Big Video Amp

[Bob Reite]

Oh, you can get a bunch of motors coupled to the tuning shafts. That's how a few broadcast transmitters have done it. Not that they were tuned up from the studio, but the designer thought it much easer to be able to have the tuning caps and inductors where they wanted them physically without having to worry about how they would be operated from the front panel.

Needless to say it will be very interesting, (as in the Chinese proverb) what kind of weird load Z you will have once you have ionization.

[Doug Coulter]

I know, another few month project with this year's HEAS only a few days away....
The issue is, not enough dedicated wires to there, and computer control at some level is iffy.
Just yesterday I test ran the mess, while there in person. A stray arc or few (which I usually have none of once things are nailed down) pretty much caused all the automated stuff to lock up - and I had to scramble to "safe" things....
The fuse on the RF amp blew for no obvious reason on one of those arcs, saving it - the circuit breaker didn't trip.
Presumably the GR oscillator is immune, but that chinese sig gen would be very suspect...
Wifi will NOT work in that room with the fusor going and the big stuff outside of the usual shield I have. I was getting 5 second response times when it worked.

So, engineering as usual in the real world - some set of tradeoffs is always in the works.

FWIW, running for 15 min at a "relatively low" 2m neuts/second with a gold coupon in the neutron oven resulted in no measurable activation, while a silver and an indium sample somewhat further away in its own oven did get warm...gold is pretty numb stuff I suppose. I was hoping to make a sample to be shown at HEAS, but unless I just run day and night, it's not happening at current output levels - I can get to 6 million or so, but then have fusor cooling issues....or run at 2 million till I run out of mains power, but of course, it pays to watch things.

Just paying off a bunch of technical debt right now. I had torn down a NAS box that used to hold the database and files for this stuff, and all that's now moved to one of the new ones (odroid HC2) mentioned elsewhere on the board, but of course some of that early software development didn't handle some things very cleanly and it's come back to bite me. I kinda had to violate one of the "normal" forms for a database to put the blobs from the cameras and audio in their own directory, and only store links to that in the db, as most db's don't like gigabyte blobs in them. Well, both the network location of the db and the blobs directory were hard coded in the acquisition and analysis code and had to be at last re-hardcoded, again, to work, again. Other issues of timing and allowing that to be changed in their various guis made it hard to just have a dropdown list of possibilities for either, with presets, but it looks like I should write a module that does that for all these programs - easy if you have nothing else to do today...

At least the new NAS is performing very well...and that's now tested and put to bed as the way forward for the fusor data. FWIW, a single odroid HC2 is faster than a far more expensive synology DS416...and I have 3 that can be used in parallel now if the network topology is set up right. There's that gigabit limit that just one will easily saturate. No issues putting in MariaDB instead of MySQL, it was transparent other than failing to ask for a root password at setup (covered in the HC2 thread). One less Oracle thing is usually good in my book.

[Doug Coulter]

I think I kinda tested load Z and reported on it, ions vs no ions, but only tried it at low levels, like VNA or with a siggen with a few watts - at those levels I didn't see a difference between 1 mw and 1W...
Using a series inductor - around 250uH - I had a resonance around 2 MHz with nothing in the tank. C to ground in the feedthrough and grid was surprisingly low.
Adding ions via the ion grid to near the density we have in real running (or at least as real as we can get with that alone, but it's voltage and current don't change much with main grid on/off so I assume we're already pretty well ionized..) seems to add some effective C - resonance went down somewhat, and effective R went up.

Quite a bit - effective load at resonance w/o ions was around 20 ohms. With it went to a couple hundred at the new resonance, which was lower, maybe some 10's of khz.

Essentially I'm driving one end of a series tuned circuit, with the fusor as a C to ground, and my inductor in series. I get a nicely high voltage stepup with no load, but that's going to be less with a load. This IS a reason I wanted to use tubes...they are a bit more resilient as they proved in an arc from the HV in an attempt to xfrmr couple both AC and DC in yesterday (which failed due to not getting enough RF coupled into the fusor winding as well as arcing). But no blood other than a fuse and no foul.

[Bob Reite]

mySQL and MarinaDB are petty much interchangeable at this point. It won't be that way forever as the fork diverges.

Yeah tubes can take a lot of abuse that solid state can't. Although LDMOS RF devices are pretty good. I had a "oops" with the 2KW FM transmitter using LDMOS transistors I'm building. The overload protection actually operated in time to save the $200 output devices.

[Doug Coulter]

Interestingly the Debian-derived distros already put in MariaDB when you ask for mysql - I guess that will save pain later. Works fine (and still uses things like the mysql command line clients, phpmyadmin and so forth).

I bet your mos output devices wouldn't have handled something like 10-20 arcs into the output from 50kv with a real high Q capacitor behind it...
I was originally going to use a broadband MOS amp we got -Taradian - but it fried on an input spike the cheap VNA generated with no load on the output, both devices went shorted gate-source...found some on alibaba, though the exact ones were $90 ea..I'm going to try the $9 ones if I can figure out how they soldered a pair of them to that bolt-down heatsink strip without frying the silicon inside....(the singles are a lot cheaper than the two soldered to the strip...).

In just RF (ac coupled as well) on the fusor, I see two modes, but not enough volts yet for neutrons, of even really getting the ions to move much at this F0 - I'm only getting a few KV on the grid with ions there to make loss with this setup. Working on that.
One mode looks a lot like the DC mode visually, and seems to match roughly the same impedance we had with DC - a little less, but meg type numbers.
If I let in too much gas (and things change wildly and quickly doing down from 2e-2mbar indicated) - grid becomes a near-short, whole tank lights up inside, volts go to nil...and so on. And it likes to stay in that mode until I shut the drive down. Fusor acts like a rectifier either way (grid is anode, cathode to ground) but is a better one in the normal looking mode (but not perfect - with 20kv peak-peak the turn on volts look like around 5kv).

