RCA Theremin Sound, Tone, and Vacuum Tube Advice

The broad range of what is generally called The RCA Theremin sound actually encompasses quite a spectrum of tone quality, from the buzzy, nasal Green Hornet sound (from the radio show of the same name), to a resonant woody low and sweet high timbre that's considered the holy grail of theremin tone.

It is the authors' observation that more RCA Theremins fall into the former category of a warm, but less than ideal sound, than do the rare exceptions—unless certain improvements are made.

Voicing your theremin for best sound

The Tone Resistor

During the course of production, RCA introduced a way to improve the tone quality by adding a carbon resistor added to the control grid of the 24A mixer vacuum tube. At the factory, the optimal value of this resistor was likely determined by trial and error (with measured progress) until a pleasing tone was achieved. You can easily tell if your RCA Theremin has this factory modification, simply by looking inside your instrument and noting whether the top of the 24A vacuum tube is connected directly to a wire, or to an exposed carbon resistor. (If you find that your theremin has the resistor at the top of the 24A, and we have your theremin on our registry without interior photos, please let us know about it).

This tone modification is mentioned in the RCA Service Notes troubleshooting section. If your RCA Theremin does not have the resistor, you may find that you can obtain a sweeter tone by adding a resistor in series with the 24A grid cap, in the range of 500 to 2,000 ohms. In other cases where there's already a resistor there, you may find that further improvement can be achieved by experimenting with this resistance.

A non-invasive way to add a resistor or experiment with a number of resistors, is to make an adapter by using a metal grid cap salvaged from a dud vacuum tube (soldered to one end of your added resistor), and a spare grid cap clip (soldered to the other end of your new resistor). The new resistor can then simply be inserted between the factory-installed grid cap clip, and the top of the 24A without disturbing the factory solder joint on the original grid cap clip. Changing a factory-installed resistor requires unsoldering the 24A grid cap clip (and/or resistor) from its wire, followed by soldering in the new or replacement resistance and reinstalling the grid cap clip. Observe common-sense precautions when soldering to avoid burns and prevent damage.

There's another, even easier way to alter the tone of an RCA Theremin, that requires no parts or electronic service training, and uses no special equipment or tools other than your ears.

The 27 Vacuum Tube

While certain improvements can only be accomplished by changing soldered-in parts, you may be able to improve the tone of your RCA Theremin based on the fact there are two pitch oscillator vacuum tube sockets which take a type 27 vacuum tube, and three type 27 vacuum tubes in every RCA Theremin (the third one is your audio preamplifier vacuum tube).

The vacuum tubes of an RCA Theremin, with the 27 vacuum tubes highlighted

Vacuum tubes as seen from through the open cabinet doors. 27 tube in position 1 and 3 are pitch oscillators; position 4 is the audio pre-amplifier.

By simply substituting the type 27 pitch oscillator vacuum tubes with different ones of the same type, you can change the tone or voicing of the instrument. Two pitch oscillator sockets, times three type 27 vacuum tubes to choose from gives you six possible permutations from which to select the best tonal result.

Six possible permutations using three type 27 vacuum tubes

Possible permutations using the three type 27 vacuum tubes that are found in every RCA Theremin.

To keep track of the permutations you've tried, you can mark an easily readable number or letter near the top of the vacuum tube with a dry erase marker (avoid putting it on the factory number markings), and maintain a log of permutations tried.

It could help to obtain a small assortment of type 27 vacuum tubes to choose from. If you acquire even one extra type 27 vacuum tube, (for a total of four to choose from, with three remaining in the instrument), it will give you 12 possible permutations. Two extra 27s (5 total vacuum tubes) yields 20 permutations, three extras give 30 possible permutations, etc.

An illustration of a globe style vacuum tube next to an ST style vacuum tube

Globe vs. ST style vacuum tubes.

Sometimes the tone change is very subtle, or you may find no discernible change at all. Other times (depending on the vacuum tubes you use), the change can be quite remarkable, for better or for worse. We've also found that the early globe or balloon style vacuum tubes can sometimes give better results than the later shoulder or ST style of vacuum tube.

Explanation

It makes no difference whether the one vacuum tube tests stronger than another. The difference, we believe, may be based on nothing more than normal manufacturing tolerances that marginally changed the spacing between the vacuum tube elements. This would result in a slightly different inter-electrode capacitance, from one vacuum tube to another, one vacuum tube lot to another, or one manufacturer to another.

