cathode-ray tube application

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lanxe

Well-known member
Joined
Jul 15, 2004
Messages
107
Location
Carson City, Nevada USA
This may be a dumb question, but
how hard do you think it would be to implement a small cathode-ray tube into a modular synth........ crt's such as this one:
913.jpg


or any crt's from this site:
http://www.aade.com/tubepedia/1collection/tubepedia2.htm

I mean, it would only be for occular pleasure........nothing scientific. Just play some frequency and watch the waveform.

I thought of using a small oscilloscope, but i cant seem to find one with a small enough crt (the back end of the tube would stick out of the back of the synth).

obviously an oscilloscope would work the best, so i was thinking.........
get an old, one channel small oscilloscope. gut the electronics and replace the crt with a smaller one.........but i am unsure if this is possible (i would guess that a different crt would need some kind of circuit change).

any way, this is just an idea i had for a synth that i am working on. i have some space for extra modules, and i thought maybe this would be cool. but it may be dumb too.

thanks for the help.
Ryan
 
Yoou have to use an elextrostatic-plate version like a scope. The TV versions use electromagnetic deflection coils. This makes the tube shallower, but the coils are tuned and will tend to resonate at a specific frequency. Scopes in X/Y or M/S work best.

Keith
 
Awhile back I ran across a site with information about using CRTs as indicators for counters instead of NIXIE tubes so I'm sure you can use them for lots of different applications.
 
using CRTs as indicators

This guy does some interesting stuff.

http://www.cathodecorner.com/

He used to have a mini CRT pocketwatch, but he seems to have taken it down.

There was something I ran into a bit back showing a pretty simple circuit for driving a CRT for use as a curve tracer, but you might be able to use it. I'll try to find it.
 
> how hard do you think it would be to implement a small cathode-ray tube into a modular synth

If you are an old tube dude, it may be trivial. I re-created a Allen B DuMont from just the case, transformer, and CRT. There was one place I got stuck: the CRT would not put a spot on screen until I demagnetized it.

The RCA datasheet on the page you cite has the basic hookup. You ground the cathode, adjust the grid to -1V to -60V to set brightness and "bloom". Put +500V on Anode#2 and on the deflection plates (R-C coupled so you can add your AC signal). Put adjustable 50V-150V on Anode#1 to get a focus.

If the deflection plate voltages are not very similar to Anode#2 voltage, focus is terrible. This is the Astigmatism control on an old fancy scope. The RCA scope plan does not have this adjustment, not does my DuMont. They are not H-P sharp, but OK for eye-candy and even general audio observation.

Note that the RCA (and my Dumont) are single-ended deflection. There are two V plates and two H plates but only three pins. This fits a simple single-ended AC-coupled amplifier, but the raster is always a little trapezoidal.

The HV is a designer-choice. Lower voltage gives less brightness but improves deflection sensitivity. My DuMont 3-inch started life with +700V, but I rigged it for about +300V. Brightness is fine and sensitivity much better (and I could use cheap 300V transistors).

The 913 CRT has about 1 inch or 25mm of usable height. Using lower supply voltages, the deflection sensitivity is 0.1mm to 0.2mm per volt. To get a 12mm swing, you need 60V to 120V peak (120-240V peak-peak) input swing! This is typical of older single-ended CRTs: if the driver is fed the same power as the CRT anodes, the driver has to flap very hard to even cover half the screen. And you will generally need a driver amp to interface to 1V-10V audio and synth signals. Traditional is a sharp-cutoff pentode, which will give a nice amount of gain. You need two for Lissajous (and why have a 'scope otherwise?). I did it with an op-amp and a pair of 300V TO92 transistors. The small power capacity means poor bandwidth: my scope just hits 20KHz, while a radio pentode driving a 'scope CRT can exceed 100KHz.

The real killer is a good horizontal sweep. In some synth work you may have a ramp signal, but in general you want a dedicated ramp-wave that will also sync to the input. You can do this with a 3-electrode gas-tube and a cap, and directly get the 200V ramp needed to drive the deflection directly. Triggered sweep is a real plus in audio work, but complicated, and also does not work well with the AC coupling you usually want in SE CRT drivers (when not triggered, the dot drifts back to the center).

Honestly: Velleman's pocket-scope is a more sane idea than messing with bottles. You will have a lot more than $200 of head-scratch, sweat, and blood in a $10 CRT by the time you get it working. But LSI LCD is so boring......
 
Thanks PRR, you always go above and beyond.

[quote author="PRR"] If you are an old tube dude, it may be trivial. [/quote]
well, i am a young newb, but i think i have a decent idea of what is going on. It may be worth a shot.

But LSI LCD is so boring......
yeah, i am definitely going for that vintage cool eye candy. so i dont think an lcd would give me that mad scientist look.

I am going to spend a little time thinking on this one........i still have a long way to go just to get the synth modules done, so i have some time.

Thanks for the help guys,
RYan
 
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