Introduction and History
The RCA 913 is a miniature high-vacuum cathode-ray tube (CRT) manufactured by the RCA Radiotron Division beginning in the mid-1930s, with the datasheet originally dated October 20, 1936 and revised January 30, 1942. It was one of the smallest practical oscilloscope CRTs of its era, featuring a screen diameter of just 1-5/8 inches (approximately 41mm). Designed for compact oscilloscope applications, test equipment, and signal monitoring, the 913 represented a remarkable feat of miniaturization for its time.
The 913 was widely used during World War II by the US Army Signal Corps and other military branches in portable test equipment where size and weight were critical constraints. Its compact metal-shell envelope and rugged small wafer octal base made it well-suited for field equipment that needed to withstand rough handling. RCA produced the 913 in large quantities through the war years, and many New Old Stock (NOS) examples from 1944-era Signal Corps production survive today in collector hands.
The tube uses a No. 1 phosphor, which produces a green fluorescence with medium persistence — an ideal combination for general-purpose oscilloscope use, providing good visibility under ambient lighting conditions while allowing reasonably fast trace updates.
Technical Specifications and Design
Heater
| Heater Type | Coated Unipotential Cathode |
| Heater Voltage | 6.3 volts (AC or DC) |
| Heater Current | 0.6 amperes |
Electrode System
| Focus | Electrostatic |
| Deflection | Electrostatic |
| Deflecting Electrodes DJ₁ and DJ₂ (upper) | Nearest to screen |
| Deflecting Electrodes DJ₃ and DJ₄ (lower) | Nearest to base |
DJ₁ is on the same side of the tube as pins No. 2 and No. 4. DJ₃ is on the same side of the tube as pins No. 2 and No. 8.
Phosphor
| Phosphor Type | No. 1 |
| Fluorescence | Green |
| Persistence | Medium |
Direct Interelectrode Capacitances
| Control Electrode (Grid) to All Other Electrodes | 8 µµf |
| Deflecting Electrode DJ₁ to Deflecting Electrode DJ₂ | 2.5 µµf |
| Deflecting Electrode DJ₃ to Deflecting Electrode DJ₄ | 2.5 µµf |
Maximum Ratings
Maximum ratings are based on a line-voltage design center of 117 volts.
| High-Voltage Electrode (Anode No. 2) Voltage | 500 max. volts |
| Focusing Electrode (Anode No. 1) Voltage | 200 max. volts |
| Control Electrode (Grid) Voltage | Never positive |
| Peak Voltage Between Anode No. 2 and Any Deflecting Electrode | 250 max. volts |
| Grid Circuit Resistance | 1.5 max. megohms |
| Impedance of Any Deflecting-Electrode Circuit at Heater-Supply Frequency | 1.0 max. megohm |
Typical Operating Conditions
| Parameter | Condition 1 | Condition 2 |
|---|---|---|
| Anode No. 2 Voltage | 250 volts | 500 volts |
| Anode No. 1 Voltage | 50 volts (approx.) | 100 volts (approx.) |
| Grid Voltage | Adjusted to give suitable luminous spot | |
| Deflection Sensitivity, DJ₁ & DJ₂ | 0.15 mm/volt DC | 0.07 mm/volt DC |
| Deflection Sensitivity, DJ₃ & DJ₄ | 0.21 mm/volt DC | 0.10 mm/volt DC |
Note: The Anode No. 1 voltage supply should be adjustable to ±30% of the value shown. Approximately 80% of Anode No. 1 voltage is required for current cut-off when, in some applications, it is necessary to use the maximum permissible grid-circuit resistance.
Note: Brilliance and definition decrease with decreasing anode voltages. In general, the Anode No. 2 voltage should not be less than 250 volts.
