40M band and the two fixed frequencies of 10MHz and 15MHz) with the aid of a converter that uses a quartz crystal controlled local oscillator (XLO) and the 1st balanced mixer. Therefore each band has an extension of 500kHz. The XLO is equipped with 7 crystals of which two are used to cover the following bands: 80M, 20M, 15M, and the two fixed frequencies of 10MHz (WWV10) and 15MHz (WWV15), only two crystals are needed to receive all these frequencies! The other four crystals cover the 10M band in four portions of 500kHz each (in the older version the receiver was equipped with one xtal, the others three were optional). The seventh crystal, of 2 MHz, is used only in the 40M band when the two (or one of them) optional accessories, the Video Bandscanner SS-1V and the Noise Silencer SS-1S, are connected (4).The crystals for the two auxiliary bands GA and GB are optional. The reception of the 40M band, which in this receiver extends from 7000kHz to 7500kHz, is simply done by replacing the fixed 5--5.5MHz Input filter of the 2nd balanced mixer with a 7--7.5MHz tunable filter. To receive the fixed frequencies of 10MHz and 15MHz the 1st mixer is employed with the crystal local oscillator and the 1MHz IF. A single conversion is adopted for the 40M band, for the two fixed frequencies (10MHz and 15MHz) and for the 5--5.5MHz band, as is evident from the description and from the block diagram. Why the IFs and the local oscillators frequencies have been chosen with numbers so "round" and unusual? The variable first IF, starting from 5000kHz, it’s also employed to receive the WWV5, the 5MHz frequency standard station, giving the opportunity to align the VLO (with C117 - HIGH FREQ) and thus automatically calibrate also the 40M band. The reception of WWV10 and WWV15, two other standard frequencies stations, is performed by using two crystals, 9MHz and 8MHz (the 2nd harmonic, 16MHz, is used), of the first local oscillator that beating, respectively, with 10MHz and 15MHz give a 1MHz IF. Then it is easy to calibrate the two quartz crystals frequencies (with the trimmer capacitors) by zero beating them with the two standard stations. Since the two crystals are also exploited to receive the 80M, the 20M and the 15M, these three bands are automatically calibrated. All this is feasible without the use of an internal crystal calibrator or an external device, ingenious! Another feature of this receiver is that the receive frequency, in hundreds of kHz, is displayed on a rotating drum (rotation driven by a 12 positions band switch) with a slide rule dial driven by the tuning knob, while the tens of kHz are displayed in two mechanical digital counters, with small sliding windows, that are located to the right and to the left in the black tuning dial front (Fig.8). In the 80M band and in the auxiliary GA band, where the tuning is reversed, the small window of the left counter opens automatically while the window of the right one, which is for the other bands, close. Thus the reading of the frequency is direct and does not create confusion. The accuracy of the display is given at +/- 1kHz maximum error, but in reality it is no more than +/- 400Hz (Ref.8). One revolution of the main tuning knob explores 10kHz, the latter has the perfect touch feeling to tune the stations, particularly those in SSB.

 

