Clearly to get an accurate, useful 1us ramp, I've got to start the charging current in say 25ns or less, and get it stabilized in a similar time. So I find the 10ns story encouraging.
Hey, Tony, great idea, why didn't I think of that? We'll have a 0.1% to 5% duty cycle at most (1kHz rep rate), so that should be just fine. I can use the DC restoration you show. It's a huge simplification, and limits the extreme HV to the plate and a few other items.
So now you run the fet source at ground, through a sense resistor maybe, and the tube grid somewhat positive, whatever it takes to get your amp plus some fet headroom.
Run the tube filament through a dc/dc (or dc/ac!) converter to keep from having a lot of capacitance in a filament transformer.
Or, build my pseudo-Marx thing with hv fets. But the fet-tube cascode is quaint, so that's more fun.
Do the Larkin-Marx circuit. Ten stages, all initially at ground, each a mosfet ramp generator on a small pc board. Charge up their reservoir caps to 1KV each. At trigger time, each ramps. Diodes stack them in series during the ramp. No HV supply, no floating gates, very rugged.
I find it interesting that my "toob" suggestion is now being seriously considered. Please be very careful in considering the tube voltage - current curves, my suggested 4CV35000's would need about 1200 volts cathode above "grounded grid" to reach cutoff (). Fortunately, the swing necessary between cutoff and 3 A is no more than 600 V for 4CV35000's.
In fairness to Ancient_Hacker (who was that comcast grg2 masked man?), I have to point out your suggestion was a non-starter. Large modern tubes are wholly unsuited, with an unattractive amount of capacitance, and with a devastating level of filament power required, they'd be an unnecessary cooling nightmare. That's in part because one has to go all the way up to the giant-size monsters to obtain a 15kV or higher voltage rating. So those are out, and never were in.
Likewise, the 1mA wimp suggested by Jim and others was a nonstarter, because we need currents closer to an amp. However, I may employ one for the low 0.1 to 5mA end of our needs. Sort of a range-switch idea.
No, it was "Ancient_Hacker" who thankfully got the tube ball rolling, with his 715-C suggestion. I wasn't able to easily find a 715, but it did cross to the 5D21, a 1944-vintage tube that comes up on eBay all the time (I bought one for $6.50), and to the Eimac 4PR60C, which is still in production from time to time (I got a one for $175). It was these 60W pulse types that made a tube not only viable, but attractive. Now we can consider 5 to 10A currents, that give us the 0.1 to 0.25us region as well, very nice!! So I say again, thanks "Ancient_Hacker"!
Very clear, and the cost of the new Eimac tubes is non-trivial. At least i may have helped jog the discussion onto a more productive line. Thank you for your honest answer.
Well, duh, go ahead and stack them! Why not try simply making the capacitive-divider stack part of the capacitive load? This would allow for a compact circuit that can be placed closer to the chamber, saving on coax capacitance. Simple!
(And there's no 50W filament heat-removal problem.)
Ackk? It doesn't sound that bad to me. A single fiberoptic data stream could trigger the module and load the slew rate for the next shot into a serial dac, all in the same burst. The micropower logic could be powered any number of simple ways. Better yet, couple the power, trigger, and data through a single, very simple high-voltage transformer: biphase data/power waveform into a Coolrunner, and a big spike as the trigger.
If you want to build a scaled down version just to get confidence, you could look at using the 6HV5 tube. It's a common CRT shunt regulator, not the 6BK4 type, shunting the huge high voltage, but shunting the boosted B+. So it's a 5KV/300ma tube. And with its 65,000 gm transconductance, you could even drive it with a 2N404.
That's another nice lead, thanks! However, I think I'll hold off from experimenting with 6HV5 for now, because they'd never handle the roughly 15kV I'm looking for. They do approach the required peak current territory, and their 11W heater power isn't too bad.
One issue that bothered me with the 715 family was removing the 50W filament heat, while keeping most of the airflow away from the 12kV wiring. Not too bad, but along with the size and shielding, a pain. So I'm setting that option aside for now.
