Selmer Clavioline
The Selmer Clavioline is an early 1950s synthesizer. It comes in two parts: a keyboard and an amplifier, which were originally connected via a thick control harness. (We modified the harness, as we’ll explain shortly.) The keyboard has a unique L-shape because it was intended to mount underneath a piano keyboard. The tone generators, located inside the keyboard, are vacuum tube oscillators. The external amplifier is the power supply for the tubes inside the keyboard, and it also amplifies the keyboard signal. Listen to some sound clips of the Clavioline here.
The Clavioline has a few interesting features:
The only volume control is a knee lever. In the photo above, we have it wedged open on the music stand. If you are proficient with a knee lever, and if you have a keyboard stand that allows you to mount a miniature keyboard at approximately knee-height, this is surely a wonderful feature. For the rest of us, the knee lever can be propped open with a thick piece of foam, and you will operate the Clavioline at full volume.
The two sliding switches at the front of the keyboard (those little silver dots in the photo above) are octave switches. So, although the keyboard only has 36 notes, it has a 5 octave range.
The white toggle switches at the front of the keyboard are effects and filters. The four on the left offer different kinds of vibrato. The rest of the switches—labeled 0-9 and O, A, B, V, and P—enable passive filters. Although some of these filters are subtle, they all have an audible effect on the sound.
There are four knobs for tuning the keyboard: two on the left that affect the bass, and two on the right that affect the treble. Of each, one knob is mounted on the outside of the instrument, accessible to the player; one is mounted inside, for the tech. The inside potentiometer brings the tuning into a reasonable range, and the outside potentiometer is for fine-tuning.
The Clavioline manual is adamant that the amplifier portion is extremely special, because—to paraphrase—it provides the harmonics and distortion that the Clavioline needs to sound good. This is actually true: the Clavioline amplifier has a parallel single-ended 6V6 output, and it sounds extremely cool.
About the Amplifier rebuild
The old speaker was no longer working, so we installed a new Celestion speaker.
On this particular Clavioline, the amplifier had been modified to add a guitar input and a volume control. This mod was a good idea, because the Clavioline amp pairs really well with guitar. However, the original design of the chassis forced the input jack into a very awkward spot: between the wiring harness receptacle and the power switch, both of which are capable of introducing noise into the signal.
This is a big weakness of the Clavioline: it is just, generally, pretty noisy. In the early 1950s, audio recording was pretty lo-fi at best, and so nobody prioritized low-noise operation when designing amplifiers. Inside the amp, noisy wires were routed near signal wires, grounding was haphazard, and the wiring harness even bundled the Clavioline signal with high-current AC tube heater voltage. (This induces hum into the signal.) Plus, whenever the harness is unplugged, the metal connectors are exposed. This is a shock hazard because the harness carries high-current AC for the tube heaters, and 150V DC for the tube plates.
To remediate this, we opted to rebuild the amplifier in a new chassis. A total gut rebuild is our preferred method of restoration for small tube amplifiers like this one, and we’ve done it many times for Wurlitzer tube amps. It’s a strategy that allows us to create an entirely new layout, so we can avoid ground loops and separate signal wiring from power supply wiring. In most cases, we reuse the original chassis. However, in this case, the poor layout of the Clavioline’s control panel made it impossible to achieve good results in the original chassis. With the harness mounted at the rightmost edge of the chassis—and the switch and pilot light in the middle—we would be limited in our ability to separate signal wiring from power supply wiring. We found a new-production black powdercoated chassis with almost the exact same dimensions, and used our drill press to make custom mounting holes.
In our rebuild, we followed the original circuit schematic closely, because we wanted to preserve the harmonic distortion that the original designers believed was so important to the sound of the instrument. We only made small modifications that would further reduce noise and increase reliability, and avoided anything that would change the amplifier’s tone. We replaced all of the original passive components with new, equivalent components. We also carefully removed the original power transformer and choke from the old chassis, and re-installed them in the new one.
The original Clavioline design had just one control harness, which contained signal wires and 6.3V tube heater wires, among other things. Inside the harness, the signal wires were shielded, which prevents AC heater noise from getting into the signal—but only inside the cable. The solder connections on the plug ends are not shielded, and, at this point inside the amp, heater wires and signal wires are mounted just millimeters from each other. So, one of our priorities was removing the signal wires from the harness. We did this by creating a 1/4” output on the Clavioline, so that the signal can travel on a regular instrument cable. One upside is that it is now possible to put pedals in the Clavioline signal path. You can also connect the Clavioline signal output to any amplifier you want (as long as the control harness is connected, to power the oscillator tubes inside the keyboard).
ABOUT THE KEYBOARD RESTORATION
The keys of the Clavioline are very small and move with a very simple rocking motion. We lubricated the action centers and ensured that the keys were as responsive as possible. We fully disassembled the keyboard in order to give close attention to all of the moving parts, including the octave switches.
As in any keyboard, it is really important for the key contacts to make a solid connection, so we spent a lot of time ensuring that the contacts were clean, unbent, and free from oxidation. The contacts are two thin wires that hit an L-shaped piece of metal when the key is depressed. The metal is L-shaped because one wire has to hit first, then the second. The way the wires are mounted makes it difficult or impossible to replace them if they are broken, so we were sure to clean them very carefully. The white rocker switches have a similar system of contacts, and we cleaned those as well.
When we received the Clavioline, most of the important capacitors inside the oscillator circuit had already been replaced with high-quality new capacitors, so we were able to leave those as-is. Some of the capacitors that provide the phase-shifting for the oscillator were still original, though. Because their values had drifted over time, the intonation of the keyboard wasn’t as precise as it could be. Once we replaced them, we were able to tune the Clavioline with a high level of accuracy. Beyond that, intonation is left to the user, via the tuning potentiometers on the left and right sides of the keyboard, which make it possible to really dial in the intonation for the key that you’re playing in.
Overall, the Clavioline is a very cool keyboard and we were very happy to work on such a historical—yet still very functional—synthesizer!