FRONT END

Antenna signals pass through a three sections low pass filter (FL1) with a cutoff at 30MHz and from there sent to a series of switchable tunable filters (Fig.9) except for the reception of the 5--5.5MHz band that uses a part of the first variable IF 500kHz wide band pass filter centered at 5250kHz (T1/T2). In this filter a low impedance output is also provided (link coupled to T2), J3-OUT 5MC, for the SS-1V and the SS-1S. The tunable input filter is a "simple" but ingenious series LC circuit with resonance (notch) to the image frequency whose reactance at the desired signal resonates in parallel with the variable capacitor C12-ANT TRIMMER. This reactance is inductive when the LO frequency is below the signal, as for almost all the amateur bands and WWV10, it is capacitive when the LO frequency is above the signal, as for the 80M and WWV15. In this latter case the reactance is still resonated by C12 but with shunt inductance added. This circuit allows obtaining a very good image frequency rejection, not less than 60dB and more than 90dB in some bands (5). These values of rejection are only obtained when a nominal source (antenna) impedance of 52 Ohms is presented at its input (see para.7). Be noted that other receivers to get close to the same rejection have at least two tuned circuits or even three (possibly with a RF amplifier intercalated amid them) between the antenna input and the mixer! Those with the "up conversion" (IF higher than the maximum receive frequency) are, of course, excluded. The inductors used are the high Q Barker & Williamson Miniductors that give, in this circuit, a RF voltage step-up of about 6 times. Only four rejection  adjustments are necessary to cover all the amateur bands, the 10MHz and the 15MHz fixed frequencies; the same rejection coils are used on several bands. The image rejection on the GA and GB bands is lower because it was optimized only for the amateur bands and for the fixed frequencies. The output of the variable filter is capacitive coupled to the grid of the first beam deflection tube, type 7360 (V1), single balanced switching mixer. A potentiometer, R18-1st MIX BAL, part of a resistive voltage divider, determines the voltages to the deflection electrodes of V1 for DC current balancing. The signal of the crystal local oscillator XLO (Fig.10), which employs a 6BH6 (V3) pentode tube, is sent through T6, a broadband balanced transformer (B & W coils - Fig.11), to the deflection electrodes of the 7360, 180° out of phase in order to reduce antenna oscillator irradiation and harmonic mixing. A crystal local oscillator low impedance output (link coupled to T6), J6 XLO OUT, has been provided to be used for calibration and for transceiving purposes (output: circa 200mV-Ref.8). The balanced IF signal is taken from the two plates of the 7360 and sent to the T1 transformer. A possible signal in antenna with a frequency same as the variable IF would undergo a strong  attenuation in the output of the mixer (T1 secondary) thanks, also, to the balancing action of the atypical primary center tapped T1 IF transformer (T1 with T3 for the 10MHz and 15MHz fixed frequencies). The local oscillator signal would be strongly present in the output if it was not attenuated by the intermediate frequency (5--5.5MHz) filter T1+T2 selectivity. It is interesting to note the unusual coupling between T1 and T2 that is via two series resonant circuits, one in the first transformer secondary and the other in the second transformer primary, connected together with coaxial cable, being T1 located on the chassis  more than 10cm. from T2! The secondary of T2 is parallel tuned by two capacitors in series, C55 and C58, their point of intersection is connected to the control grid of the second mixer V2. These capacitors act also as a divider so that the signal arrives attenuated (a bit less than 10dB) to the grid of V2. The output of V2 is also attenuated  (a little more than 10dB) in the step-down of the T4 transformer, in practice the V2 mixer gain (20dB) is nullified (only when in double conversion) in order to maintain the same signal level at the input of the 1MHz IF chain, in both single and double conversion. This suggests that some other objectives of the design were to have the minimum indispensable gain, sufficient to not degrade the overall noise figure, to the input of the second mixer (when in double conversion) and the IF selectivity "closest" to the antenna. It is to underline with how much attention the gain has been distributed in the receiver. Several capacitive attenuators are in the signal path in order to avoid the overload of some stages and making them work in their linear region. It is a succession of amplification, attenuation and amplification of the signal up to the detection! This starts with the step-up (x6) in the RF filter, the gain of V1 which is 20 dB (from the grid into the T1 5--5.5MHz transformer), the 10dB attenuation of the capacitive divider C55/C58 and so on. Special attention was also paid to the choice of ground connections, especially in the low level RF stages, to avoid “ground loops” so to maintain a low internally generated noise. This aspect has been highlighted and very well explained in Ref.5. Singular is also the combination of T3 with T1; in the WWV10MHz and WWV15MHz positions a couple of contacts of the S1C-R (BAND) switch open the junction of two inductors of equal value, which make up the primary of T1, in such a way that the T3 center tapped primary can be connected in series to T1. Thus the total inductance is the right one to resonate at 1MHz and the balance remains unchanged.
The SS-1R receiver underwent various modifications (9-10), while still in production, to improve its performances (although already very good), these modifications were applied in several phases. One of the major improvements regards the sensitivity that was increased by a minimum of 3dB to a maximum of 6dB, this has been done, probably, by modifying the RF input filter and by changing some values of its components. This assumption derived from the comparison of the SS-1R front end electrical schematic with that of the SS-1R/701 (Ref.1 and 2). I believe that a better matching to a 52 Ohms source has been realized by this modification. In the receiver Field Alignment Procedure (Ref.6) it is stressed that the sensitivity had to be better than 0.3µV (0.25µV typical!) at 10dB S + N / N, CW mode with a 2.5 kHz IF bandwidth, in the 20, 15, and 10M bands. Not bad for a receiver without an RF amplifier stage! (12) Another modification was to replace the ceramic trimmer capacitors, in all critical circuits, with precision glass and ceramic piston capacitors, to enhance frequency stability and performance.