[Doug Coulter]

I've been perfecting this thing for this use, finding out that some scope probes are "consumable cheap" imports...and got all my test equipment agreeing with each other and reality. Which is a bigger deal than one might think when trying to tweak something up.

So, down to 4 tubes (easy, just unplugged a couple), same circuit - I could increase the screen volts a little from the current 35v and not get into quiescent draw, but...for now...
At 300 ma avg plate current (arbitrary limit set by a thermal circuit breaker as Bob recommends), I'm getting 1kv pp on the plates, and around 800v into a 500 ohm load when transformer coupled with a slight step-down. I could probably have kept the 6 tube arrangement if I used less inductance in the plate circuit. This setup has a fair amount of capacity plates<>ground.

For the RF choke/transformer I used a 4" piece of .5" diameter type 61 ferrite rod, Al = 66, 35 turns primary, 30 secondary, but on the outside of a 3/4" quartz pipe with the primary in the inside....because that's going to save me some grief driving something that also has ~ 20kv of the opposite polarity DC on it - that'd be a hard to get capacitor.
This is kind of a low Q output stage that works between 1.7 and 3 mhz at more or less full power with decent waveform in this kinda-strange class B/C mode. Tubes are "off" with no signal and around -22v bias, but barely, and it takes some drive to start the "on" part of the cycle - so I get class C like operating angle. The 2020 opamps drive the control grids around 5v positive, with no obvious rounding off due to loading. Could be worse. If I need more power, I'll have to come up with another screen supply. The current one is a diode/cap off the seriesed filament windings that have one side grounded. I could maybe double off some tap of that and get close - 10 more volts might be nice and should double the ~~ 145w rms I can get out of this without heating up anything but the dummy load.

I went for those numbers as without load the fusor pi network is around 470 ohms at resonance with no ions there. It'll go up in Z and down in F0 with more ions present, but that will also (I think) cut the rather enormous voltage stepup I get from the pi network as the C goes up if nothing else. It's around a 29 pf load now, averaging around 750k ohms with minimal ions in the tank. I'm using a 113uH pi coil and 1035pf to ground on the input side. I have a beautiful 2.5 mh choke with the best numbers I've ever seen to be the DC feed isolation, but it's getting near self resonance and I may have to get a smaller one. The one I have looks a bit like this (original part must have been their last one, and was a little nicer): https://www.rfparts.com/rfc4.html
Like perhaps: https://www.rfparts.com/102-752.html
Pricey stuff. I wish there was a way to tell how good they were before buying...I need one in series with the DC bias for the main grid (~ 20kv) to avoid back feeding RF to the Spellman, I'd guess it wouldn't like nearly a kv of RF fed back into it's little ballast R. Which, as Cliff warned me, is potted and kinda hard to replace if you cook it.

So, about ready to carry this heavy deal back across the shop and try it...I think I'm beginning to understand why Hams use a separate box for the HV supply....split up the weight! This is all in out, and that's about half a cubic foot of iron/copper (the power supply is choke input!).

[Doug Coulter]

Wouldn't you know I finish up this prototype build just in time to quit for the day....but here's what it looks like before I smoke it.

DC input at the bottom, transformer, then the low impedance side of the pi network.

The pi network L coil is that big dude at the top, wound on a beaker I've drilled a hole in (glass drilling is fairly easy, I should do a post on that). It gives me a drive impedance (without ion loading) around 470 ohms at resonance at 2.9 mHz or so.
The RF amp will drive 1k no sweat, breaks a sweat below that, and I need isolation, so that's the transformer with the quartz tube in the middle, wound on a .5" piece of type 61 ferrite.
The big red caps at the bottom are bypass for the DC input, so I don't try to back drive the Spellman and fry it (it's about 5k pf total).

Lathes are nice for winding coils:

Pi coil with HV probe and feedthrough showing too.

I'm not using the vacuum cap shown here, yet. With that L it's too much for the frequency I want now...but the idea was to provide more C fixed so the ion load, which looks like more C (as well as some R) won't affect things as much. In this case I (stupidly?) added some Hysol epoxy to hold the turns on there - it gets warm - so changing the L for a smaller value to keep the F0 up will be a "try that when I know I need to" thing. It could very well be that I want a lower frequency due to the way losses seem to interact with the math...losses make the particle move slower for a given input voltage, which is probably why I was seeing at least some bunching even down at the 100's khz range earlier....at much denser (higher loss) conditions than I plan for this set of runs.

And yes, that's an oldie but goodie multi-watt neon bulb stuck in the styrofoam that's also holding the HV probe stuff...lights up quite brightly when the RF is on...

[Bob Reite]

You plan on getting that coil real hot? PVC would have been easier to work with and at HF, loss should not be an issue.

[Doug Coulter]

No, not super hot, but...Stuff happens sometimes. The pi coils I first tried with thinner wire got real hot from skin effect losses. And that's before I could really crank in the power.

Beakers are both cheaper and lighter in weight than PVC (a case of 12 was ~ $30 on amazon - some import, thin glass)...The feedthrough end does get hot, it conducts heat all the way from the grid and on out, which is otherwise a good thing, I can run more power without melting things. Both the copper and the boron nitride are pretty good thermal conductors. So if you have the tooling to use them, it's not as dumb as it sounds.

[Bob Reite]

I suspect it would be mostly conducted heat from the fusor itself. at 2 MHz, your coil should not get that hot. Now if you were at 30 Mhz, you would want to use 12 Ga silver plated wire.