Big Video Amp

High frequency, antennas

Big Video Amp

Postby Doug Coulter » Mon Jun 19, 2017 1:55 pm

So, kind of close to RF. Returning to attempt reuse of some hardware I built long ago to drive a Tesla coil, as I want a big, hard-to-fry, high bandwidth signal source to play some games with bunching, gradients, and recirculation in the fusor. Didn't seem to me like a new build in solid state stuff would be as robust, and would surely require more step-up turns ratio than this old thing (1kv or so plate supply). This uses a modification on an already odd grounded grid amateur radio linear amplifier. In this case, I'd ungrounded just the control grid, and grounded the cathodes of the tubes instead (the usual small anti-parasitic R's are there of course; I think of them as fuses at these speeds and feeds.).
The result has a rather amazing plate current vs grid bias curve, or at least, I've never seen anything quite like this before. In doing a little curve plotting, it was rock stable and never hinted at oscillating, even though the plates output lead does go under-chassis with the grid stuff. Bill and I didn't run this up to high power where there'd be big dissipation, we just wanted to find the lower end of the curve on this, so I could design a driver to go between an arbitrary waveform generator and this thing (which in turn will drive a homebrew wideband stepup transformer to the main grid, with separate DC bias for the grid via the secondary winding).

Having built this oh, 40 years or so ago..there was a bit of trepidation firing it up again, and it looked like I used good parts for the time, but was also in a hurry...at least I used a copper foil ground plane around the tube sockets. As luck would have it...it all flew. Looks like this under the chassis. There is an adjustable supply to the left to run the curves with, and various meters out of picture on the right to get the data.
100_3163.JPG
Ugly, I'll clean this up a little.


And that data, again not to full power or even close, created this unusual curve - I didn't plot every point once I realized we got to straight line land.
BiasCurve.png
1ma at -16.5 volt cut off. Wow.


Wow! I can drive this with modern fast opamps! I bought ADEL 2020's, one per tube as the input C is kind of high for one opamp to do them all. I don't think I'll need more than 20 MHz.
I can DC couple this all the way to the output if I want to and am careful. The supply is around 1kV at around 1 amp continuous, but will do a LOT more (Navy destroyer radio supply stuff) for awhile, and the tubes peak at around 1.1 amps - each - cathode current, and will easily take 10kv or so peaks on the plates.
This is why I resisted all the comments I should use some crappy old skool ham amp tubes. The're not even close to this kind of (well, ICAS) performance - opamps vs 100w driver, I'm in!
Here's what was in the box, more or less.
6kd6box.gif
traced schematic

I don't guess I'll need the bias supply, but I will beef up the half-wave rectified +/- 28v supplies (from spare filament windings) to run the opamps, so making that power supply and figuring out how to fit that in are next. Also, for reasons that will be obvious to anyone with some experience here, I want to move the plate connections fully above-chassis and make a new bottom plate to shield the input side of things.

And people on the web are hitting on my "why isn't this done yesterday"...yeah, I know you guys (members) know this isn't something you do before breakfast after writing remote control code and sysadmin-ing replicated databases for the rest of this.

This one has the potential to make a lot of pretty expensive smoke. That's one thing when you can be there in the room with it and slap the off switch at the first sign of trouble, before the tubes get "bent" or transformers stink. It's quite another when you hope you notice something is wrong in time over a time-lagged video feed that is one of many at the operating position.
So, this stuff is slower to do than it might be otherwise.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Big Video Amp

Postby Doug Coulter » Mon Jun 19, 2017 2:09 pm

For those with fewer data sheet resources, the 6kd6 is the largest ever horizontal sweep tube made, just before we went away from big CRT televisions.
6KD6.pdf
6KD6 data sheet
(383.37 KiB) Downloaded 20 times

This opamp, current feedback, is fairly special as these things go too. I'm hoping to bias them with some current into the - input to hold this at the bottom of "straight line" when there's no input.
ADEL2020.pdf
Interesting part - probably needs a heatsink
(213.93 KiB) Downloaded 51 times


Of course, posts like this are also off-site backup for me in case my design machine should crash between important backups...I'm not really this thorough, am I?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Big Video Amp

Postby Bob Reite » Tue Jun 20, 2017 12:47 am

You'll have to put in overload protection circuits like we do on broadcast rigs, as not only are they in a remote location, but nowadays, nobody is actively monitoring their health! As a minimum, overload relays for excessive grid and excessive plate current. Excessive screen current too, if you are running these as tetrodes.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Re: Big Video Amp

Postby Doug Coulter » Tue Jun 20, 2017 9:38 am

Yeah, looking for the best solution (that will fit in there, it's tight) for protection.
This is a funny config - screen and beamforming grids grounded - you'd never turn this mess off at a mere 16.5v bias if there was screen voltage. Cathodes 10 ohm 1/2w R to ground - fuses, kind of. They'll go first in a catastrophe, but in a long slow overload won't do what's wanted.

Grids will be driven by one op-amp each, which kind of limits the power there. These kids have it easy these days...why not get some too?

It'd be nice to have something better than fried resistors in cathode circuits as protection, though. Some kind of slightly delayed current relay in the HV that latches?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Big Video Amp - one more step, or was that two?