Anyway, I think I'll pursue my new MOSFET idea for the moment. If it works I'll have a nice small structure that can be placed right next to the vacuum-chamber flanges.
That's always attractive to me. :-) OK, I'll go grab a few of those beauties, just in case. Where do I find 12-pin sockets?
might have them, although I don't remember if they have just PC mount, or chassis mount as well.
FYI, I have five of those sorts of tubes, a pair 6HV5, 6HS5, and a single
6JH5. All pretty similar, IIRC, at least more similar than your MOSFETs. ;-)
Other tube geeks might have more on hand, rec.audio.radio+phono guys might have some. Be worth checking vs. e-Bay. Probably not a problem finding them for cheap though... not really any use for them!
Tim
-- Deep Fryer: a very philosophical monk. Website:
Listen up, you young whipper-snapper, have some respect for your elders!! All this talk about new-fangled tubes being better than semiconductors. All heresy, I'll tell ya, and no respect! What's the world coming to? Those of us who've been around a while know that it's the important parameters that count. Like transconductance. This is mathematically predictable for MOSFETs, within perhaps 20-30% for a given type, yet is not only all over the map for tubes (whatever those identical "target" specs of yours may imply), but badly degrades with time, as well! Dry that spot behind your ears and listen up, *that* doesn't happen with MOSFETs! Did you ever hear of us silicon guys ever needing the equivalent of a tube tester? "Ya better test those MOSFETs, they might be getting a bit weak." Nope. That's right, you won't find any FET testers in our supermarkets! No call for 'em.
Another thing, you kids may be proud of running your plates red hot, but those of us who are more knowledgeable realize that it's better to put your heat into a slab of silicon and transfer it from that to a slab of copper, and then to a huge molded aluminum heat sink. That's because we know thermal conductance beats thermal radiation, any day. And don't you forget it kiddo!
Re the heat, remember that toobs are not temperature sensitive, not even slightly, not until the glass melts or the seals overheat. maybe not even then. I've seen 6L6's with the glass melted and sucked in, shrink-wrapping the plates, and the tubes still worked fine.
50 watts can be carried away by radiation if you leave a few inches of free space around the tube. You don't have to shoehorn this into a cell-phone case, do you?
The 715's have a hard-glass bulb and is rated to dissipate the filament heat and another 60 watts from the plate with no problem.
Just eyeballing it, I suspect you don't have a problem as all the old TV's had much smaller soft-glass sweep tubes dissipating up to 40 watts with no forced-air cooling at all. Many mil-spec transmitters ran
813's, a tube only slightly taller than the 715 at over 100 watts with no airflow at all. The tubes did run very warm, but within the conservative mil deratings.
Here's a 813 running at 150 watts output, horizontal, convection cooled:
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Just a hint. Toobs can take the heat.
The only problem then is the radiated heat warming up any solid-state stuff you have around the tube!
Ooohhhh, seems to work great in a quick-look spice model. A linear ramp in about 1us. I used Fairchild's FQD2N100, a 1kV 1.6A MOSFET in a DPAK smd package,
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I used the modified spice model I previously developed and vetted for my 2.5kV precision amplifier project. Each cascode stage has a damping resistor not shown in the drawing above. Damn, it's looking good so far!
I'd like some comments on this circuit, and so have posted an Acrobat file on the simulation to my webpage. I started with a simple circuit and four cascode MOSFET stages. Spice shows a roughly 5ns propagation delay for each stage, or about 20ns for the four. If this is correct, it would correspond 75ns for 15 stages, a bit slower than I'd like.
But seriously, how about some diodes to clip the gate excursions? I know, Spice says the voltages will track perfectly. IMHO asking for
2% tracking shows an awful lot of faith in the matching of dynamic transconductances.
BTW Tektronix tried something similar for the HV regulator board on one of their early 43x scopes. A nice sedate application, with lots of zener diodes maintaining order.
A year later they sent out a new, completely redesigned PCB board, with a completely different, no series transistors, design. Hmmmm........
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