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 Post subject: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 3:41 am 
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Hi, so I've got yet another project idea in my head, and I'm currently on a break from school, so I figured I'd get to work on the schematic. I'd like to build an electrostatic TV set (Composite video in (not gonna mess with a tuner) + Mono audio) using the 3RP1A CRT I have laying around. I recently acquired a 480v CT, 65ma DC transformer from another member, and it seems like it should work well for this, with a 5Y3 for the B+ (300v) supply, and a tripler for the negative voltage for the CRT. I've gotten started drawing up a schematic, and I'd like some feedback on it. As far as I know, what I have should be enough to produce a round, focusable, bright spot on the CRT. HV should be somewhere around 1350 to 1600v at A2.

I currently have controls for brightness, focus, and astigmatism.

Approx voltages (LTSpice Sim) are next to the resistors in the voltage divider.

Please let me know how this looks:

Attachment:
3RP1 TV.pdf [29.72 KiB]
Downloaded 82 times


Thanks!


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 5:07 am 
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I recommend more B- for the 3RP1.

A voltage quadrupler would work.

Here's how I run a 3RP1 for an oscilloscope clock.

Attachment:
3RP1.png
3RP1.png [ 46.9 KiB | Viewed 1567 times ]


The NE-2 after the intensity control serves as an indication that B- is present plus it drops some voltage.


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 5:21 am 
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Higher voltage will give a brighter, sharper display but will also require more deflection voltage. I've run these CRT's with lower overall voltage.

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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 5:27 am 
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Yes it will require more deflection voltage.

But that's easy to accomplish.

The circuit I posted will provide full 3" deflection on 300Vdc B+.

I did not include the deflection circuit as its solid state and the OP wants to use tubes.

All the OP needs is two 12BH7 to drive the deflection plates and a 12AU7 as a phase splitter.


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 5:41 am 
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Location: Surry, Maine, US
1500ish volts is somewhere in the middle of the two datasheet operating points, and I didn't want to go too much higher as I'd need more B+. It seems like I might need more anyway though, so maybe that isn't such a problem. would it help to have a -150ish volt rail for the cathodes of the 12AU7s I plan to use for deflection amps?

I'd assume I can get a brighter and sharper picture with more voltage, hence the reason I went for somewhere in the middle of the two operating points.

How much voltage should a 12AU7 differential amp be able to swing? I would assume it's supply voltage - tail voltage? Hence my question about a negative rail. I can always add a pair of diodes to the other side of the tube rectifier to produce a b- of around 330 volts as well.

Tube Radio: I've been following your deflection amp project because I was curious, but I do want to use tubes. Thanks for the schematic, I'll look over it in more depth tomorrow when I'm not half asleep.

Edit: Just read the ending of your post again. Do I need to use a phase splitter to drive both grids? I was going to use one grid as the input and the other to control the positioning. Also, 12BH7s because 12AU7s don't have a high enough voltage rating, correct? I've been looking at scope schematics and they seem to use 12AU7s without a problem. I have both, so no big deal, but still.

Thanks!


Last edited by IlikeTech on Feb Mon 15, 2021 5:46 am, edited 1 time in total.

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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 5:42 am 
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Have I started a Révolution? :lol:

Not too shabby. I would have the grid being variable, and the cathode constant. A 1M resistor would be between the pot wiper and grid. The video would be fed in to the grid. 1M for the brightness seems a bit high. I usually calculate it out, or just refer to the CRT data for info if I am lazy. It also gives info on the positioning pots. However, the A2 will need to be grounded, and the deflection pots need a slight positive voltage. So if you do do this schematic, ground where it says to ground, but put a resistor between 300v and the top of the pots and voltage divider. I did the math, and math said 1.5Megohms.
https://frank.pocnet.net/sheets/049/3/3RP1A.pdf
+1 on not messing with the tuner and IF. It's Black magic! :mrgreen:

What kind of deflection oscillators are you going to use? Some that are very good are:

- Phantastrons (http://electronixandmore.com/projects/t ... index.html and https://www.cool386.com/902tv/902tv.html)
- Multivibrators (http://electronixandmore.com/projects/t ... index.html)
- Thyratron Sweep (http://www.earlytelevision.org/pdf/rtv_4-40.pdf)
- Blocking oscillators (http://www.earlytelevision.org/pdf/qst_2-40.pdf)

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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 7:00 am 
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You could use one deflection plate to feed the signal but you then need a higher B+ as one tube then has to provide a high enough output.

The phase splitter and the voltage amps after it mean you can get twice the output for a given B+

The 12BH7 is used due to its lower gain which means it can put out more signal.

The 12AU7 is wired as a phase splitter with one section for vertical and one for horizontal.

I can provide a phase splitter circuit I use tomorrow.

Given its a video display there's no real need to use position controls.

For a tube sync separator and video amp look here.

http://electronixandmore.com/projects/t ... index.html

Look at the last circuit before explanation of the video signal.

Concerning the video signal you'll need some sort of DC restorer circuit.


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 7:26 am 
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It could be done without Positioning controls, but it is rather hard. Due to imperfections in the making of tubes, and the earth's magnetic fields, the picture will almost always be off center. +1 on the push-pull circuit. However some circuits like the phantastron car directly drive the tube, without any other circuitry. I would recommend the push pull circuits for the multivibrators. Thyratrons could drive a CRT directly, but more B+ is needed. An amplifier is more economical in some cases. As for blocking oscillators, one tube amp, or push pull is fine. DC restoration is sort of a luxury in electrostatic televisions. It makes sure that the Black and White (Green) levels are set. It's not a need but it's nice.

