At 10:21 5/14/98 -0600, you wrote:
>Counterpoint to the following comments - sorry now I got too interested. I
>realize that some of W7IUV's statements are jest, but....
>
John, I am glad to see that you took my post as it was intended....
>A couple of years ago, at VOA transmitter site in Sri Lanka, a brand new
>transmitter burned up during the pre-acceptance test overnight. The
>modulator melted down. It was a very expensive failure (12 Million $
>including building damage) and they spent more than the average commercial
>interest in trying to analyze what happened. I looked at some of the photos
>and the circuit, as we had been developing SPICE models for large 3 man
>tetrodes, similar to what the XMTR had at VOA. One of the theories was of a
>parasitic in the modulator filter inductor. It is a 70 KHz PWM modulator,
>switching waveforms. And there were oil filled caps nearby to feed the
>fire. Other theories were of a loose connection that started burning. I'm
>sure that they (VOA engineering and the XMTR manufacturer) have more
>theories, but the point here is that, YES, parasitics can do incredible
>damage if not suppressed in big amplifiers. (if indeed a parasitic caused
>this)
If my post only succeded in prying loose another of your
"Three-man-Tetrode" stories, it was worth the effort! I love these.
>>7) This one outa really start the pot to boiling! Any of you guys out there
>>that work on solid state amplifier design ever seriously consider using a
>>device which has a hfe of ten or more times what is required? Sounds to me
>>like a disaster looking for a place to materialize. Why then, are most
>>(all?) current amplifier designs using tubes with guaranteed gain out to
>>blue light? No friggin wonder they will take off on their own at the least
>>provacation.
>
>Actually gridded tubes are not capable of gain out to blue light, or even
>to GHz. Typically, transit time of the E beam will eventually roll off the
>amplification. The tube is then no longer providing the plate current 180
>degrees out of phase with the grid voltage. Big tubes can go into parasitic
>oscillation in L band, or lower UHF regions. This is usually due to a
>higher order circular coaxial waveguide mode triggered by the geometry, the
>beam conditions, the RF....
John, I agree and admit to some slight exaggeration in the above. Can I get
you to agree that in a compromise design/layout, a wide-band device may
have a better chance of finding a set of conditions that will allow it to
oscillate than a lesser device, everything else being equal?
But, VHF is certainly within the gain-bandwidth
>of most modern tubes. Solid state devices are much more capabile of working
>out to high frequencies, but the gain tends to roll off smoothly. Also, the
>matching networks are TINY, so the chances of getting a big capacitor with
>residual inductance are small. And the impedances are typically less than
>50 Ohms throughout. In tubes, impedances are typically hundreds of ohms or
>more.
All those little chip caps got inductance and resonances. Fortunately for
us, ATC and the other guys did their homework and pushed them so far up
that us lazy folks can get away with ignoring them... usually.... NEVER say
never.
>As for current tubes with high gain and max usable frequency, if all new
>amplifier still used 811A (oops-bad example) or 250TH or whatever, we could
>keep the tube companies making those old bottles and no need for ceramic
>metal sealed tubes with external anodes.
Never happen, external anode tubes are generally less application specific
then glass jugs. This translates directly into profit dollars, the big
motivator!
I would be out of a job as would
>many others. Yes, I could use a 1935 tube that worked to 10 MHz, and then
>rolled off. And had a lot of inductance in the leads, and probably was
>ready to break into oscillation at the drop of a hat.
I think 1935 tubes are cute. Also getting pretty cheap since the yuppies
don't appreciate them. Besides, the old guys somehow managed to keep those
things sorta stable, are we as a group less capable. We are in the area
here which seperates the HOBBY from commercial interests.
Nope, I don't think
>that the modern tubes are any more prone to be sensitive to parasitics,
>just that they do have good GBW and that they require much care in the
>circuit component department to prevent development of hidden VHF circuits.
>One has to be prepared to do their best design, instead of sluffing over
>the lead inductance, the self resonant chokes, the filament bypasses, the
>pi network capacitor selection.
My point exactly, but you said it better. Or at least without sarcasm.
I disagree that it is all art, only that it
>is a skill which is only being improved and taugh through forums such as
>this.
I think our only disagreement here is the definition of "ART FORM".
Even the textbooks miss a lot, esp when we look for discussion of
>parasites. That's why i like to go back 50 years in the literature, when
>this was the topic of many EE's work.
One of the low point's in my 34 years with my employer was when the animals
in the Tech Library decided to shit-can the "PROC of the IRE" and I didn't
get a chance to pick the can!
This is exactly what people here have
>been debating for weeks. If it is a bore, hit delete. If it is personal and
>nasty, ignore it. However, there are pieces and tidbits here that seem to
>make sense to me and others, despite the ego's behind all of it. I have
>learned or been refreshed hearing things that I saw, read, or heard from
>some ol RF fart along the way.
I was quiet while I was learning and/being entertained by the Grate Debate.
Maybe a little external stimulus is better than the delete key. It sucked
another good posting outa you!
>
>Same problem is in solid state amplifiers, trust me. I design them also. We
>are hoping to present a paper on one in the RF Expo conference in San Jose
>next October, look for me there.
Would that I could. Some misguided manager types are promoting in-house
education again.
And we have had to pick the correct chokes
>and swamping resistors and all that, just to make them stable into VSWR
>etc. You want power, you get big devices with a lot of gain.
OH, YEAH! Couple years back I had the good luck/misfortune to be
assigned/stuck on a job where a multi-kilowatt L-band amp was to be
developed. I got kicked off the amp team because I had a tendency to laugh
out loud every time one of those $550 transistors shot it's wad. Instead, I
got to work on the development of the combiner. Consider if you will a
multi-multi-port, stripline, multi-kw, multi-layer PC board with a 1-5/8
air line launch off the board. Many a nite I went home with network
analyzer traces burned into my retina. The best was when the HAZ-MAT team
scrambled to clean up our coolant spill!
Nuf said....
>My coffee is boiling, gotta run.
>
>73
>John
>K5PRO
I'd love to take some vacation time, drive over and get a tour of your
place. I want a pix of me standing next to a "Three-man-Tetrode".
73,
Larry - W7IUV
w7iuv@axtek.com
www.axtek.com/w7iuv/
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