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• Technology

Technology - 1950s - 1960s

 

 

The next picture is a close up (cropped) of the amplifier pc board containing

the two 6GW8s and the 12AX7. The 6GW8 is a combo triode/pentode so serves as

a driver and output amp for each channel. The 12AX7 is a dual triode so it

serves as the voltage amp stage for both left & right channels. (editor's note: Notice the phenolic printed wiring board that was quite common among commercial and defense manufacturers of the period. It is not usually seen in printed circuit manufacturing today and has been all but replaced by the green fiber glass epoxy printed wiring board. The phenolic boards did not stand up too well to the heat from a wave soldering machine or to technician soldering iron repairs. I seem to recall problems with the saturation of soldering flux with the phenolic boards. The fiber glass boards cleaned up much better than the phenolic. The pads lifted on the phenolic board and in some cases were eliminated altogether causing a heck of a repair problem. Still this was the printed wiring component of that period. I cannot see any diodes in the picture so I assume the device had no solid state components. Although I cannot see any of the soldering the component insertions, usually done by hand at that time, look like good shop quality.)

 

 

 

 

Here is a view of the chassis looking at it from the rear of the cabinet

showing the in/out RCA style jacks. A view of the chassis looking down on the chassis. The control panel. The top knob is tuning for AM/FM, selector switch: OFF, 1 - Phono, 2 - Phono with filter for "scratchy records/78's, 3 - AM , 4 - FM (mono) 5 - FM stereo if used with an optional multiplex adaptor (which I don't have) 6 - TAPE input Then Volume, Bass and Treble with the balance control at the bottom.

Editor's note on the Tape input: Since the output of a tape system is not flat because the higher frequencies produce a higher voltage, the systems usually included an "equalizer" circuit within or preceding the pre-amplifier. I think that the most popular "equalizer" circuit of the day was the Williamson . The output of the equalizer was a flat audio level. Another form of equalization included to accommodate the dynamics of the human ear, was the audio taper volume controls. I believe systems today may still use the audio taper in their variable resistors. I suppose the concept is duplicated electronically in the solid state systems.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

From the editor: To the right is the Roger's Majestic system chassis. You can see the amplifier module pictured above. For the younger technicians it must be weird to see the components such as the large electrolytic capacitors. And how about the variable capacitor for tuning. Of course you can see the dreaded dial cord used to mechanically connect the tuning dial to the variable capacitor on the lower left. Most of us who have had to re-string the dial cord think of them as 'dreaded.' The one for the Rogers seems particularly challenging.

I am sure that Daniel will correct me if I am wrong but I believe the electron tube next to the variable capacitor is the detector. (Daniel corrected my memory on this stage as it is actually called the RF oscillator and mixer) The RF oscillator and mixer split the RF signal and formed the intermediate frequency (455 KZ) signal (IF) for the IF Amplifiers and IF Transformers shown on the right side of the RF Board above the variable capacitor. The Detector is the stage that transforms the IF signal from the final IF stage into the audio signal to be amplified. Daniel tells us that detector is basically a pair of diodes.

Daniel tells us:

"I'm not sure which tube is the detector as the tubes are 9 pinners and are combo tubes (two functions sharing a single glass envelope). Since I lack the back cover of the unit I do not have a tube layout diagram. I'd have to pull the tubes out to see what they are. Some units of the period even used silicon diodes for detection."

The reason for the specific frequency of the IF section is to produce a single frequency for amplification. It is easier and more efficient to amplify a single frequency. A super heterodyne IF system included four stages of IF Amplification and was the state of the art of the period if my memory serves me correctly.

Daniel continues:

"I know the RCA uses a 6AL5 dual diode as the FM detector and uses a 6AT6 triode/ dual diode as the AM detector. The triode portion is used for AVC (Automatic Volume Control). The Rogers probably has a similar set up but with different tube types."

The large power transformer was typical of such systems as electron tube equipment were "voltage" operated devices and not "current" operated like solid state circuitry.

 

About the tuner, Daniel says: "To the right are the controls of the system that included an AM/FM tuner (monaural), tuning knobs for the AM and a separate one for the FM reception. I thought this unusual since I can recall most systems using only one knob to tune either system. Because of the higher tuner frequencies for FM and the use of capacitors in this early stage of the system, the FM tuners had a drift fault. That is, it would deviate slightly from the set tuning frequency and actually would tune itself away from the station selected. This was a particularly annoying problem for FM listening. There was circuitry utilized to mitigate this although I cannot recall any of them being too effective. Daniel Thomas tells me that the unit was an early attempt at stereo broadcast in which one channel was transmitted on FM and the other on AM. Very interesting indeed and I never heard of such a thing."

"The other chassis is the AM/FM tuner sections and 2 6CG7 1st and 2nd AF

stages. The AM tuner consists of a 6BE6 Rf osc/mixer, 6BA6 IF and 6AV6

detector and AVC. The FM is a whole separarte section using a 6DT8 FM RF

osc/mix, 2 6BA6 1st & 2nd IF amps, a 6AU6 FM limiter and a 6AL5 detector."

