Did you realize P2 is an adjustable pot? The gain is the ratio of the two load resistors to the emitter-to-emitter resistance, G = 2 RL / RE, where 2 RL = R3 + R4, and RE = 2 re + R_P2. Here re is the intrinsic emitter resistance of each transistor, which is derived from the Eber-Moll formula and is re = 0.025-ohms/Ic. In this circuit you're expected to start with a signal biased to about 2.1V, and the pot P1 is adjusted to the same value plus any small offset voltage resulting from different Vbe for Q1 and Q2. We can estimate the emitter voltages are about 2.1 - 0.65 = 1.45V, so the emitter currents are about 1.45/4.7k = 0.308mA. When this current is dropped across the 220k collector resistor it creates a 68V voltage drop (ignoring the small base current), which means you'll see about 332V on each collector. That doesn't match the 300V value claimed by OZ2CPU on your weblink page, so perhaps he does not have a signal at 2.1V, but a higher voltage, about 2.8V, and his emitter currents would be about 0.6mA each. (You could reduce the value of the emitter resistors.)
Now about the gain. With 0.6mA, re = 42 ohms. The maximum gain is for R_P2 = 0, and is high, 220k/42 = 5200 (we're ignoring the Early effect, which is small for high-voltage transistors). One issue, if you set the gain that high, you'll have a nonlinear amplifier, because r_e changes with current, changing the gain. But when you use a sensible non-zero value of R_P2, say over 1k for a gain of 440 or less, etc., R_P2's steady value swamps the changing emitter-resistance and you'll get a linear output.
OK. But you'll need a negative supply voltage to create the two current sinks for the differential transistors.
How much defelection voltage does your tube require? And what average DC value is appropriate for the deflectors? Perhaps you can get by with the lower voltage.
Thanks,
- Win
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J
John Popelish
Philip Pemberton wrote: (snip)
The amplifier is a differential amplifier otherwise known as a long tailed pair. It accepts two inputs and amplifies the difference between them and outputs two signals that are anti phase (one goes positive when the other goes negative. This variable gain version has the two emitters fed by some kind of current sources (just resistors, in this case) but he gain setting is dominated by the resistor between the emitters. The gain is related to the bias current passing through the transistors, but if the emitter bridging resistor is high enough to effectively linearize the amplifier, then the gain or one side is approximately the collector resistance divided by the half the emitter bridging resistance. This is another way of saying that the differential gain between the two collectors is the collector resistance divided by the emitter bridging resistance.
This kind of circuit is a lot easier to work with if there is a negative supply below the range of input voltages the amplifier works with. This allows the emitter current sources to pull down lower than the inputs to keep the emitter current fairly constant even at the lowest input voltage expected.
Is the 5 volt supply in your design made with a single winding and a bridge rectifier, as shown for this supply, or did you use a center tapped winding with two diodes? The second case makes it very easy to come up with the negative bias needed to allow the input range to go to zero volts. If it is not convenient to come up with a low voltage negative supply, you can, alternatively, bias your deflection signals positive so that ground is more negative than they are, to use something like this design.
The important thing for deflection is to have enough differential swing between the two deflection plates, though the average voltage will affect the focus. This is why you need two outputs. If you just bias one plate with a constant voltage and swing the other, the focus changes as the beam swings. The differential approach keeps the average voltage on the two plates constant.
I doubt that many of the tubes listed need a 400 volt deflection supply (~200 +- deflection voltage) to achieve the differential voltage needed for full face deflection. But you can use a pair of high resistance dividers across your 240 volt supply and a volt meter to find out what kind of voltage difference is needed to deflect your beam. You may be surprised how little difference voltage it takes.
For instance, if you make a string of, say, 10 each 100k 1/4 watt resistors and move 4 clips from node to node (turn the power off and let the DC decay before moving clips to prevent shock) to change the tap voltages, you can adjust the tap voltages in 24 volt steps to simulate the outputs of two differential deflection amplifiers. Start with the clips all at mid setting to produce 4 plate voltages at about 120 volts. You should still produce a dot near center. Adjust pairs of these clips in opposite directions (with a volt meter connected across the plates to measure the difference) and find out about what voltage on each pair deflects the dot all the way to the edge of the screen, each way. It is okay to have a one tap mismatch between one side and the other, so you have a 24 volt step size.
This differential voltage requirement will allow you to find out what gain you need to amplify your 1.25 +-1.25 volt deflection signal up to full screen deflection voltage, and let you know whether a 240 volt supply is sufficient to produce these voltages. I think this is worth doing before you design the deflection amplifiers. Ideally you don't want to have to swing the amplifier anywhere near full supply voltage for full deflection, if you want low distortion.
