scope CRT blanking, capacitive coupling - Page 1
Forum section: electronics / projects

author: Martinn
author_rank: Frequent Contributor
timestamp: December 06, 2022, 05:31:59 pm
content: Hi all,I am working on a scope clock. I started with the deflection amplifier by TurboTomhttps://www.eevblog.com/forum/projects/maybe-risk-a-guess-what-thats-gonna-be/msg1810052/#msg1810052which works nicely, HV supply also should work (boost stage+cascade) and I think I understood astimatism and focusing correction.Remains the control grid and blanking. Controlling blanking is a bit nasty as cathode and control grid are typically 1-1.5 kV away from the deflection amplifiers. There are many solutions for this like optocouplers (+extra supply) or 3 kV capacitors combined with a brightness potentiometer.Now here is what I am struggling with: TurboTom (see link above) couples his blanking amplifier to G1 (control grid) via a (1 nF 1 kV) capacitor - without any DC reference (like usually 1M to a brightness potentiometer). So my question to the vacuum tube gurus:What potential would result on a purely capacitively coupled grid? The datasheet specifies -50 V for cutoff and 10 V for normal 10 uA beam current, so with AC coupled drive the potential should somehow land in there.What is the current into the grid? How much of the beam from the electron gun lands on the control grid? I attached the datasheet of the tube I'd like to use (D10-160).EDIT I just did an experiment. With only the heater connected (no HV, no other connection - not risking to fry my instruments by a kV spark) I connected the cathode and G1 to a power supply. With G1 positive, current drops from a few uA to 0 (hurray! We just discovered a vacuum diode). With G1 at +2 V w.r.t cathode, current into G1 is 2 mA.Of course this is without the main acceleration voltage so I wonder how many electrons make it out there. But as beam current is rated at 10 uA, it can't be much. I wonder what current flows into G1 in regular operation?Thanks - Martin

author: TimFox
author_rank: Super Contributor
timestamp: December 06, 2022, 05:41:43 pm
content: I have never considered floating a CRT grid, but I it might float negative (with respect to the cathode) until the cathode current was cut-off.Unless there is secondary emission from the grid, in which case it floats positive until something blows up.That's why a resistive circuit, including a brightness pot, is a better idea.

author: Martinn
author_rank: Frequent Contributor
timestamp: December 06, 2022, 05:54:31 pm
content: Quote from: TimFox on December 06, 2022, 05:41:43 pmCould the potential be somehow self adjusting? On a floating grid, I assume electrons would accumulate on the grid, driving the potential that far into the negative region until the current is cut off? Could this mechanism work as self-biasing?After all, I believe Tom's solution works.EDIT: Another experiment. With the heater active (and nothing else), I connected my DMM to cathode and grid 1. DMM set to high impedance (datasheet says > 10 GOhms) grid voltage stabilizes at around -2 V.

author: TimFox
author_rank: Super Contributor
timestamp: December 06, 2022, 06:32:32 pm
content: Quote from: Martinn on December 06, 2022, 05:54:31 pmThat -2V is similar to what one would get with a high (> 1 megohm) resistor from grid to cathode on, say, a 12AX7 triode (often called "contact-potential" biasing in small-signal audio). From your data sheet, -2V is not sufficient to cut off the CRT, but will cut off a high-mu triode at reasonable plate voltage.It might well work, but I haven't done that with a high-voltage tube.If the signals through the capacitor are positive pulses with a very low duty factor, then the DC (mean) level on the grid will stay at high negative potential (black), but going positive on the peaks to get local white.

author: Martinn
author_rank: Frequent Contributor
timestamp: December 06, 2022, 06:49:19 pm
content: Quote from: TimFox on December 06, 2022, 06:32:32 pmAs it's a scope clock I'd assume the opposite - long drawing periods with short moves to another starting point (so high duty factor). Tom uses a different tube, a B7S2 where g1 cutoff voltage is specified at -25...-60 V (quite similar to the -50 V of my tube). Interestingly he designed a blanking amplifier with adjustable output from 0-250 V (but maybe just because no other voltage was around).So it would be interesting to find out what potential g1 stabilizes at.I was thinking maybe I just follow your recommendation and add a brightness potentiometer setting the bias. There's jsut one nasty side effect of that: As the cutoff voltage is even more negative than the cathode (being already at -1.5 kV), I'd need an extra supply on top of that (maybe one more multiplier stage) just for driving that brightness pot.Hameg added an extra 30 V winding on the mains transformer on their HM203 scope (IIRC) just for the blanking and brightness circuitry (of course they have more functions in there, Z modulation input and an optocoupler). Maybe just letting the grid float would be a workaround to avoid that extra supply.

author: TimFox
author_rank: Super Contributor
timestamp: December 06, 2022, 08:26:43 pm
content: Could you put a Zener diode in series with the cathode so that you have a more negative voltage at the other end of the Zener, avoiding another transformer winding?

