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Vacuum tube scope

     I needed a simple oscilloscope for basic measurements and I did not want to buy it, so I decided to build it. To make my scope simple and nostaligic style, I decided to use vacuum tubes. As a saw tooth (X) oscillator I am using RF pentode EF80 (fantastron connection). As a symmetrical driver of X and Y plates I used a double RF triode 6CC42. The second tube 6CC42 serves as a two-stage amplifier (Y). Unbalanced deflection would be even simpler, but it causes poor sharpness of the line (it would be impossible to focus the bean on entire screen, but only on part of it). That's why I chose symmetrical deflection. The screen is old but unused, made in the Soviet Union (USSR). Type is 5LO38I, screen diameter 5 cm, length 19.5 cm.
     300V for vacuum tubes is obtained from a one-way rectifying the power transformer secondary (240V), negative voltage of about 650V is obtained by doubling the secondary voltage. The oscilloscope has 5 ranges, which are selected by double 5-way switch. The time base is about 100ms to 4us (frequency of X deflection approx 10Hz to 250kHz). Response time is about 2us. The sensitivity is set coarse using switch (2 steps - 1/1 and 1/20) and continuously using potentiometer in the cathode. Trimmer capacitor is used to compensate the capacity (necessary for higher frequencies). It must be set so that a square wave displays really as a square wave (only takes effect whwn the position of the sensitivity switch is 1/20). Maximum sensitivity is about 0.2 W / cm. Control elements are only brightness, range, X-frequency, sensitivity and focus. Heater of CRT and EF80 tube is rated 6.3 V 0.3 A. Heater of tubes 6CC42 is 6.3 V 0.35 A. CRT heater supply MUST be separated from the supply of tubes, because there is a voltage difference more than 700V!

     Warning! There's high voltage in this device! Prior to manipulation, it is always necessary to disconnect it from the network and make sure that the capacitors are discharged. The CRT tube can implode. You do everything on your own risk.

Simple homemade vacuum tube scope schematic.

Homemade vacuum tube scope in operation

Inside the scope

PCB during development

switching supply waveform (the blanking still not completed, the CRT is lying on the table :)

square wave

audio signal

remote controller - IR LED waveform


such a pattern occurs when the time base is set smaller than the period of the measured signal (Lissajous curve).

sinusoid, slightly cut because the transformer is loaded by one-way rectifier

Handy switching charger waveform

once more

again the cropped sine wave.

Waveform in the LED driver (dimming is already involved, the oscilloscope is now in cabinet)

Loaded SMPS ripple

Sine wave

the voltage at the 36W fluorescent lamp with magnetic ballast (measured in series with 2M resistor)

the voltage on IHVT (HV transformer) in CRT monitor

overloaded SMPS waveform

5LO38I crt tube together with 8LO29M

original datasheet of the 5LO38I crt in russian.

2nd side of datahseet.

Pulse width modulation (PWM) on tube scope

Added: 2007