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 Post subject: Log 10/Anti-log amps with tubes, can it be done?
PostPosted: Apr Wed 13, 2011 3:18 am 
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Hey oh

If I want to output the square of a voltage would I do it the same as multiplying? Sum the log of the voltage and take the anti-log. There are op-amps and complex IC's that do this for you. Is there a TUBE means to doing this? Google was a fail for me in searching returning Log Taper Pots and the like.

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PostPosted: Apr Wed 13, 2011 8:10 am 
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Take a triode, operate it the most non-linear part of its curve,
and you may find a zone where the output voltage across the load resistor changes as the square of the input to the grid.

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PostPosted: Apr Wed 13, 2011 12:51 pm 
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But don't expect it to "sound great" (whatever that means :wink:).

There are op-amp circuits for tubes as well as for transistors. You might have some luck with those.

Larry

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PostPosted: Apr Wed 13, 2011 2:46 pm 
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:lol: :lol:

The Lorenz sounds quite wild actually:

http://frank.harvard.edu/~paulh/misc/lorenz.htm is one of the "web pages" that got me into the idea a couple years ago. Lots of reading at http://sprott.physics.wisc.edu/

Image

The Mandelbrot is a similar oscillation with; sum of squares with two comparators and a logic element added.

(some pseudo code from http://www.steckles.com/buddha/)

int Mandelbrot(complex c)
{
complex z = c
int iterations = 0
while( z.real2 + z.imaginary2 < 4.f AND iterations < max_iters)
{
z = z2 + c
iterations++
}

return iterations
}


And should be just as 'doable' electrically as the Lorenz is done. However, the means of multiplying is not straight forward. While the Lorenz can be built with tube op-amps (the math is identical) doing a tube log amp is another matter, a diode feed back plate-to-grid of a triode "mostly" works, but is distorted unduly. :) so was wondering if there had been a time of log amplifiers from the vacuum tube computer days or did silicon come in first?

But yes, there is imagery and sound too, because it is amplified oscillations involved.

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PostPosted: Apr Wed 13, 2011 4:57 pm 
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Oh, good grief!

In the face of such morbid fascination with mathematics, which I despise, I can do nothing :mrgreen:.

Mox nix, life is too short.

:wink:

Larry

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PostPosted: Apr Wed 13, 2011 5:36 pm 
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Location: Pocasset, Cape Cod, MA
The Ballantine AC voltmeters had log response, but some of that was obtained by specially-shaped magnets in the meter.


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PostPosted: Apr Wed 13, 2011 6:37 pm 
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I have 2 Ballentine True Rms Beasts upstairs.

And I asked people "How come the pointer doesnt have a left hand rest spot with no signal..." Must be broken...

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 Post subject:
PostPosted: Apr Wed 13, 2011 10:42 pm 
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Location: Saskatoon
Log/antilog circuits are going to be complicated. To square a signal, all you have to do is multiply it by itself. You can do this with a balanced mixer. Since both inputs will be the same signal, and hence the same sign (both positive or both negative) the output must always be positive. That makes things even simpler, because if you full wave rectify your signal first, you only need a 1-quadrant multiplier. There should be lots of circuits floating around that can do the job. A dual control pentode or beam deflection tube comes to mind. However, you will need a precision full wave rectifier on the input which will add some complication. With a bit more complexity you can build a Gilbert Cell 4-quadrant multiplier.


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 Post subject:
PostPosted: Apr Wed 13, 2011 11:06 pm 
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:? magnets? Now that does lead to some interesting notions that!!

Bob, I googled: http://www.google.com/search?q=Gilbert+ ... nt=mozilla

LOTS of reading and LOTS of PDF files and .EDU websites. Thank you :)

HAHAHA Larry, I know, makes the brain turn into soft butter!! For me a few years ago I had to use a calculator to add and subtract simple numbers like 1 + 2 because I could NOT figure out how to do so. I do mean a hand held calculator, I kept forgetting that Windows came with a calculator program. Come a long way since 2004 recovering my noggin.

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 Post subject:
PostPosted: Apr Thu 14, 2011 1:26 am 
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All congratulations and hope your recovery continues, Keeps.

In the meantime, math and me do not agree....

:wink:

Larry

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 Post subject:
PostPosted: Apr Tue 19, 2011 6:39 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2937
Location: Seattle WA US
Keeps-
Everything was done with tubes back before the late 1950's. Here's a link to a good bibliography on analog computing. The first book on the list, by Korn & Korn is worth finding a copy.

http://web.mit.edu/klund/www/books/analog.html

Also look for anything by Philbrick - who built analog computing gear out of tubes before he moved on to transistorized op amps.

--Chuck

<edit: used copies of Electronic Analog Computing by Korn & Korn can be found on Amazon.com for as little as $9.16 including shipping !!>


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