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> CURVE TRACER!!!
Sch3mat1c
Posted: October 15, 2002 07:00 am
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It's pretty damn rudimentary, but - tracer nonetheless!

I took some pictures (I'll be darned, there's actually enough light down there for the camera to not complain), they'll be along in around 6 months.. tongue.gif
(Also took some of the bench, so I can rejoin the Bench Wars battle too. biggrin.gif )
It's still on the breadboard, I'll get a circuit along. (Gregg/Fred/anyone want to host?)

Basically, I took the 420V-no-load, 300V-at-100mA from my 6W6 PP amp (tubes except 5Y3 pulled) and ran it to the rails on the breadboard. Used a 2D21/5727 thyratron for the sweep generator, feeding plate with a 100k resistor, and have .047uF to ground. (A nicely stable relaxation oscillator!) I half-wave rectified the 6.3V filament supply for -9V bias, which I apply variably to the thy's grid with a 100k pot. This varies the plate-turn-on voltage, varying peak voltage and frequency.
I tried direct connection to a 12B4, cathode-following the thy's plate, to boost current for driving the tube-under-test. This works, but because of the low mu, output goes down to only ~80V, whereas the thyratron turns off at around 20V.
So I cap coupled the sweep to the 12B4, and using a -9 to +100V bias on it, I can vary voltage range covered, to some extent.

For current monitor, I used a 10 ohm resistor in the ground return. This obviously will only work correctly for triodes in class 1 (no/minimal grid current), as these are *plate* graphs.

Since I have my 9 and 7 pin sockets tied up with the 12B4 and 2D21, I had to use an octal tube for testing. I chose a 6SJ7, being low current.

Hooked up the scope probe (ch. 2 - X axis) (bad choice; the probe is x10, so the input had to be at lowest setting, 2mV/cm) to current monitor (10 ohm) and the probe-wire (I don't have a second probe sad.gif so use a single piece of wire running into the BNC center connection 8) ) on\\in ch. 1 - Y axis - to the plate voltage monitor (a 1M in series with 10k, output between them).

Bias to the device under test is -9-0V supplied by a 5k pot.

Long testing story short, I got the thyratron sweeping, the 12B4 amplifying, and the 6SJ7, err, testing.
Curves look right - at -6V bias, it begins at around 50V, before which it's pretty flat. Zero bias is relatively straight, though still with the upward curve typical of diodes and triodes.

Among the photos, I took some staring at the scope face - at Vg = 0, -1.5, -3 and -4.5V. I forget if I did -6V. smile.gif

Because so much voltage is lost in the 12B4, maximum voltage is around 150V peak. Not much, but enough for graphing some resistance-coupled stages, eh?

Now I gotsta wire up another 9 pin socket so I can trace 12BY7 in triode. smile.gif biggrin.gif

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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Geek
Posted: October 15, 2002 07:35 am
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Great! [img]http://www3.telus.net/small_business/Smileys/smiley0.gif[/img]

Now maybe one of the SS geeks (Blueeyedpop, you there?) can help us figure a way to translate your tracers output to a PC and code for useful SPICE analasys.....

<edit>

Sure, I can host the pic for you - just email me at dude at scorpiorising.ca.


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ZM
Posted: October 15, 2002 09:05 pm
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;D >:( sad.gif ohmy.gif 8) ??? :smile.gif tongue.gif :-[ :-X :-\ :-* :'( ;D
there is one example where sheety SS is realy useful,and easier to implement,than tubes.
personally,I have not enough time to do this,but Tim have all my support........
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Sch3mat1c
Posted: October 15, 2002 10:58 pm
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[quote author=ZM link=board=21;threadid=146;start=0#1090 date=1034712312]
;D >:( sad.gif ohmy.gif 8) ??? :smile.gif tongue.gif :-[ :-X :-\ :-* :'( ;D
[/quote]

Mood swings? tongue.gif wink.gif

[quote]
there is one example where sheety SS is realy useful,and easier to implement,than tubes.
personally,I have not enough time to do this,but Tim have all my support........
[/quote]

Huh? wink.gif

I'm thinking of MOSFET, uh, source follower. Less voltage lost. SCR could be used as well, with negative grid voltage replaced with positive gate current.

I still dunno about plate current alone. Maybe cap couple from plate? But then plate's moving too so the small current-sense voltage needs to be subtracted...
Negative supply and swinging cathode voltage instead? Better...'cept screen, etc. needs to follow cathode then...

Now to check if Gregggg is slow...

Answer your mail, slow poke... biggrin.gif

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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Geek
Posted: October 16, 2002 12:28 am
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OK Tim, she's hosted and your turn to answer your email for the URL to post it [img]http://www3.telus.net/small_business/Smileys/smiley12.gif[/img] [img]http://www3.telus.net/small_business/Smileys/smiley3.gif[/img]


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Sch3mat1c
Posted: October 16, 2002 12:35 am
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No problem...
Huh, looks like I'm only 10 minutes behind you... heh heh heh.

[img]http://www.scorpiorising.ca/Tim/Curve_Tracer.gif[/img]

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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Sch3mat1c
Posted: October 17, 2002 08:07 am
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Here's some 'scans' from the latest incarnation (Colt45's hosting this time)...

[img]http://6aq5.dyndns.org:8080/tim/6DK6_Triode.jpg[/img]

- This is the almost untouched scan of the traces on the scope. This wasn't easy because I had to hold the piece of tracing paper there, mark some divs for reference, then draw each curve, blurred as it is (you know how tracing paper is). Go over and bother the bias control to advance to the next curve, and so on.

Now - after decolorizing it, and tracing again (based on the references, and that I tried to get the center of the blurred lines when tracing smile.gif ), with Paint (using curvey line tool) to get something vaguely recignizable as a curve.

[img]http://6aq5.dyndns.org:8080/tim/Triode_6DK6.gif[/img]

The curves may be offset horizontally (which would help explain the zero bias curve) because the X pos. control on the scope is real touchy.

Other explanation for zero-bias offset seen is, the thyratron has a forward drop of around 20V...so it'll never get too low.

So...6DK6 in triode...whaddya think?
mu ~ 40, Gm a.k.a. S ~ 5000µmhos (5mA/V), Rp ~ 10k, about like 6AU6 (which does have published triode connection).

- When I get the photos of the scope I'll be able to see about 12BY7 a lil'.

Tim


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Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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