 

SECOND CONVERSION and FIXED IF

In the second conversion we find another 7360 (V2) as a single balanced switching mixer and thus has the same principle of operation of the first mixer. Here also a potentiometer, R24-2nd MIX BAL, is used to adjust the DC voltages balance to the deflection electrodes where the signal from the VLO, 6000kHz to 6500kHz, is sent in push-pull through T5. In the Test Procedure manual (Ref.8) it is specified that the VLO RF voltage in each deflector must be 1.5 Volts RMS +/-0.25 Volt, though a 3 Volts RMS per electrode was suggested by Squires in Ref. 3. Higher voltages would lead to a better sensitivity and lower distortion (Ref.9) but they have been kept low to avoid harmonic mixing and the occurrence of spurious responses, this consideration is also valid for the first mixer V1. A better explanation of this aspect is available in the last chapter of this document. The connection between the VLO output (J14) and the second mixer VLO input (J4) is via a connectorized coaxial cable (on the chassis VLO OUT and VLO IN), this cable (W3) must be removed when connecting the Video Bandscanner SS-1V. The 1MHz component at the output of V2, resulting from the beat, is filtered and balanced by the T4 transformer. Similarly the link between T4 output (J5) and the fixed 1MHz intermediate frequency amplifier input (J8) is by a connectorized coaxial cable (W2). By removing this link (on the chassis 1MC OUT and 1MC IN), the SS-1S Noise Silencer can be inserted. The variable local oscillator VLO is equipped with a double triode type 6BK7B (V11), a triode as a temperature compensated Hartley oscillator and the other as a cathode follower. The coupling to the input of the cathode follower is via a capacitive divider (for a lighter coupling) and its cathode output is filtered by a low pass filter. The LC combination of the oscillator gives a very linear frequency excursion. The inductor (L16) of the resonant circuit is made of preheated copper wire wrapped on a large threaded ceramic form with a cylindrical shield; the whole is mounted on a thick aluminum plate. The high quality precision variable capacitor (C115) of the VLO is driven by a worm and its spring-loaded gears and has a clutch system to disengage an electric AC motor (Fig.12). This motor is used to go from one end to the other of the band, in a few seconds, by pressing one of the two round buttons that are at the bottom of the tuning dial. Pushing both buttons simultaneously will not energize the motor in order to avoid its damage. The motorized tuning (6) is alternative to that manual and is much appreciated because, for exploring manually the 500kHz band in use, you must turn the tuning knob as many as 50 times! This feature can also be useful to scan the band. No electrical noise can be heard when the motor is energized. The power supply of the whole receiver is without any stabilization, this is to prevent electronic noise which is usually generated by the VR tubes and Zener diodes. This indicates, once again, a maniacal care in maintaining a low internal noise. The VLO, even though no power supply regulation is employed has an excellent stability, less than 100Hz frequency change after 5 minutes from the warm-up with +/-10% line voltage variation. How can be that this variable oscillator is so stable without any voltage regulation and no thermostatic stabilization? The trick seems to be in the feedback which is lesser than usual and in a lower output coupling (Ref.5), another inedited circuit from this exceptional receiver. In the 5kHz position, of the S4-SELECTIVITY switch, the selectivity is given by the intermediate frequency transformers T4, T8, T9 and T10. One might think that these transformers are the usual slug-tuned adjustable instead they are made of ferrite pot cores (except T10), this allows to achieve a very high Q (400 to 500). It would have been impossible to get such a high Q, at a frequency of 1MHz, with normal IF transformers. The very loose coupling between an inductor and the other, of the single transformer, is capacitive; a 0.75 pF (!) capacitor for T8 and 1pF capacitor for T9. Both of the transformers primaries are tapped down so as not to be "loaded” by the tube which precedes it and thus maintain a high Q, consequently the selectivity is at its maximum. These peculiarities ensure that the “noise bandwidth” of the entire IF amplifier remains narrow in the three selectivity positions. Another aspect that emerges from the electrical diagram is that the coupling between some stages (as those of the IF) is the most light possible (almost to not “disturb” them!) not only to avoid overload of the stage that follows, as mentioned in the previous chapter, but also to exploit at maximum the selectivity of  the circuits used. The SSB selectivity is obtained by a low insertion loss (1 to 2 dB) quartz crystals filter of multiple sections (FL2) with a 2.5 kHz bandwidth and a 1:2 form factor. In the 350Hz position a two crystals filter (FL3) is connected in series to the 2.5 kHz filter to take advantage of its excellent out of band rejection. The selectivity control is separated from that of the receive mode. The first two 1 MHz intermediate frequency amplifier stages employ two 6BA6 pentodes (V4 and V5) which are controlled by the AGC. The cathode resistors of these two tubes are of low value, 68 ohms, and without any by-pass capacitors to increase their linearity and stability. The V4 and V5 tubes plate current feed (through a resistive bridge circuit) the S Meter milliammeter (1mA f.s.) which reads accurately (in S units and dB referred to 1uV) the signal level, regardless of the RF GAIN (R59) setting, as long as the signal is moving the meter. Potentiometer R33 is for the METER ZERO adjustment and R35 for the METER MAX setting. The third IF amplifier uses the pentode section (V7A) of a double tube type 6AX8 which  amplifies the IF signal without being controlled by the AGC. The IF signal is picked up from a capacitive divider on the T9 secondary and sent to its control grid. The potentiometer R106-CAL, in the V7A cathode, determines the total gain of the receiver and it is used to set a S9 reading in the S Meter with a 50µV, 21.25MHz, signal at the antenna input. Following is V6, a 6AL5 dual diode type tube, which acts as an automatic noise limiter; the threshold can be manually adjusted by a potentiometer R83-LIMITER. This ANL operates in the IF (unlike audio limiters) on all modes, AM, SSB or CW. A switch is coupled to R83 that can exclude the ANL.

