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Old 20th April 2011, 08:26 PM   #21
mogliaa is offline mogliaa  United Kingdom
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Originally Posted by TheGimp View Post
Another set of plots from last night (my computer at home won't boot so I have to wait till now to post).

I installed a transformer for g2 with the return taken from the cathode instead of ground so the screen current is no longer going through the cathode sense resistor.

The initial plots look much better but still off a good bit from the data sheet. The knee is still wrong. I had previously wondered if the PIC can keep up with the dv/dt of the ac waveform used for anode drive. In retrospect I would expect different distortion if that were the case.

With 60hz, 400Vpeak, the dv/dt is 6.6V/us at zero crossing. If the sample rate were too slow around zero cross, I would expect compression of the waveform with a steeper onset to the knee and that is not what is happening.

In the zip is one file that is really screwy. The 6P15P just went weird with alternating plots no matter what I did with the G2 voltage.

Right now the G2 transformer is on the bench. I need to find one with a higher output voltage to mount inside.

Next up will be changing the cathode resistor from 10R to 1R to minimize cathode feedback, and increase the gain of the op amp ckt to compensate.
Hi Steven,
Back Home from my trip. Did you get my email?
I did test gu50, 6w6, el34 and other valves in pentode mode with a stabilised DC screen supply with return to ground, not cathode and got quite decent curves with the expected distortion at low voltages.
See below:

My Blog


Cheers,
Ale
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Old 26th April 2011, 03:04 AM   #22
TheGimp is offline TheGimp  United States
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I didn't get your message, and would have missed this except I have about gone as far on my lampmetre as I can and I was going to return to this thread to post my progress on my design to keep from hijacking the "Valve Testing machine" thread.

I've added the isolated G2 supply with a return from cathode to keep G2 current from the cathode sense resistor for measuring anode current. I've rebuilt the G2 supply to stiffen it. Plots look pretty good, but there are features of the system that I don't like and am going to try to address.

The GU-50 plot looks good.

By the way, I found that if one selects 3V as the max grid voltage one gets 0.5V steps which are nice for small signal tubes.
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Old 26th April 2011, 05:17 PM   #23
mogliaa is offline mogliaa  United Kingdom
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Originally Posted by TheGimp View Post
I didn't get your message, and would have missed this except I have about gone as far on my lampmetre as I can and I was going to return to this thread to post my progress on my design to keep from hijacking the "Valve Testing machine" thread.

I've added the isolated G2 supply with a return from cathode to keep G2 current from the cathode sense resistor for measuring anode current. I've rebuilt the G2 supply to stiffen it. Plots look pretty good, but there are features of the system that I don't like and am going to try to address.

The GU-50 plot looks good.

By the way, I found that if one selects 3V as the max grid voltage one gets 0.5V steps which are nice for small signal tubes.
Ok, I will resend the email. Yes I did find the same change in voltage step at low grid settings. I suggested many changed to Alain around the SW functionality.
Alain told me that a friend of his has developed the SW so needed to check with him now on May when is back from travel

Cheers,
Ale
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