Physical Specifications
| Bulb | Metal Shell, MT-10 |
| Base | Small Wafer Octal 8-Pin |
| Maximum Overall Length | 4-3/4 inches |
| Maximum Diameter | 1-5/8 inches |
| Mounting Position | Any |
Note on amplification factor, transconductance, and plate resistance: As the 913 is a cathode-ray tube rather than an amplifying valve, the conventional parameters of µ (amplification factor), gm (transconductance), and rp (plate resistance) do not apply. The relevant performance metric is deflection sensitivity, as listed above.
Pinout (Bottom View of Socket)
| Pin 1 | P₂ (Anode No. 2), S (Shell), DJ₂, DJ₃ |
| Pin 2 | H (Heater), K (Cathode) |
| Pin 3 | P₁ (Anode No. 1) |
| Pin 4 | DJ₁ (Deflecting Electrode) |
| Pin 5 | G (Grid No. 2) |
| Pin 6 | DJ₄ (Deflecting Electrode) |
| Pin 7 | H (Heater) |
| Pin 8 | Internal Connection — DO NOT USE |
The 913 is based so that the trace produced on the screen by deflecting electrodes DJ₁ and DJ₂ shall not deviate more than 8° from a plane passing through pins No. 3 and No. 7.
Applications and Usage
Oscilloscope Applications
The primary application of the 913 was in compact oscilloscopes. The RCA datasheet includes a complete typical oscillograph circuit showing the tube driven from a 500V DC supply with a bleeder potentiometer network (R₁ + R₂ + R₃ + R₄) to derive the various electrode voltages. Key circuit values from the datasheet include:
- R₁ = 0.200 megohm
- R₂ = 0.050 megohm
- R₃ = 0.030 megohm
- R₅ = 0.050 megohm (deflection plate coupling resistors)
The DC potential of each deflecting electrode should be maintained essentially equivalent to that of Anode No. 2 by connecting resistors having values not greater than 10 megohms between each deflecting electrode and Anode No. 2. This arrangement, by suitable choice of resistor values, minimizes pattern distortion and pattern drift resulting from unbalanced potentials on the deflecting electrodes. The smaller the resistor values, the less the distortion for a given beam current.
Military Applications
During World War II, the 913 saw extensive use in military test equipment, radar indicators, and portable signal monitoring devices. Its compact size, rugged metal-shell construction, and ability to operate in any mounting position made it ideal for field deployment. The US Army Signal Corps procured large quantities of the 913 for use in portable oscilloscopes and specialized test sets.
DC Amplifier Service
For DC amplifier service, the deflecting electrodes should be coupled directly to the output of the amplifier by omitting the blocking condensers. In addition, it is usually preferable to remove the associated deflecting electrode resistor to minimize the loading effect on the DC amplifier. With the resistor removed, Anode No. 2 must be returned to some point in the DC amplifier circuit such that the potential difference between Anode No. 2 and the average voltage across the deflecting electrodes will be as low as possible, to minimize spot defocusing.
Sound Characteristics
The RCA 913 is a cathode-ray tube, not an audio amplifying valve, and therefore does not have "sound characteristics" in the traditional sense that audiophiles discuss with triodes, pentodes, or beam tetrodes. It does not amplify audio signals and cannot be used as a gain stage, output stage, or rectifier in an audio circuit.
That said, the 913 does produce subtle audible artifacts in its operating environment. The electron beam striking the phosphor screen and the high-voltage power supply required to operate the tube can produce faint mechanical hum or buzz, particularly from the power supply transformer. Vintage oscilloscope enthusiasts sometimes describe the characteristic quiet "tick" or faint whine associated with CRT-based test equipment as part of the nostalgic experience of working with classic instruments.
In the context of audio work, the 913 was historically used to visualize sound — displaying audio waveforms on its green phosphor screen. Engineers and technicians would use 913-equipped oscilloscopes to observe distortion, clipping, frequency response, and other characteristics of audio amplifiers and signal chains. In this indirect sense, the 913 contributed significantly to the quality of audio equipment by enabling visual analysis of audio signals during design and troubleshooting.
