Discussion in 'Amateur Radio Amplifiers' started by W8JI, Dec 30, 2011.
Sometimes you have to know when to walk away.
That was my point about the simulation method, before that thread got locked.
There isn't a good SPICE model of a tube, not even close when we get to VHF on a tube normally operated at HF. There cannot be one, unless we measure and characterize the tube's behavior outside the range where the tube normally operates. We have to account for everything, including electron transient times that cause phase shifts.
Adding to that problem, the socket, and even sheet metal, can greatly affect the way the tube behaves. So now we have to also get some idea what a particular chassis and socket does. Only then, can we think about adding the entire path the tube connects to at each end.
Otherwise, if we look only at the anode path at the connection point, we have no idea how that impedance affects the operation of the tube.
How simple this is, or how complex, depends on the tube, chassis, and layout.
On the other hand, we can have a real unit, run the voltage up to maximize gain, and change all the varying parameters through their range, and see if the system is stable. This is a manual Monte Carlo analysis using physical SPICE components, absent the computer of course.
The entire system is far more than two or three components, and some known DC or low frequency behavior.
Too bad this is out-of-thread, because it is interesting to think about.
Tom is right and amplifies the point I made earlier.
Sorry for any confusion, the whole point was you can use modern tools to model any black box contents.
It was suggested that the PA tank network would be hard to model due to all its stray reactances etc. In fact it is a trivial task.
As you know you could just as easily get a metal box put two RF connectors on it at fit two short wire probes on the inside of the connectors.
Then fill the box with nuts and bolts and the contents of the garbage can and you could still produce a working model of the box performance in terms of impedance and insertion loss across 1MHz to several GHz. It would take less than a minute to get the model.
You could email the model to somebody and they could enter it into their RF simulator and simulate how it performs if you put resistance across the input or output or even if you connected it to a ssimulated amplifier to see if the simulation shows the black box would improve or degrade the stability of the amplifier
You seem to keep ignoring the fact there is no SPICE model of the tube, or the socket, or the chassis, or the variety of tanks and such.
Not only do we have to sweep the tank across a wide range, we have to move the PLATE capacitor and sweep again. Then again, then again.
When we are done with that we move the LOAD, and we start over on the PLATE.
When we get the LOAD done, we now change bands and repeat the entire process. The result will be the tank path for one particular amplifier.
I don't think this can be done in a few minutes, but maybe I am wrong.
So the first problem is tube behavior out through the pins, so we can do a SPICE model representation of the tube. Where do we get that information?
Tom, you started a thread about locked threads. Now you are arguing physics.
Someone once said, if you are not part of the solution . . . .
This indeed can be the case--and is a common complaint heard from a wide variety of members. Sitting on the staff side of an internet resource such as this, we see a different picture than our readers do. Posts are reported with regularity for perceived and real abuses. Email's and PM's are launched to us by members as to why things are a certain way. And then there are metrics--solid data and statistics through Google Analytics that show exactly what is happening in every board of the forum, and every thread contained within.
As the hackneyed phrase goes, I have no dog in this technical fight. My role and the role of other staff and moderators is to make the QRZ.COM forums a useful and enjoyable experience. When what should be grounded discussions turn to personal innuendo, outright insult, or any sort of argumentative nonsense--the results are predictable and repeat themselves in a formulaic manner. First, a curiosity over the impending train wreck of personalities draws a spike in readership. As the matter escalates and the 'discussion' becomes circular--readers drop out quickly--and do not return. Visitation for an overall category usually takes a dip too. This sort of thing is simply at odds with the intent and mission of QRZ.
Aside from such matters, heavy threads that run over 100 posts are unwieldy to read--and by that time are usually recitations of the same arguments over and over. Threads anywhere on QRZ that have not been posted to in 90 days are automatically closed by the forum software.
I did not close the modeling thread--and given an element of interest will reopen it and see what happens. If it turns into a personality based discussion it will join the other two. Perhaps something useful to the average amateur operator will emerge from the discussion--but my money is bet the opposite way...
See Y'alls in the morning I needs my beauty sleep Can we just re title this thread Vacuum Tube Physics and modeling and let it live?? Please... Pretty Please..... With an anode cap on it.
As an Addendum.... Patrick does have the science down on forum behavior and could with little effort model it. I would be one to vote for reopening of the modeling thread. Now with my final two cents in I shall go for my beauty sleep regardless of how futile that may be. Y'alls have a good evening.
OK. Let's say I tell you that the reason some amplifiers occasionally make a big bang is due to tube gas that caused an arc inside the tube. Whereupon you counter that arcs inside a vacuum make little noise. I say this is nonsense - which itself is pretty obviously just that. Your move Tom.
Value depends on veracity, not on politeness. Where's the value in politely letting your opponent get away with specious reasoning in his vain attempt to prove you are wrong when any high schooler who has had Physics-4 knows that sound does not travel very well through a vacuum ?
• Rich, ag6k
Sound travels quite well in metal & glass,though ,Rich!