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Old 31st October 2012, 10:38 PM   #301
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Yes, very close I have problems with ac noise as well, especially with my home HPNA network and the microwave. I think Ale used a filter on his unit.

you must be tracing a small signal valve, what happens when you trace something with more plate current, it should clean up.

It appears as if there is some resistance affecting your current sensing circuit as the cutoff signal is not laying on top the zero current horizontal line.
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Old 2nd November 2012, 08:15 AM   #302
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Quote:
Originally Posted by avincenty View Post
Yes, very close I have problems with ac noise as well, especially with my home HPNA network and the microwave. I think Ale used a filter on his unit.

you must be tracing a small signal valve, what happens when you trace something with more plate current, it should clean up.

It appears as if there is some resistance affecting your current sensing circuit as the cutoff signal is not laying on top the zero current horizontal line.
Good eyes! So far, I have only tried 12AX_ tubes and kept the plate voltage pretty low, based on your comment, I stuck in a 5881 and cranked it up, the curves were indeed much cleaner!

Here is the latest shot (5881):
Click the image to open in full size.

I still need to work on the volts/step, the grid bias circuit and of course the chassis as they are still not to my liking, but at least it is another step closer to completion.

Thanks,
Jaz
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Old 2nd November 2012, 08:24 AM   #303
mogliaa is offline mogliaa  United Kingdom
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Jazz, looking good!
My advice is to try some different types of valves to see whether you may experience any oscillations or issues with your driving circuit. Just experiment more before moving into the final build stage. I used plenty of ferrite beads between sockets and also from grid and anode drivers to the test sockets
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Old 2nd November 2012, 01:41 PM   #304
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Looking good indeed!

I would also suggest some plate current limiting resistors. Not sure what the screen scale is but it appears the 5881 is passing some serious current near the top traces. The slope of the 0v trace is starting to slope down at the end. You dont want to cook the tube. Or it could just be your plate supply is running out of current to give.

Afredo
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Old 3rd November 2012, 04:45 AM   #305
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Default Remaining Issues

Thanks, guys! Couldn't have gotten this far without your guidance... I am still struggling with the grid drive circuit, the leftmost curve that you see in fact is not quite at Vg=0, since the circuit breaks into oscillation as Vg approaches zero let alone going above zero.

Ale, on your schematic, you showed the IRF9630's as two source followers, is that what you are using? In my version, the second 9630 is driven from the drain of the first 9630 instead, otherwise I could not get the swing required.

Alfred, I do have plate limiting resistors like the Tektronix (and your schematic), I just did not turn it up far enough when the shot was taken.

Here is the latest gate drive circuit (taken from one of drawings from TheGimp), it sims fine, but does not work as is in my setup...
Click the image to open in full size.

TIA,
Jaz
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Old 3rd November 2012, 07:37 PM   #306
mogliaa is offline mogliaa  United Kingdom
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Hi Jaz,
Here is the driver. I think I ended up adding a drain resistor in the output mosfet to protect it in case of grid shorts. Other than that, this works brilliantly for me...
Attached Images
File Type: png Curve tracer-grid driver.png (113.8 KB, 151 views)
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Old 3rd November 2012, 10:43 PM   #307
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Quote:
Originally Posted by mogliaa View Post
Hi Jaz,
Here is the driver. I think I ended up adding a drain resistor in the output mosfet to protect it in case of grid shorts. Other than that, this works brilliantly for me...
Ale,

Thanks for posting the updated schematic, I will give it a try and report back, hope to have it sorted out soon, thanks again for your help.

Jaz
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Old 4th November 2012, 12:47 AM   #308
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Ale,

I tried your values just now, and the result was the same... Vg=0 gets distorted when the grid current begins to flow. No problem with the small signal triodes since they don't draw much grid current (at least the ones I tried).

Also, even with 15V on the source, it still does not pull above 0, so no way to trace Vg>0, I'm still not sure why that would be the case, as the sim showed that it should be able to pull above 0.

With about 250V on the plates, here are some shots of the grid drive:

With 5881 (see the bow on the Vg=0 step):
https://dl.dropbox.com/u/1326040/IMG_1669_sm.jpg

with 12AX7 (clean Vg=0):
https://dl.dropbox.com/u/1326040/IMG_1674_sm.jpg

Thanks,
Jaz

Last edited by jazbo8; 4th November 2012 at 12:58 AM.
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Old 4th November 2012, 01:33 AM   #309
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I do see that your drawing shows the grid signal being taken from the Drain. Ale shows his on the Source. It probably makes a difference.
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Old 4th November 2012, 02:23 AM   #310
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Quote:
Originally Posted by avincenty View Post
I do see that your drawing shows the grid signal being taken from the Drain. Ale shows his on the Source. It probably makes a difference.
It is taken from the source, I just flipped the FET's on the drawing. If I can get the LME49810 and an appropriate xfmr, I may give your version a try.

Thanks,
Jaz
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