Equivalent or Substitute Types
| Type | Notes |
|---|---|
| CV2691 | British military (CV-series) equivalent designation for the 913. Direct substitute with identical specifications and pinout. |
The 913 was a somewhat unique tube in its size class. While other small CRTs existed (such as the RCA 914, which was a similar tube with a different phosphor), direct pin-compatible and electrically identical substitutes are limited to the CV2691 military equivalent. Builders should verify that any proposed substitute matches the 913's specific deflection plate arrangement, phosphor type, and voltage ratings before substitution.
Notable Characteristics
- Extremely compact CRT: At just 1-5/8 inches in diameter and 4-3/4 inches in overall length, the 913 was one of the smallest practical oscilloscope tubes available in the 1930s and 1940s.
- Metal shell construction: The MT-10 metal shell bulb provided excellent shielding from external electric fields and added mechanical ruggedness compared to all-glass CRTs.
- Dual deflection plate pairs: The tube features two pairs of electrostatic deflection plates (DJ₁/DJ₂ near the screen, DJ₃/DJ₄ near the base) with different sensitivities, allowing X-Y deflection for standard oscilloscope operation.
- Asymmetric deflection sensitivity: The lower deflection plates (DJ₃ & DJ₄, nearest the base) have approximately 40% greater sensitivity than the upper plates (DJ₁ & DJ₂, nearest the screen) at both operating voltage conditions. This is typical of CRT designs where plates closer to the electron gun deflect the beam through a longer path to the screen.
- Any mounting position: The 913 can be operated in any orientation, a significant advantage for portable and field equipment.
- Green P1 phosphor: The medium-persistence green phosphor was the standard choice for general-purpose oscilloscope work, offering good visibility and a reasonable decay time for observing repetitive waveforms.
- Low heater power: At 6.3V and 0.6A (3.78 watts heater dissipation), the 913 was economical to operate and compatible with standard 6.3V heater supplies common in vacuum tube equipment.
Usage in the Audio Community
While the RCA 913 is not an audio tube per se, it holds a special place in the audio community for several reasons:
Vintage Test Equipment
The 913 is most commonly encountered by audio enthusiasts as the CRT in vintage oscilloscopes from the late 1930s through the 1950s. Restorers of classic audio test equipment — particularly compact, portable oscilloscopes from manufacturers like RCA, Dumont, and others — seek out working 913 tubes to bring these instruments back to operational status. A functioning vintage oscilloscope with a 913 CRT is both a practical tool and a conversation piece in any tube audio workshop.
Visual Audio Display Projects
In recent years, DIY audio enthusiasts and makers have used the 913 in custom "oscilloscope music" displays and audio visualization projects. The tube's compact size, simple drive requirements, and attractive green glow make it appealing for building small, self-contained audio visualizers that display Lissajous patterns or waveform traces driven by audio signals. These projects combine the aesthetic appeal of glowing vacuum tube technology with the visual representation of music.
Collector Appeal
NOS (New Old Stock) examples of the 913, particularly those with US Army Signal Corps markings from the 1944 production era, are sought after by tube collectors. The compact metal-shell form factor and military provenance make these tubes attractive display pieces. While not as valuable as rare audio tubes, the 913 commands modest collector premiums, especially in original military packaging.
Amplifier Builders' Workshops
Serious tube amplifier builders and technicians who maintain vintage test equipment often keep a stock of 913 tubes for their bench oscilloscopes. While modern digital oscilloscopes have largely replaced CRT-based instruments for precision work, many experienced audio engineers prefer the analog display of a CRT oscilloscope for certain tasks — particularly observing clipping behavior, crossover distortion, and the overall "shape" of audio waveforms, where the continuous phosphor trace can reveal subtleties that a sampled digital display might miss.
Aesthetic and Artistic Use
The warm green glow of the 913's P1 phosphor has made it popular in artistic installations and custom audio equipment faceplates. Some boutique audio equipment builders have incorporated small CRTs like the 913 as visual indicators or level meters in high-end preamplifiers and integrated amplifiers, adding a distinctive retro-futuristic aesthetic that appeals to the vacuum tube audio community's appreciation for vintage technology.
