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Old 5th November 2012, 12:44 PM   #321
TheGimp is offline TheGimp  United States
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This is where an adjustable current source in the drain of the driver FET might be useful. However, one would necessarily adjust it depending on the allowable grid current for each tube to be tested.

It would become another adjustment that if forgotten could result in a damage tube.
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Old 6th November 2012, 08:31 AM   #322
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Quote:
Originally Posted by TheGimp View Post
This is where an adjustable current source in the drain of the driver FET might be useful. However, one would necessarily adjust it depending on the allowable grid current for each tube to be tested.

It would become another adjustment that if forgotten could result in a damage tube.
More knobs!

Nothing new to report... but here are some shots I took today with and without power grid/RF interference, quite a big difference as you can see...

Click the image to open in full size.

Click the image to open in full size.

Jaz

Last edited by jazbo8; 6th November 2012 at 08:31 AM. Reason: typo
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Old 6th November 2012, 08:53 PM   #323
mogliaa is offline mogliaa  United Kingdom
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Jaz,
Second trace looks really good. What is the difference between the two, just interference?
Have you tried removing the grid stopper or using 10/100 ohms? Lower value will definitely help specially if you are using a ferrite bead in the grid cable just right at the socket pin. If you are careful enough not to overheat the grid you will be able to see a proper curve for 0V grid bias.
There is an interesting thread about mains filtering opened this week. Have a look.
I added a mains filter classic common choke with X2 caps before and after. It has actually helped when the wife uses the microwave
What is your sensing circuit? I'm using a differential op amp arrangement to provide x1/x10 and inversion as well to be flexible for future analogue scopes I may end up with. The sensing cable from the sensing resistor to this amp should be shielded to avoid interference. Think when you are plotting low anode current valves.
Have you tried plotting power valves? Different beast btw...
Ale
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Old 6th November 2012, 11:31 PM   #324
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Quote:
Originally Posted by mogliaa View Post
Jaz,
Second trace looks really good. What is the difference between the two, just interference?
Have you tried removing the grid stopper or using 10/100 ohms? Lower value will definitely help specially if you are using a ferrite bead in the grid cable just right at the socket pin. If you are careful enough not to overheat the grid you will be able to see a proper curve for 0V grid bias.
There is an interesting thread about mains filtering opened this week. Have a look.
I added a mains filter classic common choke with X2 caps before and after. It has actually helped when the wife uses the microwave
What is your sensing circuit? I'm using a differential op amp arrangement to provide x1/x10 and inversion as well to be flexible for future analogue scopes I may end up with. The sensing cable from the sensing resistor to this amp should be shielded to avoid interference. Think when you are plotting low anode current valves.
Have you tried plotting power valves? Different beast btw...
Ale
Ale,

Nothing is different other than the time of day - during business hours, I get crazy interference, basically rendering the tracer useless. I tried the line filter too, alas it's no use against such outrageous level of interference...

BTW, the valve shown is a 5881/6L6 type triode-strapped and the vertical scale is 50mA/div. My plate transformer is already running out of juice, so I can't go much beyond 300V at the moment. All the cable for the sense lines are shielded, and ferrite beads are place close to the pins of the individual tube sockets. The grid stop resistors are selectable 100, 1K or 10K, but I need to move them closer to the tube pins when I re-do the layout. When there is no interference, I can almost get away with no stopper, with the darn interference, even 10k can't do anything...

Below is my current sense amp based on the INA128, with x10 or x50 gain, the sense resistor is selectable 1R or 10R:
Click the image to open in full size.

Jaz

Last edited by jazbo8; 6th November 2012 at 11:34 PM.
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Old 7th November 2012, 01:46 PM   #325
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Have you tried sensing just with a resistor to the return of the plate drive like in the tek570? Is the noise being injected to the Plate drive or to the grid drive? In my case when i have noise is just the plate drive.

One item that i added to mine is a momentary switch that grounds the grid of the DUT to give me a reference of the zero curve and adjust the bias level to match the top curve of the family of curves with it, same as Tek570.
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Old 7th November 2012, 10:05 PM   #326
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Quote:
Originally Posted by avincenty View Post
Have you tried sensing just with a resistor to the return of the plate drive like in the tek570? Is the noise being injected to the Plate drive or to the grid drive? In my case when i have noise is just the plate drive.

One item that i added to mine is a momentary switch that grounds the grid of the DUT to give me a reference of the zero curve and adjust the bias level to match the top curve of the family of curves with it, same as Tek570.
I'm think it's coming from the plate drive as well, since the grid is very clean without the plate drive turned up. I think I tried the sense resistor on the return leg of the Plate Sweep earlier and it did not seem to make a difference, but many things have been changed since then, so I will take another look to see, thanks for the suggestion.

I think I will add the Zero Bias button, and the the Start Adjust pot as well, but as mentioned, I can't get the steps above zero - still more work to do. Do you have some shots of the grid drive with the positive steps when tracing curves? May be like:

Click the image to open in full size.

Jaz

Last edited by jazbo8; 7th November 2012 at 10:21 PM.
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Old 8th November 2012, 12:09 AM   #327
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Hi Jaz

I do have control in the positive area although I rarely use it. I use the bias level all the time to match against the zero curve.

I have three pictures, the zero reference, one step family below zero and one above it. I do have individual control of step size an steps per family. Vertical is at 20V/division.

I also recorded a short video so you can see the operation.


link to short video
Attached Images
File Type: jpg Curve Tracer Grid Drive Range.jpg (24.5 KB, 110 views)
File Type: jpg Curve Tracer Grid Drive Range-001.jpg (25.3 KB, 18 views)
File Type: jpg Curve Tracer Grid Drive Range-002.jpg (25.2 KB, 19 views)
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Old 8th November 2012, 05:09 AM   #328
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Quote:
Originally Posted by avincenty View Post
Hi Jaz

I do have control in the positive area although I rarely use it. I use the bias level all the time to match against the zero curve.

I have three pictures, the zero reference, one step family below zero and one above it. I do have individual control of step size an steps per family. Vertical is at 20V/division.

I also recorded a short video so you can see the operation.


link to short video
Awesome, that's exactly how it should be.

Jaz
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Old 9th November 2012, 07:21 PM   #329
mogliaa is offline mogliaa  United Kingdom
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Built a simple source follower to provide screen bias. Here is a sample of a 24a tetrode with its characteristic kink:


http://www.bartola.co.uk/valves/2012...etrode-curves/

Click the image to open in full size.

Cheers

Ale
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Old 11th November 2012, 11:06 PM   #330
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Did not make too much progress this weekend, I tried a few different things but in the end, still not quite there...

Alfredo - I tried using a simple unregulated supply for the Vd and Vs for the FETs, like the ones used for your LME49810, but I end up with large ripples on the grid steps, so that's no go (the actual ripple on the supply line was pretty low). With the bench supply, the steps are ruler-flat, given the relatively low current for the grid drive, I still do not understand why it would be the case.

I now have spend more than a week trying to get the grid driver amplifier to work properly, for such a simple circuit, it sure gave me a lot of grief. Anyway, back to the breadboard...

Jaz
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