In QST. Yay the scopeclock! If anyone plays with that and does not use the amplifiers shown, and hooks up the 'clock' board directly to the innards of a scope, beware the DAC chip is fragile and can be easily popped when connected to the capacitor coupled Z axis, or other things with high voltages on the other side that provide a discharge or charge. The schematics in the article look like they isolate the board from this issue. The DAC can be replaced but it is small SMT stuff.
The reference pages for the article assume to limit the choices of CRTs, based on quite limited operating voltages. The typical setup is deflection plates near goround or a few hundred volts above, and the cathode -600V to -1000V. It is done that way so that the deflection amplifiers' plates or collectors are at a convenient voltage. (If the K were grounded, those would be at +600 to +1000V)
because it gets more and more impractical to operate the cathode, grid, etc. at more than -2KV in order to have the deflection plates near ground yet have a whopping accelerating voltage, some tubes have a separate anode called the post deflection accelerator. PDA, 3rd anode, Ultor anode, whatever it may be called. I like Ultor. ULTOR!
While it is true that some of the bigger CRTs are dim when run with the PDA at the same voltage as the 2nd anode (around the same as the deflection plates), it's because those tubes are higher performance and need +1.5KV to +3KV more for the separate accelerator. The reward is usually a sharper trace too. There's just no way around that. The accelerator is away from the deflection plates and does not interfere with them, and because it is -after- the deflection angle has been produced, increasing the PDA voltage does not so badly shrink the image as on older types.
To practical matters, one thing I have faced is that all the low voltage tubes, those that need only a cathode voltage of -600V to -1000V or so, are scarce any more and command higher money. There are many, many 3" and 5" tubes that can use the existing scheme shown, but also need a high positive voltage to work. No one wants to go to the trouble of a multiplier to make the 2-3KV. It is microamps, very simple. So don't buy the negative remarks about those tubes with the PDA. It's worth a few more stages of multiplier to get them going, and spares are pretty cheap.
In the supporting website
http://www.electronixandmore.com/projects/simplescope/index.html, it says some 3" and 5" ones work but are dim. That very tube type is the one to get for $10-20, instead of $40-75 for the low voltage ones, and give it the boost it needs. The 3" ones need only +1000V to +1500V, the 5" more like +2KV to +3KV. Some will take more, don't hold back. The tube manuals like the 1950's Sylvania ones have the data, and the HB-3. The TCA tube colectors' association sells a JEDEC CD set with all. I got one of those.
http://catahoulatech.com/#!/~/product/category=9646441&id=36990228 shows the voltages made by the CRT driver board. It's easy to see how to match up a variety of tubes to that.
Here is the trick. It is just a tube.. so the voltages can fudge a bit. The best pat is that all you have to match up is the electron gun. His design lets you fudge around the bias, focus, etc. quite a bit, way more than an instrument scope's strict voltage dividers would.
The PDA is mostly separate, even in real scopes. So almost any tube with a gun that will work at those voltages is plug-and-play, once you rig up a positive supply for the accelerator. If the 'found' tube takes 3KV max, it'll run on 2000, and if you can see there is -670V in the offered circuit, then you need only make a +1400V supply at less than 1mA.
Finally, the tube manuals give all voltages in relation to Cathode. The scope manuals give them in relation to GND. Excel is your friend. Plug in the tube manual voltages, and subtract the negative (cathode) voltage, and there's a typical list of actual voltages referenced to GND, as derived from a manual, Like so:
element , in-book , in-scope
A3 , 3000V , 2000V
A2 , 1000V , 100V
def plates , 1100V , 200V
A1 , 450V , -550V
G , -100V , -1000V
K , 0V , -900V
Not sure I expect anyone to play with this stuff. I like it though.
last word - unless you can make +5 to +8 extra KV above ground, don't use super high performance TEK or HP tubes as they sometimes have aluminized screens and that adds 4-5KV to the requirement. Check the scope manuals to find operating voltages for any tube.