P.S. I like the 3RP1, but I like the 3AP1 better. It can be driven as low as 400V!! The downside is though, the 2 deflection plates are connected to the A2, and the filament current is hard to work without a transformer. With this, only a 1 tube amp is possible.


Edit: Something like this for Push Pull:
Image

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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 7:34 am 
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Without dc restoration the picture won't be all that great.

I've used a 5" TV as a video display which didn't have dc restoration as a second computer monitor and everything was fine until I tried to display a window that was mostly white on the screen when the desktop background was of a night scene.

The background became darker as I moved the white window to the display.


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 7:40 am 
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If you want to be cheap, you can leave out the DC restoration. But if you want a stable black level picture, you can use it. I never was bothered by no DC restoration. However in color TV it truly is a necessity,

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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 2:52 pm 
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Okay, so now that I'm not almost asleep I'll outline what I was planning.

I had already found this:

Image

And was planning to use that design for the oscillators and video amplifier, as it's simple enough for me to understand it, as it's just a pair of multivibrators and I have a ton of double-triode tubes.

Also, I was planning to use push pull deflection, as I know that holds advantages. I'd prefer to have positioning controls, as Mr. Highlander says, it's not going to be perfectly centered. Also, I plan on using DC coupled deflection, so positioning controls will need to be built into the amps themselves. Looking at this schematic:

Image

I see how the DC restorer works, and it shouldn't be too much harder to add it. However, I did notice that the CRT in that design is being run with the cathode grounded, which means that I'd need to increase cap voltages I think. I feel like it probably needs some other mods too, please let me know.

Mr Highlander: Thanks for pointing out the 1M pot for the brightness control, that was a mistake when I was copying my math into the schematic. That's supposed to be a 100k pot. Is there any real reason not to couple the video onto the grid and control the brightness with the cathode?

Also, the A2 grid is adjustable between GND and +300 volts, as that allows for astigmatism control.

Thanks!


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 3:51 pm 
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Don't see a reason to dc couple the deflection plates if only using it for a video display except to eliminate four capacitors.

However for a position control you might be able to use a pot in series with the 12BH7 plates with one end going to each plate and the wiper going to B+.

That would vary the B+ slightly on each plate which would provide a small shift in the position.

My experience with the 3RP1 is that the dot is pretty much centered good enough to where I see no real reason to worry about centering the image. However the tube I used was a flat faced 3RP1A.


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 4:08 pm 
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Okay, I might do that. I think I have a 3RP1A actually too, I need to double check. Can you give an example of the circuit you were thinking of if it’s not too much trouble?

Thanks!


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 4:13 pm 
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I can draw up something simple for the position control but I dont know if it will actually work.


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 4:16 pm 
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Hmm okay! If you don't mind!

Thanks. I'm going to attach the circuit I was looking at from an oscilloscope, as it seems to use the noninverting input of the differential amp to do the position control.


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 4:25 pm 
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The design I was looking at was along the lines of the way the vertical is done in this oscilliscope. It's done with a 12AU7 differential pair to drive the deflection plates. It uses a B+ of around 400 volts and drops 90 volts on the tail resistor, but since my rectifier won't output that much, I was planning on running the tail so that the cathode is near 0 volts, which would allow me to use the full 300ish volts for swing, if I understand correctly.


Attachment:
page36.png
page36.png [ 365.73 KiB | Viewed 1522 times ]


Thanks!


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 4:50 pm 
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That circuit will work.

It may work ok on 300Vdc B+


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 5:11 pm 
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Okay, any reason to do it with BH7s instead of AU7s? I have about 3 BH7s, 6 AU7s, and probably 10 NOS AT7s. As far as I know, BH7s and AU7s both have a gain of 17, so there shouldn't really be much difference, at least I wouldn't think so, except for maybe max voltages. That circuit uses a B+ of 400 volts, and I think my CRT requires a bit more swing, so I plan on using the +300 rail, and then adding a -300 rail which I can drop down to more like -100 or -150. Would that work?

Part of the reason I wanted to use that scope circuit is the CRT is running at very similar voltages to mine in the same sort of configuration.

Right now, this is what I'm thinking for tube count:

1 6AU6 (Video), 5 12AU7s (Horizontal and Vertical video out, multivibrators, and sync separator), some tube for audio out (probably 6AQ5), and the 5Y3 rectifier, plus the CRT.

Thanks!


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 5:27 pm 
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Suggest ditching the 5Y3 unless you just want to use it.

With no 5Y3 you then have the opportunity to make two separate B+ supplies.

400V for the final deflection tubes and 300Vdc for the rest.

I'll modify your schematic as best I can to show you what I'm talking about.


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 Post subject: Re: 3RP1 Based TV Set
PostPosted: Feb Mon 15, 2021 5:49 pm 
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I kinda wanted to just use it as I have the 5v winding to do so, but if it's a better circuit without than I'll definitely consider it.

I've been looking over other schematics and I think I'll start drawing up something soon.

I'll take a look at the mod when you post it, thanks!


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