PART TWO - DC POWER SUPPLIES. Step one, determining the required power for the tube and its supporting circuits.

---

Nothing will operate without the DC Power Supplies

       At this point, we want to design our power supply. But, we need to first figure out what voltages and approximately how much power the camera will need to operate. There is a heater in the iconoscope tube, so we are already 3.6 watts in debt! In the real world, there is the problem of the ideal part we desire and the available part are not going to be a perfect match. The design then becomes a comprimise between perfection and reality. Let's review the 5527 tube itself...

FYI: [RCA 5527 DATA SHEET]in PDF format.

       In the follwoing discussion, all of the voltages called out will be "relative to the cathode" (K). For instance, if I say A2 is at +800V. Assume the cathode is zero volts for this discussion. The cathode most likely will be at some arbitrary voltage dictated by practical circuit considerations and yet the K to A2 differential will remain 800 volts. Though the absolute voltage at any given pin may differ, they must always maintain the differential. For the iconoscope tube to operate, it must have a voltage differential, from the cathode (K) to the second anode (A2), of 800 volts. At the same time, grid 1 (G1) will be a zero to a few volts negative so that elecrons flow and the first anode (A1) will be operating between 100V and 200V and will always more negative than the second anode (A2).

       The cathode, via the aforementioned heater, creates a cloud of electrons. These are passed through a tiny aperture (hole) on their way to the highly positive second anode (A2). This electrode is called grid 1 (G1) and it controls the intensity of the electron beam. Anode 1 (A1) is responsible for focusing the electron beam to a fine dot at the back side of the image mosiac.

       Here are some hard rules for operating this electron gun: G1 must absolutely always be the same voltage or more negative than the cathode. Never positive relative to the cathode. G1 operates as a pin hole camera. It produces an itsy bitsy teeny weeny (electron, not light) image of the hot end of the cathode and projects it to the front of the tube. The less negative that G1 is the more electrons that pass through it. The more negative G1 gets has the effect of reducing the number of electrons in the beam (beam current). Around -20 volts or more, electron flow ceases altogether. When this happens it is said that "the beam is blanked". I take advantage of this effect later when I add the beam blanking circuit to cut off the electron beam during scan retrace.

       Finally, the first anode (A1) in conjunction with the two parts of the second anode (A2) forms an electron lens. Its function is to focus the beam to a spot just behind the light sensitive mosaic at the front of the tube. A1 will be normally operate between 100V and 200V, relative to the cathode (K) and will always be more negative than the second anode (A2).

       Got that? There will be a test later. Now let's get practical...

Figure 1. A practical Ike bias circuit

       Looking at figure 1, we see my practical answer to the 5527 electron gun bias. Note that the voltage divider stretches downward from +90V all the way down to -710V at the bottom of R8, the beam control. Take note that all voltages are now called out realtive to system ground. Why start at 90 volts north of ground? Because of a real world problem in electrostatic deflection, if the deflection plates (DJ1 - DJ4) have an average DC bias much different from the second anode (A2) voltage they can defocus and distort the electron beam as it passes between them. My deflection drive is approximately 120Vp-p and 90Vp-p respectively (anyone notice the 4:3 ratio of these voltages? What could that mean?), each on a DC bias of +90V above ground. Tada! If I make the second anode +90V, then the difference between A2 and the deflection plates is mostly zero! More about that later.

       Adding up R5, R6, R7, R9 and R8 we get 2.02 megohms between the +90V and the -710V points. This is a range of 800 volts. (Recall absolute vs relative voltages?) 800 volts divided by 2.02M ohms is 396uA or approximately 400 micro amps running in the divider chain. To be perfectly frank, the gun is really dropping only 780V cathode to anode 2. The other 20 volts is accounted for as the drop across R8, the beam control. This is still close enough to make the tube operate and we can actually trim the range effectively with R11, the -710V SET control, should that become necessary.

       The transformer I obtained for the high voltage in this project produced 930V after rectification and filtering. I dump the unnecessary extra voltage across the 1M ohm variable resistor R11. R11 functions as a rheostat, a two contact variable resistor, Adjusting the absolute current for the rest of the resistor divider. I calculate that is set to approximately 55.5K ohms.

       Who remembers the purpose at the start of this page? We set out to discover how much power this part of the design uses. We have yet to perform that last calculation. So, here goes. Power equals volts times amps. 800V times 396uA gives us 317mW (milliwatts), abround a third of a watt. So far, the heater is winning....

Figure 2. The final power supply.

       At this point, we know we need less than a watt for the electron gun, we need 6.3V @ .6A or 3.6 watts for the tube heater. I got these two voltages, heater and high voltage, from a surplus oscilloscope transformer which conveniently had also sported a nice low voltage 13.8V center tapped output as well. That winding provided a rectified output of plus and minus 18 volts. (Square root of 2 times the RMS voltage of the winding. 1.414 times 13.8V equals 19.5V theoretical) I dropped this to plus and minus 12 volts using standard 7912/7812 linear voltage regulators. From the minus 12V we used a second 7905 negative regulator to get minus 5 volts. These are standard low voltages used by almost all comon components. All of this is generated from the estimated 30 watt oscilloscope transformer.

       Using a second transformer we get +180 volts for deflection and blanking as well as the +90 volt bias and another 6 volt winding which I convert to 5V DC to comlpete the low voltage set. Once upone a time, this transformer was extremely common and cheap. It's referred to as a 'table radio' power transformer in the texts of the day. The output rating is 125VAC @ 25mA and 6.3V @ 1A. Grab these when you find them in old table radios, UHF converters, etc. Observe how clean this finished design looks. It is art at this point. I am a firm believer in the garbage in garbage out theory. If the schematic looks bad, the design probably is as well.

       In the final analysis, this project will use less than ten watts of power.