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Calculating how much V and I a CF can deliver
Calculating how much V and I a CF can deliver
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Old 22nd October 2018, 05:58 PM   #11
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by fozzy215 View Post
I almost feel guilty at times, picking you's guys brains. Then again, no one is forcing anyone to answer our newbie questions.
If our ancestors had hoarded their knowledge, our species would probably have died out. "I figured out an easy way to kill a wooly mammoth, so I have lots of food, but I won't tell anyone else how I did it, so the rest of my tribe will starve to death, ha ha!"

So evolution wired our brains to share knowledge, and to enjoy doing that. If we weren't wired like that, we wouldn't be here.

Well, most of us. Evolution has no ethical preferences, whatever strategy lets you survive long enough to reproduce will let your genes continue. So some greedy hoarders also survived, because they got enough food for themselves, even though the rest of their tribe starved to death.

These days, their descendants do very well in our more capitalistic societies. Some of us even hold up those greedy hoarders as examples for our children to follow.

Back on topic, yikes, Michael Soldano certainly loved sprinkling extra triodes everywhere!

-Gnobuddy
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Old 23rd October 2018, 05:54 AM   #12
Max999 is offline Max999
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There will be much more questions Gnobuddy, a lot of things still need to click here. Learning this stuff,to me, feels similar to practicing guitar: sometimes there is little progress for quite some time and then suddenly you have small breakthroughs that in the end all add up and cross pollinate.

The beauty of evolution should be that it is self correcting. There wil be interesting ethical questions raised when humanity gets good at this DNA manipulation thingy. Its beautifull that people share their knowledge here, it must indeed attract the more altruistical people, although not every forum is as pleasant as this one.
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Old 23rd October 2018, 10:33 PM   #13
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Max999 View Post
Learning this stuff,to me, feels similar to practicing guitar: sometimes there is little progress for quite some time and then suddenly you have small breakthroughs that in the end all add up and cross pollinate.
I know that feeling well. I was stuck for a decade at one point, while going through college and having no spare time, money, or energy. Eventually I finally plucked up my courage and went looking for a good guitar teacher to get me unstuck.


At the same music studio, I later saw a "$5 for 5 drum lessons" flyer. I signed up, and was surprised some weeks later to find I was starting to use the thicker strings on my guitar as the bass drum, and the thinner strings as the snare drum, so my rhythm guitar patterns were starting to borrow a little of the sound and feel of drumming.


After that I tried putting occasional drum fills into my rhythm guitar playing, for instance, after a silent pause in the song.


More recently, when I began trying to sing harmonies, sure enough, that started to influence my guitar solos, and little bits of harmony and countermelody would show up in my solos.


Music is such a fascinating thing. There is no end to how far you can go, how much you can improve, how many new things you can learn!
Quote:
Originally Posted by Max999 View Post
The beauty of evolution should be that it is self correcting.
There are major mistakes that have not been corrected in hundreds of millions of years, though. For instance, having our food-pipe share an opening with our breathing-pipe: a disastrous recipe that has led to millions of deaths by choking during the history of amphibian, reptilian and mammalian life on land. Insects don't have the same evolutionary mistake, as far as I know.


Back on topic, I'm attaching a screenshot of an LTSpice simulation of the Soldano common-cathode stage/ DC coupled cathode follower / FX send circuit we've been discussing.


In the simulation, I turned up the input signal until the CF was just starting to clip. The FX send waveform is shown in the graph, and sure enough, reaches to pretty much +/- 500 mV, i.e. it is 1 volt peak-to-peak, almost exactly matching our quick estimate based on drawing a 100k load-line.


Of course an LTSpice simulation like this is only as good as the mathematical model of the 12AX7 we're using. But it is nice to see the simulation in pretty good agreement with the back-of-the-envelope calculation.


-Gnobuddy
Attached Images
File Type: png Soldano_FX_Send_001.png (54.3 KB, 51 views)
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Old 24th October 2018, 12:54 AM   #14
Max999 is offline Max999
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I also did a LT Spice simulation using your values, but got a different result. Looking at your sine wave input I see 1.5V. Output is slightly clipped. I had to input 2.25V to get a similar result. Also there is more output swing in my simulation, around 1.4V peak to peak.

Do I need to add the V3 from your schematic to do a proper transient sim?

I have added my spice outcome below.
Attached Images
File Type: png lt spice loop send.png (28.0 KB, 49 views)
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Old 24th October 2018, 05:42 PM   #15
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Max999 View Post
I also did a LT Spice simulation using your values, but got a different result.
I think the only difference is that we happen to be using two different mathematical models of the half-12AX7 triode. Honestly, I don't know which one is closer to reality.


