The curve doesn't show grid voltage increasing past 0 Volts. But I can now see, if I use the 20,000 volt line, with -12V on the grid 0 ma of currents flows. with -10V on the grid 0.06 ma of currents flows. with -8V on he grid 0.375 ma of currents flows. with -5 on the grid 1.35 ma of currents flows. with -3V on the grid 2 ma of currents flows.
So as the grid voltage goes less negative (positive direction) more current flows. Thanks, Mikek P.S. Tube, socket, 2N7000s, some 2W, 1.5MΩ High Voltage resistors ordered. Resistors are for voltage divider to my used with a multi voltage experiment.
They stopped graphing, but the slope is steep at 0 volts. It ain't going to flatten out at +1.
Positive on the grid will accelerate electrons towards the plate. And draw a little grid current.
What you really need is lines for +1000 and +250 on the plate, but the people who designed this tube never expected it to be used as a saturating switch.
It would be interesting to test it as a switch with positive grid voltage. I guess you will!
Composition resistors ARE rather fat, compared to metal-film traces on a ceramic base. To see corona discharge, did you examine your 12kV supplies in the dark? That's how you check for corona: I've noticed it in seventies-vintage car ignition wiring (glows real pretty, time to buy aftermarket silicone wires).
I'm relieved you're not teaching electronics. I've had quite a few CRT focus resistors fail, enough to diagnose (and fix) the sister's TV from seeing the familiar symptoms. That's only a 4kV component, usually, and the failure is from corona. Early decay of the component, not immediate failure that a kid would notice.
On Friday, May 12, 2023 at 4:32:33 AM UTC-5, whit3rd wrote: That's only a 4kV
I'll keep that in component decay in mind, but these are short tests of 15 minutes at most, all in open air. If this every becomes part of useful system, it will be long past me helping my son do tests, and the design will being someone else's hands. That's a good idea, turning out the lights to see corona. Seeing where it is, gives us a chance to reduce it.
Last night I set up the transformer for halfwave output. I have two 8kV diodes in series in case we go over 8kV. I monitored the output with my scope, expecting to see a halfwave waveform. I didn't, as I raised the variac voltage, the scope show a distorted waveform with small halfwave peaks. but over a second or two it diminished to 0. Another bump up on the variac and you could see the distorted wave form again, but it diminished to 0. I finally put a 2.5MΩ load on it and then waveform looked normal halfwave waveform. Then I put only an 18pf on it to simulate the actual load. This brought the waveform back to the distorted waveform situation. The obvious solution is to add a resistive load in parallel with the vessel. But, I'm curious, is this some characteristic of the HV diodes that is causing this? I.e, the HV diodes need a certain amount of current to work properly.
I'm wondering if the tube solution is going to leave a +150V to +7kV pulse, i.e. won't go to 0V? I don't know if 150V will cause enough electric field to have control over water droplets, but I would like to get as close to 0V a possible. I think part of why pulse may be better in oil/water separation is because of the rest time allowing coalescence of he water bubbles, but that is just my speculation. Mikek
There is very little to be googled about using tubes with positive grid voltage. Most people just ignore the possibility, or say DONT DO IT.
I did find an interesting study on the subject. It's from 1933 and it's still paywalled after 90 years.
But I do still have the 1964 edition of the RCA Transmitting Tube manual, nice bedtime reading. Driving big transmit jugs with positive grid voltage is the way to get RF power. Some data sheets have curves that don't show anything below zero but go to +300 on the grid. The current enhancement, as compared to Idss, gets up into the 20:1 and even 50:1 range.
Based on these curves, I wild-guess that my imagined circuit could enhance a wimpy 6BK4 by something like 2:1 or maybe 5:1, which it probably needs.
A tube will certainly not swing to ground. One might be lucky to get a
6BK4 to go down to 500. But as I understand it, the sample is in a glass centrifuge tube, so the system is inherently AC-coupled and all you need is peak-to-peak pulse voltage.
My assumption is that John Larkin starts off with circuits that he gets from other people, and tweaks them until he gets a result he thinks he can sell, at which point they become "his circuits".
Of course John thinks that somebody else should contribute the first schematic - he should have had time to work out that Mikek hasn't actually specified his problem in a way that would let anybody design anything from scratch.
Neon sign transformers do seem to be part of the kind of solution that Mikek has on mind, and without a fairly detailed specification of the particular neon sign transformers he's planning on using, working out a detailed schematic that might work is pretty difficult. If John actually designed his circuits he'd know about that.
I've tried to make helpful suggestions. So has John. What's fairly clear is that is isn't so much a design problem as a case of improvising around what Mikeke and his son have to hand - and Mikek hasn't posted any kind of detailed list.
Mikek doesn't want 27 kV, and he's trying to set setting up 10msec or wider high voltage pulses. Drain capacitance isn't an issue. Valves/tubes do have advantages in specialised jobs, but they tend be bulky and fragile and keeping the electron-emitting filament warm is always a pain. MOSFETs should work fine, though perhaps not with neon sign transformers.
Mikek's experiments with his neon sign transfomer produce strange results - the fact that he has to load the output with 2.5M of resistance before he can see a half wave suggests that there are capacitors embedded in his set-up. The fact that 18pF killed it off again says that it might just be the interwinding capacitance in the transformer - easy enough to measure if you know what you are doing.
I don't doubt there is a large self capacitance in the secondary, there are lots of turns. Well, I thought I knew what I was doing, I used C1-4C2 / 3. But C1 = 3,208pf @ 75Hz and C2 = 945pf @ 150Hz.
3208 - (4x 945) / 3 = 3208 -3780 / 3 That doesn't compute, I did it twice and got very similar results. Any help?
John, you keep saying this, but, compared to the cathode it is a negative voltage. I've looked at several schematics and the grid is negative to the cathode. The more negative the grid goes the less current. Where's the disconnect? Thanks, Mikek
Yes John, I need two low voltage supplies one for filament and the other for bias. I'll be building on a metal box I hope, transformer and Tube on top, LV supplies, and timer inside. If I don't have a 6.3V transformer, I plan to build a 6.3Vdc supply for the heater. What voltage should I build the Bias supply? Or adjustable, up to what? Thanks, Mikek
Sure, it'll be fine for that. I was addressing the more general question of why you don't want to run too high a cathode current for a given tube, if you're making 50,000 All-American Fives and don't want a lot of warranty returns. ;)
But their arrangement can make a lot of difference. A secondary tat is wound as a series of layers, one on top of another, has a higher interwinding capacitance than one that is banked, wound as series of multilayer coils, each one stacked on top of one another electrically speaking, while physically side by side.
But what do you think you are doing?
Every last bit of capacitance inside the transformer has got to get charged up when you apply a voltage across the transfomer. You've thrown a few diodes into the system and it looks as if you are charging some of those capacitances with DC and revers biasing the diodes inot thier non-conducting state for most of the time.
You need to think harder about what you are looking at.
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