I am doubtful that you will need more than the 240 that is available, but a test will prove this pretty quickly.
P
Philip Pemberton
Hi, Having spent a few days playing around with Ohm's Law and splitting apart and rebuilding the focus chain for my DG7-32 CRT, I've managed to finally get a decent spot on the tube face. For those who don't know, the DG7-32 is a 7cm round oscilloscope CRT with electrostatic deflection.
Now I'm having a problem with the deflection circuitry. Initially I used the "OZ2CPU" deflection amp design from . I've had a few problems with that - not least of all its strange input signal requirements and the need for a 400V deflection power supply. Can anyone tell me how to calculate the gain of the circuit, or maybe even if there is a specific name for that style of amplifier (so I can go looking for the info myself)?
I also found a similar circuit in Sascha Ittner's CRT clock project. When I say "similar" I mean "same circuit, different resistor values, +250V HT instead of +400V and +12V LT instead of +5V".
What I want to build is a deflection amplifier that will take a max. 2.5V input. If the input is 0V, then the spot is deflected full left, 1.25V centers the spot, and 2.5V moves the spot to the far right. Ideally I'd also like to be able to tweak the X and Y gain (=size) and offset (=position).
I've got +240V available from the anode circuit; adding an additional +400V PSU would involve adding another voltage doubler, and the one that's on there now isn't exactly small.
Well, let's put it this way. I'm a transistor user. I have precisely no knowledge of vacuum tube (valve) circuitry, and even getting the CRT working was a pain in the ass.
I've finally got the deflection circuit working properly using R=4k7 from Q1.E to Q2.E in the drive circuit. The 10k carbon film resistors in the Y circuit are hopelessly out of spec, and in dire need of replacement with some meatier (and more accurate) resistors though.
I've also added a simplistic astigmatism control - a 220k pot from +240V to GND, with a 100k resistor in the ground path. Careful tweaking can get the spot size down to less than 0.5mm - even better resolution than my Tek 466. Then again, comparing a storage CRT to a normal CRT is a bit like comparing apples to oranges...
Heh, 2MHz out of a DG7-32. Nice. I didn't know the deflectors could go that fast. Guess it will make a nice vector display testbed. I wonder if Asteroids will run properly on it... :D
Later.
Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/ | Panasonic CF-25 Mk.2 Toughbook
No software patents! /
... The universe is laughing behind your back
P
Phil Allison
"Philip Pemberton"
** You need a nice pair of 6GH8s to drive a DG7-32 CRT ;-)
As used in the 2MHz CRO I built from a published design in 1970, while a high school student.
I still have it and it still works just fine.
......... Phil
P
Phil Allison
"Philip Pemberton" "Phil Allison"
** You gotta learn to love 'em.
** Yes - a basic ES deflection CRT display is * amazingly * sharp.
The fancy PDA and screen storage ones are relatively blurry.
LCD is unspeakable .......
** In 70, I connected the vertical and horizontal amps of my new CRO the deflection ccts of a B&W TV set to get a 625 line raster, then coupled the video signal to the Z (intensity) input and wound up with a viewable bright green pix.
Way cool, back then !!!
.......... Phil
F
Fred Bloggs
Filling in the blanks- and using a single supply oa: View in a fixed-width font such as Courier.
Great minds think alike - I did that too and it was great showing the kids the pic on that little 3inch screen
David
Phil Allis>
P
Phil Allison
"quietguy"
** Early TV sets all used ES deflection, same as a CRO.
They avoided the need to generate many watts of power to drive the H and V coils of a yoke AND the need to generate circa 20 kV EHT and thereby avoided nearly all the reliability problems that still plague TV sets ands monitors .
........ Phil
P
Philip Pemberton
Heh, now that would be cool. The big problem is sorting out some form of Z-axis amplifier - working out how much I need to pull down G1 to visibly dim the spot, then figure out how much gain I need to design in to convert a 0V/2.5V signal into the right pull down. "Fun with single-transistor amplifiers" :)
At the moment all I've got is a digital on/off control for the Z axis (pilfered from the "Sascha Ittner" CRT clock design). Kinda useless for analogue control of the brightness. Or maybe I should try pulling UP the cathode instead.. might work better.
At the moment I'm trying to find some way of triggering a xenon strobe tube without using an SCR or triac. I've got some MOC3023s but they don't trigger reliably. I would use an SCR or triac driver with gate wired to the 3023, but I don't have any spare SCRs or triacs :(
Later.
Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/ | Panasonic CF-25 Mk.2 Toughbook
No software patents! /
... Happiness: a perfume you can\'t give away without getting some yourself.
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