 

AGC CIRCUITS, DETECTORS, BFO and AF

After the last IF transformer (T10) we have a 6AV6 (V8) which replaced the 6AL5 of the older version of the SS-1R. The 6AV6 is used in an unusual combined circuit  for the development of the AGC and the isolation of its circuitry. The triode section acts as a cathode follower to isolate the two diodes unit, that are used as AGC rectifiers, from the IF amplifier (to avoid pulling of the BFO) and from the AM diode detector (CR4) type 1N34A. This diode is operated at high-level (above 2 Volts) for low distortion. The twin diode contained within the 6AV6 tube have the following functions; the first one rectify the IF signal and hence generates the AGC negative voltage to control, with fast attack and slow release, the gain of the two V4 and V5 intermediate frequency amplifiers (first loop), the second diode generates another negative voltage to control the gain of the first mixer V1 in case that very strong signals are present in antenna (second loop). In the old version of the receiver also the V2 mixer was controlled by the second loop AGC. The second V8 diode is also used to clips noise peaks and prevents them to produces "bursts" in the AGC. Another 1N34A (CR10) allows to apply the second negative voltage to the first mixer from a predetermined bias level and hence with a delay from the first AGC intervention. This results in a very efficient double loop automatic gain control system. The selection of  the time constants is by the AGC SLOW-FAST-OFF S5 switch, which are for the Attack 0,001second and for the Release; 1,0 second for SLOW and 0,1 second for FAST. The OFF position is normally used for special tests or for alignment purposes. The SSB product detector (V9), which in the earlier version of the receiver was a 6BE6, employs a 6BY6 (dual-control heptode that was used mainly in television sets) in a sophisticated circuit designed to achieve extremely linear detection. The BFO frequency is sent to the first grid and the IF signal, taken from the capacitive divider in T10 primary, feeds the grid N°3. This divider is exploited too as a low impedance IN/OUT connection (TP2). Looking at the schematic we can note, among other, a negative feedback path from the output to the input (grid N°3) of V9 and a particular bias arrangement, presumably, for a better linearization of the detection and for a lower spurious responses output content. The combination of this detector circuit with the automatic gain control and, of course, with the two 7360s give a dynamic range of 1µV--1V! (7) The BFO with the 6AU6 pentode (V12) is very stable thanks to a carefully designed oscillator, this stability has been further improved, in the newer version of the receiver, by the addition of two crystals (Y10 and Y11), one for LSB and another one for USB, keeping the variable oscillator (+/ - 4kHz) for CW. For the audio amplifier we have the triode section of the 6AX8 (V7B) as a preamplifier driving a fixed biased 6AS5 beam tube which provides a maximum power output of 2 Watts on a load of 4 or 500 Ohms. The SS-1RS matching cabinet houses a loudspeaker with a 4 Ohms impedance voice coil. The cabinet neatly houses also the SS-1S Noise Silencer when available.


