1. Introduction and History
The EM87 is a specialized vacuum tube developed by Philips in the late 1950s as part of their Miniwatt series. It belongs to the family of "magic eye" tuning indicators, which were popular visual aids in radio receivers and audio equipment from the 1930s through the 1970s. Unlike its predecessors that displayed a glowing green fan or eye pattern, the EM87 featured a more modern vertical bar display format.
Philips introduced the EM87 as a more compact and versatile alternative to earlier tuning indicators, designed specifically for the emerging generation of smaller radios and tape recorders. The tube gained popularity throughout Europe and was manufactured primarily in Philips' facilities in the Netherlands. Its production continued into the 1970s before semiconductor-based indicators gradually replaced vacuum tube technology.
2. Technical Specifications and Design
Physical Characteristics:
- Envelope: B9A (Noval) base with 9 pins
- Height: Approximately 55mm
- Diameter: Approximately 22mm
- Construction: Glass envelope with internal fluorescent screen
Electrical Parameters:
- Heater Voltage: 6.3V
- Heater Current: 300mA
- Maximum Anode Voltage: 250V
- Typical Anode Voltage: 200V
- Maximum Target Voltage: 250V
- Typical Target Voltage: 200V
- Control Voltage Range: 0 to -22V
- Display Sensitivity: Approximately 8V for full indication
Pin Configuration:
- Pin 1: Internal Connection
- Pin 2: Heater
- Pin 3: Target
- Pin 4: Internal Connection
- Pin 5: Anode
- Pin 6: Internal Connection
- Pin 7: Cathode
- Pin 8: Control Electrode
- Pin 9: Heater
Design Features:
The EM87 employs a unique design where the fluorescent display appears as a vertical bar rather than the radial pattern seen in earlier magic eye tubes. The internal structure consists of a triode section that controls an electron beam directed at a fluorescent target. As the control voltage becomes more negative, the illuminated portion of the vertical bar decreases, providing a visual indication of signal strength or tuning accuracy.
The tube's display screen is coated with a green phosphor compound that glows when struck by electrons. A special feature of the EM87 is its relatively linear response to input voltage changes, making it particularly suitable for precise tuning applications.
3. Applications and Usage
The EM87 found widespread use in various electronic equipment:
Radio Receivers:
The primary application of the EM87 was in FM and AM radio receivers, where it provided visual feedback for optimal station tuning. The vertical bar display would reach maximum height when a station was perfectly tuned, making it easier for users to find the clearest signal.
Tape Recorders:
Many high-end tape recorders incorporated the EM87 as a recording level indicator. The tube would display the audio signal strength, helping users set appropriate recording levels to avoid distortion while maximizing signal-to-noise ratio.
Audio Equipment:
The EM87 was used in various audio amplifiers and preamplifiers as a volume level indicator. Some audiophile equipment manufacturers favored the warm, analog appearance of the magic eye display over mechanical VU meters.
Test Equipment:
The tube appeared in various test instruments, including signal strength meters and audio analyzers, where its relatively linear response made it suitable for measurement applications.
Typical Circuit Implementation:
In a typical application, the EM87 would be connected to an audio detector or AGC (Automatic Gain Control) circuit. The control grid would receive a negative voltage proportional to the signal strength, causing the illuminated portion of the display to change accordingly. A voltage divider network was often employed to adjust the sensitivity of the indicator to match the specific application requirements.
4. Equivalent or Substitute Types
Several tubes can serve as direct or near equivalents to the EM87:
Direct Equivalents:
- EM800: A direct equivalent with identical electrical characteristics and display pattern
- 6E2: Eastern European equivalent (particularly from Tesla)
- 6HU6: North American designation for essentially the same tube
Functional Substitutes (requiring minor circuit modifications):
- EM84/6FG6: Similar vertical bar display but with different sensitivity characteristics
- EM80/6BR5: Earlier vertical bar indicator with comparable function but different pinout
- EM81/6DA5: Similar to EM80 but with different sensitivity
- EM71: Predecessor with comparable display characteristics
Modern Alternatives:
For restoration projects where original EM87 tubes are unavailable or prohibitively expensive, some enthusiasts have developed solid-state replacements using LEDs or small OLED displays that mimic the appearance and behavior of the original magic eye display.
5. Notable Characteristics
Display Pattern:
The EM87's most distinctive feature is its vertical bar display pattern. Unlike earlier magic eye tubes that showed a radial pattern resembling an eye closing, the EM87 presents a vertical illuminated bar that changes in height according to the input signal. This vertical display made it easier to read from various angles and allowed for more precise tuning.
Sensitivity and Linearity:
The EM87 offers excellent linearity in its display response, with the bar height changing proportionally to input voltage changes. This characteristic made it particularly valuable in applications requiring precise visual feedback. The tube typically requires about 8V of control voltage swing to move from minimum to maximum indication.
Longevity:
When operated within specifications, the EM87 demonstrates impressive longevity compared to some other indicator tubes. The phosphor coating is relatively resistant to burning, and many examples continue to function well after decades of use. However, like all vacuum tubes, they eventually suffer from cathode emission degradation and phosphor fatigue.
Collectibility:
The EM87, particularly those manufactured by Philips Miniwatt in Holland, has become highly sought after by collectors and vintage audio enthusiasts. NOS (New Old Stock) examples that pass AVO testing are especially valuable. The distinctive green glow and vintage aesthetic appeal make these tubes popular not only for restoration projects but also for modern tube audio designs that incorporate visual indicators.
Warm-up Characteristics:
The EM87 typically requires 10-15 seconds to reach operational temperature after power is applied. During this warm-up period, the display may appear dim or uneven before stabilizing to normal brightness and response.
Power Consumption:
With a heater drawing 300mA at 6.3V (approximately 1.9 watts) plus modest anode current, the EM87 is relatively efficient compared to power tubes, making it suitable for inclusion in equipment where power budget is a consideration.
In conclusion, the EM87 represents an elegant example of vacuum tube indicator technology that combines functional precision with aesthetic appeal. Its enduring popularity among audio enthusiasts and collectors speaks to both its technical merit and the nostalgic charm of its glowing green display.
