This relatively small organ uses vacuum-tube-based circuitry to both produce and amplify its sound, containing 30 tubes of 8 different types in total. No transistors are used whatsoever; after all, the first transistor radio only predated this unit by around two years! This particular example was made in 1956 based on its serial number as well as date markings on the original tubes. Somewhat like an accordion, it has a section of chord buttons, 96 to be exact, normally played with the left hand, and a 37-key manual generally used for playing melodies with the right hand. As well, it has two foot pedals for playing bass notes; the left plays the root of the chord, and the right plays the fifth up (or down, depending on the chord) from that. It has no expression pedal like most organs, but instead uses a lever moved with the right knee to control its volume.
Despite its age, this unit required relatively little attention to make it fully functional. It had clearly been well cared for by the previous owner(s).
When received, the bass pedals and the solo voice were not producing any sound. Two of the 12AU7 tubes—the solo preamplifier/pedal control and the pedal divider driver—each had one dead heater, so these were replaced.
The lovely rectangular "tablet" switches were stiff, so I lubricated those, and gave the whole thing an overall cleaning.
Some of the notes of the organ section were a little too out of tune, so I tuned them based on the procedure in the service manual (see Links section), which involves tapping the cores in or out of the tuning inductors with a hammer and screwdriver.
Lastly, and most importantly, I added a fuse in line with one of the power cord wires, since for some bizarre reason, Hammond did not include one! Given the spec of 158 VA on the builder's plate, at 120 V, the current would be 1.32 A. Thus, considering also that tube electronics have a high initial surge of current while the heaters heat and capacitors charge, and 1.32 A is not a standard fuse value, I used a 1.5 A, 250 VAC slow-blow fuse in an inline holder tucked in the power pack. This should hopefully prevent damage should anything short out. I also replaced the decayed rubber power plug.
The following is more of a refurbishment than a repair: In June 2015, I decided to finally clean the chord buttons and the button area of the chord switch cover, which were thickly coated with light-brown dirt. The process to remove these pieces is a bit tricky. First, adhesive tape must be applied across all of the tops of the chord buttons, otherwise they will fall out when the plastic piece is lifted. Then, the thin plastic strip with the chord names (C, G, etc.) has to be removed by some small flat-head screws. Next, the chord bar must be removed, first by removing the two screws holding it in place, and then by removing the single screw holding the reed switch to the chord bar; the service manual says to desolder this switch, but this is unnecessary as long as you keep the pieces of it held together (with tape, for example) while the screw is removed. Finally, the remaining four screws can be removed and the chord switch cover lifted off. Here are some images of the condition before cleaning:
When cleaning the chord buttons, the chord switch cover holding them must be removed, which reveals the array of 96 thin metal plates, each corresponding to a chord button. These makes the necessary contacts for each button when pressed down, and are impressive to look at.
The chord switch cover was cleaned mostly in a large sink with a cloth soaked in soapy water. Hand soap was used. After being run under tap water to wash off soap, the remaining dirt was then removed with moistened cotton swabs. The chord buttons were placed into a tub full of soapy water and individually scrubbed, then placed in a regular water tub to remove the soap, and then left to dry. As well, the metal hardware was cleaned and polished using Brasso, having been removed for the cleaning of the top piece. It all turned out quite well, I would say.
The finished product:
As a final repair note, I did order and receive high-quality modern replacements for all of the electrolytic capacitors, since they are now about 60 years old and likely quite dry. However, I really haven't found a need to perform the replacement yet, since everything is functioning quite well. The organ's output is typically very low in noise and hum, with only occasional slight "popping" noises that may be indicative of aging electrolytics. One of these days...
Looking at the back, the outer features of the electronics can be seen, including the massive number of vacuum tubes! There are 30 tubes in total, mostly of the 12AU7 dual triode type. There are three main electronic chassis: the generator chassis (largest), organ chassis (to the bottom left), and power pack (with the very large transformer). Also visible are the two 10" alnico speakers, which are wired in parallel, and which are of curved- and straight-cone types according to the S-4's service manual. (Note that the S and S-1 have one 12" field coil speaker instead.) The builder's plate is also on the back. Coincidentally, this S6 is of "form B3", which is of course the model name of one of the most famed Hammond tonewheel models.
Powered on in the dark, it is even more of a beautiful sight:
Anyway, let's have a look at the electronics, beginning with the generator chassis. It holds 21 of the 30 tubes, all of the amplifier circuitry, and all of the solo and chord circuitry. One user adjustment is provided on this chassis: the two "solo" voice tuning knobs, fitted to rotary switches.
Removing the two screws holding the top wooden panel down from below allows it to be removed. Another two screws being removed will then allow the generator chassis to be rotated 90 degrees into servicing position, making the components very easily accessible.
The long row of identical-looking components above the generator chassis are the tuning inductors for the "solo" voice, which are put in series depending on the key pressed. The solo voice behaves essentially as a monophonic synthesizer (only the highest key pressed is actually played). On the generator chassis itself, there are quite a few plastic film, electrolytic, ceramic, and perhaps some wax paper capacitors, wire-wound and carbon composition resistors, as well as quite a few tube sockets, inductors and transformers. It works quite well even with what appears to be all original components. Some of the components would be tricky to replace, such as the tuning inductors and capacitors, since they were specially selected and matched at the factory.
The construction of the organ chassis is fairly similar to the generator chassis; there is a drawn diagram of the inside of the S-4's version in the service manual. The power pack doesn't have much in it besides the transformer and the fuse holder that I installed. Nonetheless, I'll take pictures of them at some point, as well as taking more pictures of the front of the unit. This will likely happen along with replacing the electrolytic capacitors.
Since this isn't one of Hammond's tonewheel-based models, its sound is unlike what you would expect from a typical "Hammond Organ". But, like a Steinway piano could never sound like the Archibald Ramsden, I can assure you: a tonewheel-based Hammond will never sound like this.
The solo voice, being essentially a monophonic analog synthesizer, sounds exactly like that. There is a single solo oscillator, the waveform of which can be altered from vaguely "sawtooth-like" to "square-like" by activating the "solo woodwinds" tab. While there is only one oscillator, there are two dividers, such that the oscillator has bass, tenor, and soprano registers. Further tone control is achieved with the five timbre tabs (deep tone, full tone, first voice, second voice, and brilliant). These turn on or off the outputs of basic passive filters through which the solo signal passes, and of course without the resonance/high-Q-factor that you might expect from, say, a Minimoog's filters. The highest note pressed is the one played.
The organ voice is polyphonic, with a tone much like the solo voice without the woodwinds tab activated, and in the "tenor" register. Interestingly, it has only 16-note polyphony, despite having 37 keys. The reason for this lies in the organ oscillator chassis. It has eight 12AU7 dual triodes, so 16 triodes in total, each part of an organ oscillator. These oscillators are assigned to either two or three notes each, always directly beside each other. The designers figured that semitone and (lower and highest) whole tone intervals were so seldomly used that it wouldn't matter! See the Service Manual, available near the bottom of the page, for more info on this.
The chord voice has six oscillators, with most chords having three notes played at once, and with a "sawtooth-like" tone. A "chord bar" can be set to either slightly or completely mute the chord output when the bar is depressed, using the "sustain cancel" tab. The notes played by the pedals (the root and fifth of the chord being played) are simply from the chord oscillators divided down two octaves, plus lowpass filtering.
In the future, there will be a demonstration video showing off the sounds here, so stay tuned!
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