POWER SUPPLY and AM-SSB-CW SWITCHING LOGIC

The power supply transformer has a primary for 105 to 125 Volts 50/60Hz (on request a 220 Volts version was available) and two secondaries; one has a center tapped high voltage winding and the other has a 6.3 Volts winding for the tube filaments. The two high voltage half-wave of the secondary are rectified by two silicon diodes and the resultant voltage is filtered by a Greek PI circuit with choke. The DC output voltage is +140 Volts and is used for the B supply. From one branch of the H.V. secondary two negative DC voltages are derived in a rectifier circuit with a silicon diode. The two negative voltages are -45V and -75V and are used for; the audio tube bias, the RF GAIN, the LSB and USB switching logic, the Muting and for the Noise Silencer. A potentiometer R84, with front panel control (NOISE SILENCER), is used to adjust the negative voltage, derived from the -75V rail, to the circuit of the SS-1S Noise Silencer  when it is connected to the receiver. As already mentioned no stabilizers are used so as to help maintain a low internal noise in the receiver. To reduce the internal (and external) stray magnetic fields effects, mainly due to the transformers, in the two mixers and therefore their unbalance, both 7360s are fitted with  mu-metal shields. The switching logic required to provide the right frequencies from the VLO and the BFO, when switching between AM, SSB and CW, and thus maintaining the correct reading frequency, is rather complex, this is due to the choice of the IFs and crystals frequencies that allow WWV auto calibration of the bands. Germanium diodes (1N34A) are employed as switches to shift the VLO and BFO frequencies for proper LSB or USB operation. The logic involves the S7-AM-LSB-USB-CW switch and the S1 BAND switch. The switching is of DC voltages only.

 

EXTERNAL CONNECTIONS

Another special feature of this receiver is the multitude of coaxial connectors (Fig.13), for external
connections, which is provided. In addition to the BNC antenna input connector (J1) it has the following
RCA/Cinch jacks (female):
(with reference to the electrical diagram)

- J3    Output (and Input) 1st IF, 5—5,5MHz (T2)
- J4    Input to the 2nd Mixer for the VLO (T5)
- J5    Output 1MHz IF from 2nd Mixer (T3)
- J6    Output (or Input LO 1st mixer) XLO (T6)
- J8    Input 1MHz IF amplifier chain (T7)
- J9    Output AF from detectors
- J10  Output AF 500 Ohms (T11)
- J11  Mute
- J12  Loudspeaker 4 Ohms (T11)
- J13  Phones (T11)
- J14  Output VLO
- TP1 In/Out low impedance (cathode) 1st Mixer
- TP2 Output 1 MHz IF amplifier (V7A) and
          Input last IF transformer (T10)

          and :

- J7    11 pins socket for accessories (SS-1S, SS-1T, VHF converter, etc.)

          2 extra AC outlets for the accessories.
          
          

Some of these connections are for a normal use, see Speaker, Mute, etc. Connectors have also been provided for the connection of accessories such as the SS-1S, the SS-1V, for the SS-1T transmitter, which, unfortunately, was never marketed (Ref.7) and for a phantomatic VHF converter of which little or nothing is known. These and the others interfacing connectors lend themselves to testing purposes and to facilitate the alignment, but probably they were also provided to make tests at different stages of the receiver and possibly modify them to improve their performance. A note of disapproval regards the use (for some of the connections) of a coaxial connector type that is more suitable for AF than for RF (excluding, of course, the BNC Antenna connector). This also makes the interface with RF test instruments more complicated. This receiver, that had a sale price rather high (justifiable for everything else), would deserved something better, although this type of connector has been used in many radio amateur equipment from other brands.

4-CONCLUSION

Despite more than forty-six years have elapsed since its birth, the SS-1R/701 holds very well the comparison with recent receivers. Although one might argue that its SSB selectivity is not the best, the frequency stability, albeit very good, is not that of a synthesized receiver, but when you listen to it you notice that it’s truly unique in that the weak signals coming from the antenna, which in other receivers are audible but not very understandable, in the SS-1R/701 they are perfectly decipherable although if strong in band signals are present. The two 7360s, performing an excellent job, combined with low phase noise local oscillators, a very effective AGC and a correct gain distribution, contribute largely in giving to this receiver a very low internal noise and offer a "relaxing" reception. All this is thanks to a careful and ingenious design, nothing has been left to chance! Unfortunately this receiver has not had the commercial success it deserved maybe because its selling price was rather high (in 1963 the cost of this receiver was one and a half times that of the Collins 75S3) or maybe because of the strict characteristics requirements of the antenna to be used (see chapter 7). We must remember that many (but not all!) contemporary receivers were “panting machines” with an amplification of “a lot of dB” and that was enough to connect a piece of wire as an antenna to listen to “everything”, even what was not to be in the actual band in use! For the technical solutions adopted and for the refinement of the circuits used, in my opinion, the SS-1R/701 can be placed at the apex of the radio amateur HF receivers with tube technology. This supremacy was maintained even (more than ever!) with the advent of receivers with solid state technology and lasted until the arrival of those of recent generations. I believe that this receiver was simply ahead of its time. One of the receiver's slogans, “in a class by itself “, was really guessed! 