Since there is a "Balance" control I thought that it is an early stereo system. I was wrong as Daniel points out. At one time, stereo was only available on the FM bands and not all stations offered the feature. Usually, when the system was tuned to a station with a stereo output, an indicator light shown on in the display.

This system includes a loudness control and controls for Bass and Treble.

 

 

 

 

 

 

 

The next picture shows the inside with the lid off (I'd previously mentioned that the lid needs some repair to stay on it's hinges) the Phillips record changer and the control panel.

 

Playing a 45 RPM record on a Record Changer (a history lesson)

Since we do get visitors who are young and therefore may only be familiar with music on compact disks (CDs) or their IPod, I thought it may be interesting for them to understand the process of playing a 45 RPM record and something about a "Record Changer" like the one to the right.

To begin, the record was 7" in diameter and had a one inch hole in the middle to maintain center on the Turntable, the mechanism that spun the record. The player to the right was called a "changer" because it enabled you to "stack" about ten records on the top (red zone) section.

After you stacked the records on the hub, You would turn on the record player using the on and off switch which was part of the volume control. Turning on the player in addition to powering up the enclosed amplifier would start the turntable to spin at 45 RPM or Rounds Per Minute. You would allow sufficient time for the vacuum tube (valve in UK) amplifier to warm up. This could take about ten or more seconds.  You would then press the "Record Drop Button" which would start the playing process. The little tone arm would raise itself off of a rest and one of the records would automatically drop to the turntable. The tone arm would then move over to the outside edge where the starting track would begin and move down. The record would begin playing and when it reached the end, it would move out into an exit area. The tone arm mechanism would react to this by beginning the playing process all over again: drop new record on top of the old and then move to the playing surface. A standard play 45 RPM record seldom had a recording length of five minutes on each side. An Extended play may have seven minutes of recording time.

The tone arm contained a little "picturek up", a sensor into which a player needle would be inserted. The player needle would ride in the records groove (a still popular term that many do not know of the origin).

I am sure that the more astute among you would say, wouldn't one record playing on top of the other scratch their surfaces. Well in fact they did at times, but the record label and sometimes a little ridge in the label area would keep the record surfaces away from each other.

For the technician, such a player as described above would contain a "series string amplifier," that is it would not have a power distribution transformer (too costly to include for the manufacturer) but would string the electron tube filaments in series with a rectifier having a filament voltage of 50 volts, the first audio would have a 35 volt filament, with the first amplifier being (if I am recalling correctly) 24 volt filament and the final amplifier (output) being twelve volts. The rectifier would supply the voltage for the cathode, grids, and plates of the tubes. Unlike solid state circuits the amplifiers were voltage as oppose to current operated devices.

The player above was manufactured in the USA by RCA Victor (I believe the Indianapolis manufacturing facility).

I hope this bit of nostalgia was interesting for you. I am sure that there are many of you who have memories of these little systems and maybe I am wrong about some facts. Feel free to write me and share your knowledge about 45 RPM systems.

Wes

More on systems of the day

 This is a frontal shot of an RCA Stereo AM/FM, Record Player from the late 1950s to early 1960s. Daniel put a few LP covers from our Hero in the photo. I am sure that all of us older Mantovani fans will recognize the LP covers and have had the LPs in our own collection. 

Here is a bit closer shot of the console showing the Garrard Turntable and the tuner/preamp-amplifier controls (more about these later). Daniel put an album cover of an LP of a venerable Canadian Orchestra leader, Mr. Guy Lombardo (and his Royal Canadians). The LP seems to have an incredibly large Center space (less music for the money. Daniel corrected my perception on the recordings, "

Here is the Garrard Automatic Turntable with the supplied components

 (accessories) to serve the variety of recordings available at the time. Here is the description of the accessories:

(1) rubber topped turn table

(2) removable center spindle for use in automatic record drop operation

(3) removable automatic center spindle for playing 45 RPM records it dropped one record at a time

(4) removable center spindle for playing single records (one at a time)

(5) center spindle for 45 RPM single play (fits over #4)

(6) The "sensor arm" ( I do not know the actual name of the feature) that swings out and detects, the diameter (7, 10, 12 inches) of the record for accurate placement of the tone arm (7) that contains the cartridge containing the needle that actually rides in the record groove.

(8) Twelve inch record drop mechanism. Drops one record at a time on to turntable or previous record.

(9) control for turntable speed in RPM (16, 33 1/3, 45, 78 RPM)

I do not know the purpose of the controls in the lower right hand position of the pictureture. Some turntables allowed for a slight reduction or increase in the turntable speed, I think this is called hysterisis and I don't know how to spell the term. I do not know if you can have this feature with a  CD. Later models of turntables used a neon bulb as a timing light feature on the turntable so that the user could return the turntable to the correct  speed.  The two picturetures below show the turntable playing a 33 RPM disk without the changer mechanism. The second photo shows the player in the 45 RPM "Changer" mode with records "stacked" for automatic release after each play.

