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Old 27th January 2012, 05:56 PM   #31
mogliaa is offline mogliaa  United Kingdom
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Attached is what I'm using as the grid driver. I feed the output of the CD4520 counter resistor ladder on to the op amp. With this circuit you can easily generate all ramps required starting from 1V steps. Is handy enough and reduce complexity of the overall circuit.....

Still experimenting
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Old 27th January 2012, 05:57 PM   #32
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Quote:
Originally Posted by avincenty View Post
Still no adjustments to the volt/step. Steps are around 4.5v/step. First one at 0v. I think that the IC works fine, it has a large voltage swing for power tubes and can source some current for the positive grid region.

Now I need to fine tune things (bias method as well) and build it. By the way, the step generator on this one is a little different. Based on your recommendations I tried the 4 bit ripple counter. I still used a 240hz clock that I derived from the mains frequency (using PLL and divider). I can still use a comparator to reset the counter and control the steps per family. I used a AD7528 8 bit dac I had laying around, works just fine. I will play a little more with it as it has two outputs that can be enabled separately. Be changing the connection of the 4 bits to the DA I can change the output voltage of the ladder. So I can basically have an enable toggle that doubles that selects from either DAC output. One output could be 1/2 of the other.


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Nice ones!, is looking really good
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Old 28th January 2012, 11:15 PM   #33
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Quote:
Originally Posted by jackinnj View Post
I used a variant of this supply from "The Art of Electronics" for my tube tracer article in AX about 9 years ago
Hi Jackinnj

Is your article available somewhere for download?

On your drawing, are you talking about biasing Mosfet itself or about shifting the ramp up or down within a range (with no amplification).


Alfredo
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Old 28th January 2012, 11:21 PM   #34
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Quote:
Originally Posted by mogliaa View Post
Attached is what I'm using as the grid driver. I feed the output of the CD4520 counter resistor ladder on to the op amp. With this circuit you can easily generate all ramps required starting from 1V steps. Is handy enough and reduce complexity of the overall circuit.....

Still experimenting
I think i need something similar to this, but need to play with the LME49810 rails.

I think the ground connection needs to be virtual otherwise I would loose the benefit of the full voltage swing.

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Old 29th January 2012, 03:05 AM   #35
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Quote:
Originally Posted by avincenty View Post
Hi Jackinnj
Is your article available somewhere for download?
On your drawing, are you talking about biasing Mosfet itself or about shifting the ramp up or down within a range (with no amplification).
Alfredo
1) I don't think that they have PDF's of the article -- it's ancient history at this point and I know a lot of things I would do differently.

2) The steps were generated by a 10 bit Maxim DAC and I just used a bit-banging RS-232 program to generate the steps -- quite slow back then. There several ways to bias the first MOSFET -- but you can just get a DAC with an outboard reference provision.

It's nice to see the output on a computer screen, or CRT -- what made the program more useful was that the data was downloadable to Excel. Today there are a number of programs to take the data and create the SPICE parameters.
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Old 30th January 2012, 12:33 AM   #36
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On a Tek curve tracer, the cathode of the DUT (Device Under Test) is always grounded. The current sense resistor is in the ground leg of the plate supply. Current waveforms are inverted, but that is easy to fix. Also, the grid drive is referenced to ground and that is handy.

For testing pentodes the screen supply also returns to ground via a separate current sense resistor. (not 100% sure on this)

I don't recall if the 570 does it, but on the 576 you can select 2x the grid step rate so the grid drive changes at the peak of the plate supply waveform. This reduces the flashing on the display. The 576 is limited to 10 steps of grid drive. On the 570, the right/left switch used for matching DUTs connects the step generator to one device, both devices get a large negative voltage to bias them off when they are not selected. On the 576, the switch disconnects both base and collector (grid and plate) supplys from the DUT.

On the 576 there is a selectable voltage limit imposed on the base current drive so that it doesn't go to the rail if the specified current does not flow. This is especially important since you can dial in an offset to base current or voltage drive.

The first thing you do when testing a tube on the 570 is adjust the heater voltage. That's what the large meter on the front is for. You can also watch what the effect of changing heater voltage has on the curves. Think those power tubes are matched? Make sure they get exactly the same heater voltage!
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Old 30th January 2012, 01:25 AM   #37
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I am actually following a few of the 570 concepts. I am sensing current as described above. Also use the 2x by using the 240hz rate, hence the PLL. I am also using the same phase matching between the grid and plate supply, using a .05uf cap and a 250k pot, works really well to clean up the xy display. Also i am planing to use the same plate series resistors as the 570. The series resistor is what makes the curves end in the diagonal seen on my pictures.
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Old 30th January 2012, 01:24 PM   #38
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Old 30th January 2012, 02:11 PM   #39
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Nice!

I was able to play with the negative bias configuration. Very similar to post 31. I had to float the PS of the LME49810 and inject the negative bias to the common of the floating supply. I had to remove the feedback cap on the chip so that it also amplifies dc. This way with a small dc shift of the input signal I can move the output easily. I did this test with my signal generators dc offset function. For the real circuit I would need to change my buffer staircase op amp at the output of the DAC from a single supply to a dual supply one so I can give it some negative offset.
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Old 1st February 2012, 07:13 AM   #40
mogliaa is offline mogliaa  United Kingdom
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Hi can you post your PLL circuit? I'm keen to see whether I can add a simple stage to increase clock above 100Hz and improve refresh cycle.
Thanks
Ale
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