In other cases, a great sounding vacuum tube may be nothing more than one that has been accidentally dropped, thus bending the internal supporting structure of one element in relation to another. That being said, we do not recommend intentionally damaging your vacuum tubes. In general, you want to avoid vacuum tubes that have visibly bent or at-angle internal structures, as these may also have internal short circuits which can damage your theremin. A test for vacuum tube shorts should be part of every vacuum tube test. See Testing Vacuum Tubes under the Troubleshooting Advice section.

In any other type of electronic equipment this difference would be inconsequential, but in the sensitive circuit of the RCA Theremin where fractions of picofarads cause sweeping changes, these minuscule variations in manufacturing can result in a notable difference to the tone.

The 24A Vacuum Tube

You may find that a slight additional improvement in the tone quality can be obtained by experimenting with different 24A mixer vacuum tubes, although this is more subtle than pitch vacuum tube substitutions, if noticeable at all. Briefly, each of the two pitch oscillators sends its output to one of two grids in the 24A, to be equally blended. The screen grid in the 24A is physically much larger than the control grid in that same vacuum tube, so in order to equalize the level of the two inputs, RCA placed a 10,000 ohm resistor in line with the screen.

Theoretically all vacuum tubes of a certain type (for example, 24A) would be manufactured to the same specifications, regardless of the maker. In reality variations can occur from one manufacturer to another, and sometimes within the same manufacturer's product. If the total grid area differs substantially enough from one vacuum tube to another, it can have an audible effect on the quality of sound.

Warning: For the protection of your instrument, do not attempt to swap the position of the 24A vacuum tube with any other position. The purpose of the 24A mixer vacuum tube is to blend, or mix the waves generated by the pair of 27 pitch oscillator vacuum tubes that stand on each side of it. The 24A is the only other 5-pin vacuum tube in the RCA vacuum tube complement, easily identified by the terminal and connecting wire at the top of the vacuum tube, and is located between the pitch oscillator sockets. Only the 24A should be installed in the mixer socket, and the mixer socket should have only the 24A installed in it.

The vacuum tubes of an RCA Theremin, with the 24A mixer vacuum tube highlighted

The vacuum tubes of an RCA Theremin, with the 24A mixer vacuum tube highlighted. As seen from through the open cabinet doors.

The 56 Vacuum Tube

A type 56 vacuum tube can be substituted for the type 27. If your RCA Theremin has a particularly edgy, raspy tone quality that you'd like to smooth out, we've found that a mellower but more muted sound can sometimes be obtained by trying a type 56 vacuum tube in place of one or both of the type 27s in the pitch oscillator sockets. While smoothing out the shrill highs (a desirable improvement), a result of this particular substitution can be that the lows can sound too mushy and lack the definition and woody, cello-like timbre that's more desired. You can also experiment with using a 56 in one pitch oscillator socket and a 27 in the other. It's all about wave-shape of the pitch oscillators, and the blending of those waves in the mixer vacuum tube, all of which are influenced by the particular oscillator vacuum tubes you have plugged into those two sockets.

While electrically somewhat different, the type 56 vacuum tube shares the same internal pin connections with the type 27, and the voltage of the heater is the same while consuming less current (1.0 amp for the type 56 heater, vs. 1.75 amp for the type 27). Likewise, the voltage and current specifications for the grid and plate are within safe limits for substitution purposes. The type 56 vacuum tube has a higher amplification factor than the type 27. Therefore if using the 56, it may be desirable to conduct further experimentation, as outlined under Voicing Your Theremin above.

Saturation Points & Sweet Spots

You may find that after trying a few permutations, your ears reach a saturation point that reduces your ability to discern a change (especially the more subtle improvements). It's best to give the exercise a rest at this point and come back to it later. If you have access to an oscilloscope, you can also employ a visual aid, by consulting the waveforms provided below. These are representative of one of the better sounding RCA Theremins that we've encountered and can serve as a target wave shape to achieve as you try different permutation of pitch oscillator vacuum tubes.

The challenge of voicing any vacuum tube theremin is that the very characteristic that can give you sonic richness in the lower register, can make the upper register shrill and annoying (even if you are playing more or less in tune). Likewise, if the upper register is made too mellow and sweet sounding, it can result in a poorly-defined, muddy bass response.