5-NOTES

1 - Before Squires there have been other designers who had the idea of using a beam deflection tube as a RF mixer for receivers. There are also patents from the 40s and 50s for mixers and converters with this type of tubes. There is even a patent of the thirties (!) where it is suggested to use a “cathode ray” tube type as a super heterodyne mixer and as a CW detector. Nevertheless no merit is denied to Squires because in reality he was the first to have used a beam deflection tube as a HF receiver switching mixer in an original and functional circuit, to have described its operation and to have highlighted its unique properties with respect to the intermodulation and low noise, what his predecessors have not done.
On August 23, 1963 Squires applied for a patent with application serial No. 304,006.

2 - The 7360 beam deflection tube was developed for applications in balanced modulators, balanced frequency mixers and converters more specifically for SSB and DSB transmitters up to, at least, 100MHz, so as to achieve a high degree of suppression of the carrier. It was William K. Squires who used it as a HF receiver mixer, with the results that we know, though, almost contemporaneously, a certain Mr. Guy Herbert Smith, Jr. proposed it, in a thesis, as a low distortion mixer for receiver. However he injected the local oscillator signal in the first grid and the antenna signal to the deflection plates. As far as it is known Squires has not developed further the 7360 mixer instead he used a double triode tube, 6DJ8 type, in an unusual and very interesting balanced switching circuit (Ref.10). After that he designed a low noise switching mixer, with two FETs, similar to that with the 6DJ8 tube (Ref.11). Evidence suggests that Squires-Sanders utilized these two mixers; one in a military HF receiver, AN/URR-58, and the other in a marine receiver for the U.S. Coast Guard. This marine receiver could "... accept signals up to 7 Volts RMS with no notable distortion." Rumors indicate that only three AN/URR-58 were built and that this military receiver had a vague resemblance to the Collins R-390(A)/URR. I have no way to confirm or to invalidate this (for now) then it remain only the "rumors".
NEW (March 2011) - See note 13.
 

3 - Custom factory modifications were available if the SS-1R was to be used consistently for frequencies other than the amateur bands. This customizing could include performance optimizing (particularly image rejection) and special dial scales for a variety of bandswitch-frequency arrangements. Receivers with such customizing could be obtained only on special factory order.

4 - For the operation of the SS-1V and/or the SS-1S a wideband IF output of 5--5.5MHz is needed from the receiver and since a single conversion is adopted for the reception of the 40M band (with V2 mixer) this wideband IF is created by using the first mixer V1 and the 2MHz xtal in the XLO. The 40M signals coming out from the RF filter are splitted in two outputs; one goes to the V2 mixer that gives an IF of 1MHz for a normal reception use, the other goes to the V1 mixer that generates a wideband IF of 5--5.5MHz only for the operation of the SS-1V and SS-1S.

5 - A filter with a similar working principle was employed in tube television sets of the fifties.
It was used to bypass to ground the audio carrier frequency in the video IF amplifier.
It must be know that W.K. Squires worked, among other, for the Sylvania Electric Products -
Radio and Television Division and several patents covering the television field were released to him.

6 - Some receivers with a motorized tuning:

- Zenith Model 12S265 (1938)
- Phonola Model 567 (1940)
- Telefunken E52 Köln (1942)
- Hallicrafters R45/ARR7 Receiver (1944)
- Scott Radio Laboratories Inc. Model 800 B-6 (1947)
- Collins R-389/URR (1951)
- Saba Mod. Freiburg - Automatic 100 Stereo (1959)
- Philips 8RO 5/501 (1963)
- JRC NRD-11E (1968)

7 - In the SS-1R Test Procedure (Ref.8) for the Overload Test it is specified that no overload, and so no audio distortion, should occurs up to a 3 Volts (!) signal at the antenna input (without SS-1S inserted).

8 - An antenna tuner, the Broadband Antenna Matcher, was then produced by Squires-Sanders as an external add-on for the SS-1R and the SS-IBS (11) receivers to remedy to the impedance matching problem between the antenna and receiver.

9 - A retrofit 1 MHz IF T-Notch filter was later available for the SS-1R and SS-IBS, it was a passive but very effective circuit. A high Q ferrite pot core was used for the inductor of the tunable rejection circuit.

10 - Several modifications were made to the SS-1R receiver, during its production life cycle, to improve its performances and to get the best from each circuit. These modifications were implemented in different times and in almost two years (November ‘63/September ’65), so various versions of the receiver have been sold. The version that incorporates all the improvements is the 701 Series SS-1R (the previous version was the 700 Series). After that Squires-Sanders produced (1967?) an IF T-Notch filter board to be factory retrofitted, so we may find, per example, a 1964 version of the receiver with this filter installed. It could be that towards the end of its production life the receiver has been manufactured with the built-in Notch filter, but this is not sure. Nowadays it is not easy to come across a SS-1R with this filter. From the indications and information in my possession I can deduce that the receivers of the older versions could be updated and retrofitted, with some of the modifications/improvements, by the manufacturer. It also seems that the SS-1R receivers of the 701 Series, with higher serial numbers, have an identification paper sticker with a green background instead of the black and silver riveted photo-etched aluminum plate.