author: Martinn
author_rank: Frequent Contributor
timestamp: December 07, 2022, 04:36:28 pm
content: Quote from: TimFox on December 06, 2022, 08:26:43 pmThanks, sounds like a good idea! I'd have to add some buffering for when the trace is off. The question is what voltage is needed (is it really 50 V) to blank the beam completely? I'd rather not try this out with my lab supply (I doubt it would appreciate having 1.5 kV between its channels). Seems I need another test PCB to try this out.Do you think the CRT might be damaged if g1 potential is too high? I wonder why the grid can be pushed to +10 V at all - without acceleration voltage, at 2 V I already get a current of 2 mA into the grid.Can vacuum tubes act like a diode if the grid is biased too high (sorry, total tube newbie)? Probably the grid is always at some safe working point (like the -2 V you mentioned for the 12AX7).

author: TimFox
author_rank: Super Contributor
timestamp: December 07, 2022, 10:26:28 pm
content: Yes, if you bias the grid positive with respect to the cathode, the grid-cathode diode will conduct in the same way as a small vacuum diode (6AL5, 6H6, etc.).Some tubes were intended for such operation, and their data sheets show the I-V curve for the grid.Grids are fragile, and should not be subjected to high currents: the data sheets often have maximum ratings.

author: TurboTom
author_rank: Super Contributor
timestamp: June 30, 2025, 07:04:06 am
content: Only recently I noticed this thread on some details ofmy scope clock design, and since there obviously is some confusion about how the blanking works, I simply had to chime in, even if it's rather late...The answer is very simple: Inside the B7S2(-01) CRT, G1 is connected to several pins (actually 3), basically for mechanical reasons since these connections align the electron gun system axially. The datasheet denotes these additional G1 pins as "i.V." ("interne Verbindung" = "internal connection"). It just happened that one of these pins was positioned very comfortably on the PCB to feed it the output signal of the blanking amplifier without any length of copper track. Hence, I feed the DC bias and do the clamping via pin 3 of the CRT socket and apply the blanking signal to pin 6 (another G1 connection is pin 13) which are all on the same potential.So sorry for the confusion, there's nothing special or unconventional about the blanking circuitry I utilized...Cheers,Thomas

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 30, 2025, 07:51:40 am
content: This is a good occasion to share some notes on my own experiments on this topic, in case they are of use to anybody.I did a scope clock design a few years ago, and started with a blank sheet as a challenge. The power supply was the most difficult part, and I ended up using an off-the-shelf flyback transformer (made for 5 V from rectified mains), driven by a baxandall oscillator, with a buck converter wrapped around it to regulate the deflection +300 rail. All voltages were doubler-derived, for good cross-regulation. I also ended up using a tapped resonant capacitor, and rectified the voltage across part of it to get a ground-referenced negative 12 V rail for the deflection LTP biasing. The filament was driven from a gate-drive transformer hooked in parallel to the mains transformer, with inductive current limiting for slow-start of the filament and to maintain a healthy loaded Q for the baxandall during startup with cold filament. Having a secondary voltage higher than the filament voltage also allowed me to generate sufficient negative voltage for G1 cutoff with a moderate number of multiplier stages.I had some issues when using a resistive chain to derive all the tube voltages, the bias voltages ended up drifting around more than I expected relative to each other with brightness, focus and astigmatism adjustment. I suspect that this was because the cathode current was a lot higher than the A2/A4 current. I also ended up buffering the A2/A4 astigmatism voltage with an emitter follower, to minimize variations in this voltage as a function of the brightness control.For the G1 drive, I experimented with using a digital gate drive isolator. These are very fast, and they can operate from reasonably high supply voltage, allowing them to directly drive the grid. A friend ended up taking this project a lot further, and he went back to capacitively coupled G1 drive with DC recovery, to do analog brightness modulation using an amplifier running off the deflection rail. I also proposed dynamic defocusing for more drawing options, which he also ended up implementing with a similar circuit.For the control side, I ported the cathodecorner code to run on an RP2040. I was able to approach the original drawing rate with a slower and cheaper DAC, by optimizing the SPI transfers. It turns out the original code had significant time overhead by having to reconfigure the SPI peripheral between 8 and 16 bit mode for each transfer, due to using a DAC with a 24 bit command word.I can share some schematics if there is any interest.

author: CaptDon
author_rank: Super Contributor
timestamp: July 01, 2025, 01:39:43 pm
content: Floating the grid is a bad idea. The highest resistance in the grid circuit should be limited to 1 meg. Look at the retrace blanking circuits of the old Heathkit and Eico kit type oscilloscopes of the 60's and 70's for ideas. The schematics are easily found online.

author: Martinn
author_rank: Frequent Contributor
timestamp: July 06, 2025, 03:00:39 pm
content: Quote from: TurboTom on June 30, 2025, 07:04:06 amHi Thomas,thanks again for sharing that design! Fitting the whole PCB below the socket is fantastic. I tried without success to replicate your power supply. The switchmode controller is a rather unique part (and unavailable), I have not found any replacement which would allow for an equally compact design. Same for the transformer, no good off-the-shelf option (of course you can always wind your own, which I tried to avoid).BTW the schematic you posted is just a bit too low resolution to be able to read all values, would be great if you could re-upload it with maybe double the resolution. Thanks again for publishing!- Martin