The most fascinating type of Tubes are the indicator Tubes
Their green shimmering light is a guaranteed eyecatcher rarely seen today. A few decades ago almost every Radio had one built in as a tuning indicator, but now these tubes become more and more expensive and hard to find.
The Circuit described here can be used for a number of Tubes, it was testet with EM800 and EM84, These Tubes are still available and easy to drive.
Layout / Component Placement
The circuit is operated from 250V DC, connected to X1. Filament Voltage is connected to X3.
An auxilliary Voltage of about 30 Volts is generated by R4, D1 and C2. R1 should be chosen to allow for about 1 -2mA of current through the Z-Diode.
The Resistor R4 has about 220 V of Voltage drop, which means about 400mW of power dissipation. Use only resistors rated for this high Voltage and Power dissipation!
This resistor becomes quite warm, but DO NOT TOUCH IT! You might suffer a electric shock!
If the circuit is to be operated from a higher voltage than 250V, R1,r2 and R4 have to be chosen correctly.
Transistor Q1 amplifies a AF signal connected to X2. The negative parts of the signal are decoupled by C3/D2/D3 and charge capacitor C1 to a voltage proportional to the signal level. This Voltage drives the indicator Tube. C1 is slowly discharged through R3, ensuring a smooth display.
The value of C1 controls the tubes behavior,
Larger values make the tubes reaction slower.
Componet values for 250V Operating Voltage
(input sensitivity aproximately 1V)
R8 adjustable 100k
D1 30V Z-Diode
Preamp Stage design:
The tube needs about -10 V for full illumination, so the preamp stage must be able to provide this level.
The component values given above give about 20X amplification.
If you need a high impedance input, you can replace the input transistor with a Mosfet and suitable resistors.
All Tubes similiar to EM800/EM84 can be used. The Circuit was tested with both types. Other types may have diferent pinouts / Display orientation!
Operating Voltage 250V DC/ca 3mA typ
Additional Information,Basics,DIY Kit, Links for Schematic