11 - The SS-IBS receiver is basically an SS-1R/701 but for the International Broadcast Shortwave bands reception. It does not have the CW 500Hz quartz crystals filter that was replaced by an LC one with 8kHz bandwidth. Listening to this receiver in the AM mode is an unforgettable experience; if the received AM broadcast station is of good quality and the signal has an adequate level then just switch from the 5kHz to the 8kHz bandwidth to have "High Fidelity" on the shortwave! The SS-IBS makes "blush of shame" many other renowned and "noble" AM receivers! This receiver was used by the VOA as a monitor and probably as a relay receiver.

12 - Is it correct saying that the SS-1R (or the SS-IBS) is a receiver without an RF amplifier? Not really. We must considering the 7360 mixer as a "double" tube or as a two stages circuit that is; a good RF pentode amplifier followed by a very good switching mixer. An excellent combination in a single 9 pin bulb! All this was clearly stated by Squires in his article (Ref.3).

13 - From recent information (March 2011) : the "Military" receiver and the "Marine" receiver are the same! The AN/URR-58 was developed specifically for the U.S. Coast Guard. Its resemblance to the Collins R-390(A)/URR is very very vague (almost inexistent), the only similar part is the mechanical digital readout. The AN/URR-58 is an hybrid receiver, it uses tubes and transistors. The RF mixer is the one with the double triode tube, so a sort of mystery arises; where or in which receiver the two FETs mixer has been used?

 

 

7- THE SHADOWS

So far I have described this receiver highlighting its qualities and virtues, but what about its
"shadows" or its defects?

A- The SS-1R was designed for an RF input (antenna) impedance of 52 Ohms, like so many other receivers, but if used with different or unknown antenna impedance (Long Wire, Whip, etc.) creates several problems; reception of not in band signals, "deafness" in the actual band in use, etc.
This is due to:

a- the absence of an RF amplifier stage that separates the mixer from the antenna and therefore presents always the same impedance at the input of the mixer.

b- the RF input filter, which being of particular conception, do not performs its function properly due to the impedance mismatch while having excellent characteristics of image frequency rejection when it is matched. Therefore it is imperative to use only antennas with 52 Ohms impedance or to match them with an antenna tuner (8). This seems, to me, not being a defect, but a requirement.

    B- From the SS-1R/701 (or SS-1R) manual:

“Special Note: The SS-1R is a receiver of exceptional sensitivity, signals as weak as 0.1 to 0.2 microvolts produce usable copy. At the same time, exceptional image and spurious response rejection (greater than 60dB) is provided. But this means that occasionally spurious signals not in the actual band in use may be heard (they may not be heard on other receivers even with poorer image rejection because the other receiver is not as “hot”), for example, with a multi-band antenna, 20M signals might be heard weakly when tuning 80M, since the 80M image lies on 20M. This in no way reduces the receiver’s effectiveness, since the “not-in-band” signals are far below the desired in-band-signals; it is simply the consequence of owning an unusually “hot” receiver.”

 

This means that, since the receiver is “so hot”, it could receive (though occasionally and in particular conditions) signals "not in band" ???
A very unclear and discordant note for a receiver of such a class!
It is obvious that behind this note a receiver malfunction is hidden.
What could it be?