Postby Doug Coulter » Sat Jul 01, 2017 5:12 pm

Coming along in between life-interruptions.
I didn't like the really cheesy +/- low volt supply in the amp, so I built a slightly less cheezy one and put it in there, hope it's enough for this:
LowVoltPS.JPG
+/- 15v regulated if half wave will get it done here.

It's still half wave, the bridge is likely a lot more ampacity than I need, but is nicely its own heatsink. The 7815 and 7915 I hope will do...
Bigger capacitors but still only half wave (but +/- 28v no load).

This is meant to run 6 of these, one per tube on each tube's control grid. here's the prototype, using the circuit from the datasheet above for gain = +10.
I added a resistor from the - input to the positive power supply to bias it for a dc coupled input that only goes positive from ground, eg with ground on the input, the output of this is at the negative limit (good for resting the tubes).
VidPreProto.JPG
Amazing it doesn't oscillate or misbehave built like this.


Now, those teeny 270 ohm resistors get kinda too hot...and the waveform is just about perfect (clipping a little on the negative half is intentional for this test), so perhaps what I should do here is increase all the R values to higher ones, as I don't need the full performance this thing appears to have.
VidPreampInOut.png
Scope trace at 1 MHz signal from that DDS project
VidPreampInOut.png (4.76 KiB) Viewed 698 times


I suspect I'm only going to need about 1/5th the frequency response this delivers right now for the real deal, so changing the resistors to be a bit easier on them and the power supply looks like it's in the cards here - I may have to order some more (for things like this I like them to all be from the same lot and match..OCD), but so be it.

You kids have it too easy. Anything even close to this "hot" would have instantly been some kind of oscillating transmitter with this sloppy construction, back in the day. Of course, now I have it too easy too - nice little bench, killer signal generator I built, scope, computer...all the toys right at hand. Makes this easy once you know what you want and how to design and what to expect.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Big Video Amp

Postby Doug Coulter » Sat Jul 01, 2017 6:19 pm

For testing, I changed the feedback resistor to 1k, bias to 1k, - input to ground 100 ohm and am sweeping from 5 KHz to 2.5 MHz and it's perfect.
~ 10 mA draw on the negative supply, around 20 mA on the positive supply (per amplifier, no load).
If I read the specs right, about 2 amps would do this and I may try that.

Now to figure out how to get some overload protection going...
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Big Video Amp

Postby Doug Coulter » Fri Jul 07, 2017 7:09 pm

So, as Joe Piezynski would say (and did in his threading video, it's about time to clench those cheeks and apply power and RF to this preamp and see where the smoke is the prettiest.
Schematic once it works. This is 3 channels to drive 6 tubes, with a relay on the input that either selects the panel BNC or a resistor to -15v to turn things off. ADEL 2020 opamps will go in the sockets.
100_3186.JPG
One side

100_3187.JPG
The other side
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Big Video Amp

Postby Doug Coulter » Sat Jul 08, 2017 3:12 pm

Well, after finding one bad solder joint ( :oops: ) it seems to work perfectly, though the opamps do get warm even at no load (not surprising for these specs).
https://youtu.be/QUX1c1CrTBU

shows a nice little sweep from 5kHz to 2.5 MHz, flat as can be - note source is DC coupled and puts out positive from ground only, and the preamp here is also DC coupled but outputs the negative rail voltage (around -15v) at idle or with an input at ground, which is a good resting bias for this amplifier in this configuration. A relay connects the preamp to either the input or a slightly negative bias so I can "blank" the setup for when the FeelTech Arbitrary wave generator is glitching while I'm changing its parameters (there is no way to tell it to shut up while doing that).

For those curious about the black box making the test signal, see here: viewtopic.php?f=60&t=1001
Yet another project...
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Big Video Amp

Postby Doug Coulter » Sat Jul 08, 2017 3:24 pm

Testing above is with a sine wave for the reasons all engineers know.
But in real life, I don't want a sine (which is why I'm calling this a video amp - it's broadband) but an arbitrary waveform, generated by this thing for starters:
viewtopic.php?f=61&t=1033&start=0&hilit=feeltech
Which I have somewhat working programmatically at this point.
I have some far better Hantek DDS 3x25's but they only run with crappy windows software and I can't get them to tell me the format to input fancy waveforms into them, which should be LOTS quicker and cleaner than with the Feeltech (which takes 20 seconds and glitches while you do it). So, unless someone steps up to the plate and documents the Hantek format (which appears to be talking directly to all the USB endpoints - 16 of them as seen in wireshark via windows in virtual box), it's gonna be this guy we actually use here.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Big Video Amp

Postby Doug Coulter » Sun Jul 09, 2017 3:46 pm

Well, here's hoping Bill "Grumpy" Jenkins was right when he said "Purty don't make it run good".
All wired, tests tomorrow or the next day. The sharp-eyed might notice I pulled the screen grids off ground with bypass caps and then grounded the 6 parallel ones for now...I might want more like the full available current out of these at some point - which will necessitate some other changes to get more control grid bias at rest, so we'll see how it is this way first.
100_3188.JPG
RF deck

100_3189.JPG
Interior with cover off

100_3190.JPG
Topside
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