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Here is how it works. The 5Y3 is the full-wave rectifier tube that provides about 400VDC, and is filtered by the filter system.
The 6DT6 tube is used as a vertical amplifier, so the signals fed into VERT INPUT are amplified. The V. GAIN control controls the vertical amplification. Touching your finger on the V. INPUT with the switch on EXT will make a small sine wave, compared to a larger, rough sine wave with the switch on AMP and the V. GAIN turned up.
The 6J6 is a dual triode that is wired uniquely so it oscillates. The capacitors in the RANGE section determine the frequency range the tube oscillates at. The larger capacitor (.33uF) will make the tube oscillate slower, while the smaller capacitor (.001uF) will make it oscillate faster. The smaller the capacitor, the faster the tube will oscillate, allowing you to look at waveforms at higher frequencies. However, there is a point where the oscillations will falter and become screwy. In the RANGE section, one or more of the switches can be turned on, connecting one or more capacitors to the circuit. The 6J6 tube will oscillate its slowest with all the capacitors switched on (when you parallel capacitors, the capacity increases). The FINE FREQ control allows you to change a small amount of the frequency of the 6J6's oscillations. The INT OR EXT HORZ OSC switch allows you to either use the oscilloscope's internal oscillator (6J6) or provide the oscillations through the HORZ INPUT. The H. GAIN serves a similar purpose like the V. GAIN to the horzional oscillator circuit.
Notice the 5Y3's filaments are fed with 6 volts. This is one prime example of tubes' flexibility. They can take in a few volts more, but generally it shortens their life. If you wish, a small dropping resistor from 0.8 to 2 ohms can be used to reduce 6 volts to 5 volts.
Also notice the homemade 11-pin tube base for the 2AP1 CRT in the pictures below. I used the pins from two octal tube bases and put them in the holes. The CRT plugs in nicely (it's plugged in half-way on purpose).
The 3.3M ohm resistors from B+ to the vertical and horzional plates of the 2AP1 CRT can be changed out with a 2.2M ohm resistor and a 1M variable resistor in series for centering.
The power transformer should be mounted behind the CRT to prevent distortion, or metal shielding around the CRT is recommended.
A Z-intensity connection can be added on this scope. All you need to do is add a .1uF capacitor to the first grid of the CRT, and feed the signals through the capacitor. You also might want to increase the 470K resistor from the brightness control to the first grid.
Some of you might have noticed, there is a flaw in the power supply in the original schematic. The 5Y3's filaments have high B+ voltage, therefore the other tubes' filaments are supplied with that B+ voltage. Those tubes have very low filament to cathode voltage ratings. This will reduce tube life or short them. However, since I have not experienced any negative results, it is better to provide a separate filament supply for the 5Y3 or modify the power supply for a different tube. Below are schematics of two modified power supplies.
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This circuit uses a 6X5 rectifier and is strongly recommended. A 6X4 miniature equilavent can be used instead of the 6X5.
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This circuit uses a 6SN7 dual triode tube. I currently use this setup since I didn't have a 6X5. This is based on an economical design in the 1930s when designers used a triode tube, usually type-37 as the rectifier. The grid and plate on the 37 were connected. Although the 6SN7 will probably have shorter life, it is easier to replace compared to a 2AP1 CRT. A 12AU7 miniature equilavent can be used in place of the 6SN7.
If you experience lower vertical gain from one of the modified power supplies above, you may replace the 6DT6 with a 6AU6 or similar tubes.
Here are some pictures of my ultra-simple homemade oscilloscope. Click the thumbnails for a larger picture.
Side view 1
Side View 2
Oscilloscope in operation with sine wave display on CRT
(camera seems to have slow shutter speeds at night)