A couple of things: one, the difference between 1 Vpp and 1.4 Vpp is only about 3 dB, which is barely noticeable by the human ear. Two, real 12AX7s vary as well, and its quite likely that a real one may exactly match the datasheet.


So, though the difference between your sim and mine is annoying, both results are in the ballpark. At least we know that we didn't make some sort of huge howling blunder in our back-of-the-envelope estimations.


With tube simulations, this seems to be about the best we can hope for. (Fairly accurate models for semiconductor devices are much easier to find.)
Quote:
Originally Posted by Max999 View Post
Do I need to add the V3 from your schematic to do a proper transient sim?
V3 was there for something else. One way to find the output impedance of a circuit in LTSpice is to feed an AC voltage into the output, and then measure the amount of current that flows back into the circuit. Dividing (peak) voltage by (peak) current then gives you the output impedance.


That's why I had V3 and R6 in my schematic.


(The unsurprising result, by the way, was 500 ohms, as nearly as I could tell from the graph. Just as we figured in our back-of-the envelope calculation earlier.)


-Gnobuddy
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Old 25th October 2018, 04:28 AM   #16
Gnobuddy is offline Gnobuddy  Canada
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Arrgh. I meant to write "... real 12AX7s vary as well, and it's quite likely that a real one may not exactly match the datasheet either.

Sorry about the error.

-Gnobuddy
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Old 26th October 2018, 02:45 AM   #17
Max999 is offline Max999
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Ow yeah I use a similar trick to measure output impedance. I inject a current source with an ac amplitude of 1 to a node of interest. Then you can add an V/I expression on the plot.

I am interested in finding the limit of the load a 12ax7 can drive. Opamps usually want a load not heavier then 2K. Is there such a limit for a 12ax7?
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Old 26th October 2018, 04:22 AM   #18
PRR is offline PRR  United States
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Many op-amps are current-limited; the figuring is different.

The popular TL071 opamp has internal resistors like 300 Ohms. It drives 2,000 Ohms fine.

The 12AX7 with the customary 100K plate resistor is an effective source like 40K Ohms. >100X higher than TL072.

Why do you need to make a 12AX7 work hard? TL072 is 19 cents.
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Old 26th October 2018, 05:26 AM   #19
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by PRR View Post
TL072 is 19 cents.
And they are wonderful things, no longer appreciated as the miracle of technology that they are. It took about 250,000 years for homo sapiens sapiens to go from hunter-gatherer in the African savannah to the first op amp.

The typical op amp has 30 V between supply rails, the typical 12AX7 stage has 300 V. The op amp happily supplies 10 mA, the half-12AX7 will manage 1 mA.

So the tube runs on ten times as much voltage, and delivers one-tenth the current. Impedance being voltage/current, we can make the ballpark estimation that the triode will only drive a load that's a hundred times stiffer than the op amp...two hundred kilo ohms instead of two kilo ohms.

That's not too far off base, though voltage gain is limited and precious when it comes to tubes, so we tend to use loads that are even higher impedance than that. Which is why you see 500k and 1 meg volume pots wired to 12AX7s.

It has crossed my mind that the early history of electricity was all about static electricity: very high voltage and very low current. Today's semiconductors go to the opposite extreme, with the CPU for your computer drawing tens of amperes at maybe 1.3 volts. And the century-old vacuum tube technology fits neatly in the middle, right between static electricity and what is sometimes loosely called current electricity.

Incidentally, if you want a half-12AX7 to deliver lots more drive, the solution is simple: wire a suitable high-voltage, N-channel MOSFET to the triode in source-follower mode. The MOSFET will be audibly transparent, but work as a very effective buffer.

-Gnobuddy

Last edited by Gnobuddy; 26th October 2018 at 05:29 AM. Reason: Fix a typo
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Old 28th October 2018, 03:49 AM   #20
Max999 is offline Max999
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I seriously thought about adding an opamp or a mosfet to act as an extra buffer, but wanted to see how far I would get by using what is already there in the design.
What I do know from playing with opamps and a too heavy load is that they usually do work, but they start to behave funky ( clipping in my case). I thought that maybe a tube had a similar behaviour.

I have another question ( they keep on coming!)
So on this 12ax7 buffer circuit I drew ..is it true that since there is no plate resistor there will also be no ac voltage at the plate? I am feeding it ac at the grid. In my simulation I see no ac voltage at the plate.
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