The cartridge that contained the needle was either dynamic (used vibrations from the groove/needle to modulate a crystal thereby producing an electrical current for the preamplifier) or magnetic (the needle was connected to a magnet that vibrated close enough to an electro magnet with a coil that produced the necessary current for the preamplifier. The crystal cartridge produced a voltage that remained at the same level regardless of frequency and could literally be (and often was)  connected to the volume control (potentiometer) rotor and ground. The magnetic cartridge produced a higher voltage for high frequencies than for lower frequencies and therefore was not musically, "flat" or equalized. This problem had to be corrected using an equalizer circuit that preceded the preamplifier. I can recall that the "Williamson" equalizer being the most advanced of such devices. Since the output of the magnetic cartridge produced a lower voltage, a slight amplifier had to be included to send the signal to the preamplifier. Since tape heads are also magnetic, they require (and still do) an equalizer circuit prior to being connected to the "Line" or the signal carrying conductor going to the preamplifier. 

Here is a pictureture of the turntable/changer playing 78 RPM records. For the best sound the playing needle (the part of the "picturekup" in actual contact with the grooves of the record) had to be a 3 millimeter diamond or sapphire. The 45 and 33 RPM records required a 1 millimeter needle. Since it was the needle riding in the groove that produced the electrical signal that went to the preamp of the system, it had to be of the highest quality, usually a sapphire or diamond tip. The quality of the needle determined the wear on the record. As you will note on this website, and as we see in correspondence from Mantovani fans, they literally wore out their Mantovani LPs and were so happy to see them re-released by Vocalion and other producers.

In addition to an "entry" track that moved the tone arm into the playing area of the record, each had an exit track leading from the playing area into a circular track that held the tone arm until manually moved to the rest. Automatic turntables sensed the tone arm in the exit track and this caused it to move automatically to its "rest area". In the automatic "changer mode" the tone arm was moved to an area that would allow the next record in cue to drop to the playing are. This continued until the last record was played and then the tone arm would move to its rest area. Believe it or not but there actually were records produced previous to the automatic changer that began with the tone arm in the middle of the record and as it played it would work its way to the outside track.

To the right are the controls of the system that included an AM/FM tuner (monaural), tuning knobs for the AM and a separate one for the FM reception. I thought this unusual since I can recall most systems using only one knob to tune either system. Because of the higher tuner frequencies for FM and the use of capacitors in this early stage of the system, the FM tuners had a drift fault. That is, it would deviate slightly from the set tuning frequency and actually would tune itself away from the station selected. This was a particularly annoying problem for FM listening. There was circuitry utilized to mitigate this although I cannot recall any of them being too effective. Daniel Thomas tells me that the unit was an early attempt at stereo broadcast in which one channel was transmitted on FM and the other on AM. Very interesting indeed and I never heard of such a thing.

About the tuner, Daniel says, "

Since there is a "Balance" control I thought that it is an early stereo system. I was wrong as Daniel points out. At one time, stereo was only available on the FM bands and not all stations offered the feature. Usually, when the system was tuned to a station with a stereo output, an indicator light shown on in the display.

This system includes a loudness control and controls for Bass and Treble.

To the right is the preamp/amplifier and what appears to me to be a 5 inch by 7 inch speaker, no doubt one of two for stereo. You can see the power supply to the left that appears to use a 5U4 (I think) rectifier tube (valve) that supplied the necessary voltage to the system. Tube (valve) systems were "voltage" driven while solid state systems are current driven. The tube systems required a relatively high voltage supplied between the tube cathode and plate. Any of us old times who worked on these systems can tell you, since we were crazy enough to work on them while they were on, that coming across several hundred volts was not fun. At least one time I was knocked out by the shock. I never told my parents because it would have ended my fun.

You can see the power transformer that supplied the necessary variety of voltages for the system including the required filament voltage. Many of the later tuner/amplifier systems were built with no power transformer with the voltage divided by using tubes filament resistance. 

Because systems used a rectifier (5U4 in this case) that did not produce a pure Direct Current, but a "rippled" one, there is a large "can type" electrolytic capacitor shown just to the right of the power transformer to "flatten" out the voltage. All of us technicians can remember the terrible hum produced when the electrolytic capacitor failed. The electrolytics would hold their voltage even after the system was turned off and therefore they could give you a very uncomfortable shock as personal experience will tell you. I believe that there may be one dual preamplifier tube to handle signals for both sides and one or two main amplifier (final) tubes. I see two speaker output transformers that were the components connected to the speaker inputs. Many solid state amplifiers do not use output transformers and connect the output from the final power transistor to the speaker.

We'll add more to this page later. I hope you techies enjoyed the page and that you will send in your comments, articles, and suggestions. . Again I would like to send a very hearty Thank you to my Canadian friend, Mr. Daniel Thomas for inspiring so many fond, techie memories.