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Old 17th January 2012, 01:50 AM   #21
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I did not use the dc coupling capacitor. I will probably post the results of the class A2 test as a new topic.

Alfredo
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Old 17th January 2012, 02:10 AM   #22
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Quote:
Originally Posted by SpreadSpectrum View Post
It will be interesting to see how much current that thing can source and how much distortion results.

I have been considering a curve tracer but would make it microcontroller based(I want to record curves to import to excel) so I am interested in how this high-voltage chipamp performs.
The first tube curve tracer I built was based upon a Basic Stamp -- and I reconfigured a Heath HV supply with a 10-bit DAC to drive the grid voltages etc., etc. The Stamp is quite easy to program, but data exchange to Excel via the serial port was pretty slow. A few years ago easily configurable USB interface became available.

A diff amp can be easily configured as a precision current source -- see the Linear Tech data sheet for the LT1990. With a resistor, the current source becomes a voltage source, although a buffer might be required....
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Old 17th January 2012, 06:51 AM   #23
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Moderator, can you fix my post from yesterday at 8:56PM

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thanks

alfredo
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Old 21st January 2012, 12:47 PM   #24
mogliaa is offline mogliaa  United Kingdom
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With help (and a lot of patience) from Steven Parfitt (TheGimp) the belated project of my curve tracer is coming along. Still a breadboard setup but works.
Clock is derived from rectified mains (clock of 100Hz in UK) and 8 curves are plotted. Refresh is acceptable, but not perfect. However, the circuit is much simpler this way.

Attached is a sample test of a 45 valve

Will post more details later...
Cheers,
Ale
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File Type: jpg curve tracer test1.jpg (71.4 KB, 753 views)
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Old 22nd January 2012, 07:41 PM   #25
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Looking good Mogliaa

I hooked the LM49810 to he step generator today. I am only using the bench supply with +-40V. The picture below is 60Vpp. Now I need to work on injecting bias to it and having control of the volts per step. In the picture, there is 30V above ground and 30V below ground. The volt per steps are at 5V. For the bias, I am thinking about floating LME49810 power supply and injecting a variable negative voltage to the transformer center tap. Is there an easier way of doing it without affecting the grid of the tube under test?

For the volts per steps i need to pre amplify the step to get double what I am getting now, this will give me up to 10V per step. I have a spare op amp in the cuircuit. This will also invert the steps.

I am still using the same control of steps per family as before so I can go to a very large number of steps per family. The problem is the refresh rate. But.. I gave myself a Christmas present, a 100mhz DSO. Shouldn't have problems capturing the data to a the PC.

More later...

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Old 22nd January 2012, 08:52 PM   #26
mogliaa is offline mogliaa  United Kingdom
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Originally my plan was to produce a negative and positive grid driver to enable testing of power tubes with positive grid current. TheGimp helped me out in designing the driver with op amps, MOSFET stages and feedback loop. I gave up when trying to stabilise the circuit so ended up with a simplified version with op amp to set bias and a couple of P MOSFETS.
The staircase is a CD4520 counter from the mains clock as per Merlin's design.
Anode sweep is also mains rectified and a MOSFET in source follower driving the valve. Have a pair of 230/24V back to back transformers which after rectification give about 350V max sweep.

At the moment the blurred lines or double lines on the curves are due to the breadboard noise and long cables. My current sensing resistor is 10 ohms which then provides the Y signal for amplification via a differential amplifier and inverter as per Goote's design above.

Need to etch the PCB and see if gets ok in the end, with a sharper set of curves once everything is tightly wired and shielded.....

Cheers,
Ale
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Old 22nd January 2012, 11:21 PM   #27
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mogliaa, how to you wire the op amp to setup the bias? High much bias is possible like this?


I added a few pots to the board. Ended up having to use a preamp and a pot to control the gain of the LME4910. I am able to get volts/step from .5V up to 10V.

I am hooking up a tube next.


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Old 27th January 2012, 08:23 AM   #28
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Did a test with a 1619 trioded. I dont have a bias circuit yet so basically the lme49810 is putting out a symmetrical step centered around 0V. In the picture below the volts per step are not calibrated,they are around 10 volts. The third curve is very close to the zero volt step.

The tube is going to around +15v on the grid, pulling 10ma from the lme49810.

I tried biasing the grid steps by using a blocking cap and a resistor, it works but it wont source current on the positive grid steps. When i connect the grid directly to the output of the chip it does feed current to the grid. I will try to float the chip and inject the bias on its ground pin. Hope it works.


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Old 27th January 2012, 04:16 PM   #29
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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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Old 27th January 2012, 05:29 PM   #30
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I used a variant of this supply from "The Art of Electronics" for my tube tracer article in AX about 9 years ago -- but I explicitly set the bias for the first MOSFET -- if you work with the loop compensation you can get a very linear HV supply which will swing 400 or 500 volts. I used a 10 bit DAC for the ramp generator:
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