If there's a sweet spot to pursue when you're working on tone improvements to the RCA Theremin (whether by trying different vacuum tube permutations, altering the resistance of the mixer grid inputs or taking other measures), the goal should be to achieve a woody, resonant bass response that's so cello-like that you can practically hear the rosin on the bow, progressing to a pleasing mid-range that's smooth and moderate, and a sweet upper range. This requires patience, the ability to discern minute changes in timbre, and a logical way to chart the changes. It can be a time-consuming process but almost any RCA Theremin is capable of rewarding your efforts with a pleasingly rich quality of sound.

Audio Waveform Spotlight

The desirable traits described above are graphically depicted in the shape of the waveforms shown below. We took these readings directly from an RCA Theremin in Philadelphia that possesses a nearly ideal quality of sound. Notice how the harmonically rich, cello-like waveform of the lowest A (A2, shaped like a cursive lower-case letter r), gradually loses its peak and slope as we ascend up the scales. As the upper register is reached and A6 is heard, the waveform and its corresponding sound has lost much of the edginess appropriate to the lower notes, and has become almost a sine wave. However the waveform still retains enough character to impart some warmth and life to the high notes, neither shrill nor hollow sounding. This is the near-singing quality that is so sought-after.

Key to graph: The first five waveforms shown are all of the same note (A) over a four octave range, with the A above Middle C (A 440) in the center of the range. The sixth waveform is a repeat of A6, but expanded 60% so the waveform characteristics can more easily be compared to the lower notes.

Oscilloscope views of the audio waveform at the speaker terminals of an RCA Theremin

Oscilloscope views of the audio waveform at the speaker terminals of an RCA Theremin. All are shown at 10 volts per division (amplitude, vertically). The first five entries have a horizontal time-base progression of 0.5 milliseconds per division (0.0005 sec/div.), and the last entry (the prolonged A6) is shown at 0.2 ms/div. to facilitate waveform comparison.

RCA Theremins & Speakers

Your Speaker Makes a Difference

RCA’s preferred theremin speaker (or loudspeaker to use the standard terminology of the day), was the model 106, which came in a wood and tapestry-trimmed floor model cabinet with turned corner posts. The 106 was among the best sounding consumer speakers of the day due to its large size and house-current-powered field coil. When RCA introduced the theremin in October 1929, the 106 (which was carried over from RCA’s 1928 radio product line) was the logical choice, given the musical instrument nature of the theremin.

The theremin buyer however, was not required to buy the 106, and if they were willing to settle for a narrower audio with less bass, could opt for the smaller and less expensive RCA loudspeakers 100B or 103, or the discontinued 100 or 100A. Likewise, almost any speaker made by any manufacturer in the mid to late '20s would do the job. The only requirement was that it be compatible with the high impedance of the theremin’s audio output jacks.

Today, you have a choice of speakers, including 1920s vintage or modern. As it was more than eighty years ago, the RCA 106 remains one of the best sounding speaker options for the RCA Theremin. As noted above, you can expect a less full-bodied sound when you opt for one of the smaller model loudspeakers. In general, we do not advise using large, inefficient modern hi-fi speakers (requires matching transformer to avoid damage to the theremin), as they are usually designed for high-watt sound systems. When combined with the low actual audio power of the theremin (less than one watt, true RMS), your large hi-fi speaker won’t respond with adequate volume and tone quality. If a permanent magnet, voice coil speaker must be used (with matching transformer), you will want to select as efficient a speaker as possible to avoid these losses.

Bleed or Ghost Tone

A possible downside to using even a carefully selected, high-efficiency modern speaker is that it will have a higher cut-off frequency than a vintage one (will reproduce higher notes and harmonics), and you might hear the famous RCA Theremin ghost tone, even when your volume hand is nearly touching the volume antenna and silence should be expected. This undesirable artifact is a residual background sound that mimics the pitch of the note that you’re meant to hear (responds to hand position), but is very quiet, thin sounding, and distorted. The bleed tone always remains at the same background level, and doesn’t respond to movement of the volume hand. Once the volume hand is raised and the normal amplified sound emerges from the speaker, the bleed tone is no longer noticeable. A side-by-side test using the same RCA Theremin with different speakers shows that the bleed tone isn’t generally heard when using the inherently filtered RCA 106 or other vintage loudspeakers.