INQUIRY and ANALYSIS

Analyzing the first 7360 (V1) single balanced switching mixer we note that the local oscillator is injected in push-pull, through T6, to the deflection electrodes and then the two 180° out of phase signals (and with the same frequency!) that could arrive on the control grid single electrode, being in push-pull and equal in amplitude, would be canceled at perfect DC and RF balance, hence the LO rejection in the RF port (antenna) is very high. Then if we add the step-down and filtering of the RF resonant circuit it becomes excellent, could reach 100dB, therefore no spurious signals can be created at the mixer input. Similarly the IF rejection is very good being the IF frequencies, 5--5.5 MHz, balanced in the T1 tuned transformer. This means that any signals in antenna with the same frequencies of the variable IF (and not with any frequency as one might think, not being T1 a broadband transformer) would be attenuated by > 60dB in the T1 secondary. But for other RF signals present in antenna, including the desired one, they will appear at the output at the discretion of the RF and IF (T1 + T2) filters selectivity! Not only that, they would also be amplified by the tube itself! The same thing happens with the local oscillator signal which we find it amplified in the T1 primary! To understand how this is possible we must remember the operation of the 7360 mixer tube; the RF signal on the control grid, filtered by the antenna filter, modulates the electrons ribbon beam emerging from the cathode, this beam (or current) is alternately switched between the two plates (and hence varying their current) depending on the instantaneous LO RF voltage polarity impressed on each of the deflectors electrodes. The switching rhythm is set by the frequency of the local oscillator itself. This switching action is nothing but a mixing process that gives the difference (and sum) between the LO and the RF, the result is the value of the IF. The instant when the beam is centered on one or other of the two anodes the tube operates, as well, as a conventional pentode amplifier. There is also amplification (lower) from the deflectors to the plates. This is why we have the RF and LO signals amplified in the output. The T1 IF transformer having the two LO signals in the primary, because present and amplified on the plates, in anti-phase and with the same amplitude are canceled in the center tap but, unfortunately, appear as a single frequency in the secondary, and hence at the T2 output (and to the V2 grid) this frequency (as per any other undesired RF signals) is attenuated only (but fortunately!) by the filter selectivity and the capacitive attenuator C55/C58. It is obvious that all these undesired signals may create extra spurious responses. The second mixer V2 do not contribute to create other unwanted signals because they are stopped by the narrower filter selectivity that follows T4. Where and how the extra spurious responses are generated? These extra spurious signals arise in the T1 primary coil (and consequently transferred in the secondary) where a mixing process takes place (see APPENDIX 1). The amplified anti-phase LO frequency cyclically interrupted in the 7360 plates, and hence alternately switched between the T1 primary ends, easily mixes with the amplified RF signal(s) and with the IF frequency (already formed in the tube) that are in the primary coil. All the combinations within these signals are possible! This is one of the reasons why a relatively low LO level has been chosen to be injected in the deflectors of the 7360s because a higher level would facilitate the undesired mixing process and lead to higher spurious. The antenna filter that has, as already outlined, excellent rejection at the image frequency, good selectivity, at least more than enough for a mixer like the 7360 with a high immunity to strong adjacent signals, but having a single tuned circuit has a low out of band attenuation. Why a multiple cells RF filter has not been foreseen? Because a filter of this type would have introduced a higher attenuation at the expense of the sensitivity and perhaps it would not have achieved a very high attenuation at the image frequency. Beyond any consideration the original RF filter remains very valuable not only for its high image rejection but also for its low insertion loss (and good voltage step-up) that permits to obtain a very good sensitivity to the receiver. We can consider its combination with the 7360 mixer, guessed. However a solution had to be found to maximize the performance of the 7360 although this anomaly occurs rarely and in certain situations. What can be done to eliminate or mitigate it?
 

POSSIBLE SOLUTIONS

A very effective solution is to block the offending signals (in particular the LO signal) at the mixer plates, before they reach the T1 primary and therefore before any undesired mixing action takes place. How this can be realized? It can be done by inserting, in the V1 plates paths, low pass or notch filters, these must be exactly identical in each plate so as not to degrade the balance (also any extra added capacitance should be taken into account). It should be sufficient to notch the 9MHz and 16MHz LO frequencies as these are used to receive most of the bands. This could be more than enough not to create spurious responses being the LO signal the most insidious in that point.  This solution has not yet been performed. A sort of phase cancellation system of the LO on the V1 plates has been tried but with no success. Any further solution and/or test results will be published on this site.



APPENDIX 1

This is really a very interesting mixing process! Is this 7360 mixer anomaly which inspired Squires for the development of his two other mixers (Ref. 10 and 11), since it is exactly their principle of operation?! However this principle has inspired me, in the late eighties, when I was searching for the "Ideal RF Mixer"(!) Among the various mixers investigated, I had built and tested an inedited 7360 mixer, see Fig. A1-1. Considering the 7360 tube as a good pentode amplifier followed by a very good switching mixer; if we "remove" the weak link, that is the amplifier, and we leave the switching mixer, the latter can be fully exploited. This is what has been done with this experimental unedited circuit. The tests results were very encouraging but need further investigation, I hope to be able to resume the tests in the near future (time permitting.......).

 

 

 

 

SLIDESHOW PICTURES

 

 

 

 

 

 

www.radiopharos.it

Copyright© 2010--2021 All rights reserved
Tutti i diritti riservati
 

1st Issue - January 2010
Revision C: Last Update - March 2011
Revision D: Modified page background - May 2015