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Old 9th April 2012, 12:12 AM   #161
TheGimp is offline TheGimp  United States
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Most data sheets call out the maximum static voltage. When driving an inductive load, the voltage swing will double. So you are below the theoretical 2*250V =500V maximum swing voltage.
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Old 9th April 2012, 12:48 AM   #162
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Thanks Gimp.

What is curious is that the datasheets for the PL36 show pentode curves that stop at 250 or less. I guess since the tube was not intended for audio that there was no need to extend the plots.

Alfredo
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Old 9th April 2012, 05:11 PM   #163
mogliaa is offline mogliaa  United Kingdom
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Default Tracer first test

After a lot of work (more than I thought it was going to take), managed to bring the valve curve tracer to life.

Never thought I'd ended up wiring and soldering so many cables! Anyway, it gave me some headaches with the phase adjusting RC stage. It worked brilliantly on the breadboard, but now it seems not to be sufficient level of correction. I used same Pot=100K (10T) and C=50nF. When I drive down (or up?) the pot all the way through can't move the retracing down to the bottom of the curves, so probably need to increase R but adding a series resistor or perhaps reducing the capacitance? I guess need to try and see what happens...

Other minor hiccup was the neon lamp. It's a classic russian IN-1, and wired to the +80V with a 30K resistor I think to enable 0.5mA for an operating voltage of 65V as read on the specs. It worked for some time but then it went dead. Any idea what can be?

Well, couldn't resist the afternoon and had some red wine whilst playing with the tester....

Cheers,
Ale
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File Type: jpg IMG_2601.jpg (102.7 KB, 200 views)
File Type: jpg IMG_2602.jpg (97.3 KB, 200 views)
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Old 9th April 2012, 05:25 PM   #164
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It's a beauty! Congratulations.

The wiring and the soldering was also more than I expected! I need to buy new meters that fit without doing more modification work. Currently I am trying to play with "shunt, schade, parallel" feedback. Not really sure what the best setup is. I will post some pictures later.

Once everything is complete I will post the as-built circuit.

Alfredo
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Old 9th April 2012, 08:14 PM   #165
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Default Tracing parallel feedback curves

I want to properly trace parallel feedback curves but I am not sure if what I am doing is correct. Can somebody validate that my setup is acceptable? It appears I am getting good curves.

Thanks

Alfredo
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Old 9th April 2012, 09:50 PM   #166
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Quote:
Originally Posted by avincenty View Post
I want to properly trace parallel feedback curves but I am not sure if what I am doing is correct. Can somebody validate that my setup is acceptable? It appears I am getting good curves.

Thanks

Alfredo
I can't explain the big tetrode-triode transition. That's not the way I think about it's in-circuit behavior.

It would help if you labeled the resistor values for each case and voltages on the plot.

The first thing I wonder about is the series load resistor. What is the purpose of this, the value, and where are you measuring plate voltage?

Are you quoting the voltage divider ratio as the feedback %?

Also there won't be a curve with grid voltage starting at zero because the feedback will pull the grid positive from there. The leftmost curve would have grid voltage =0 at it's top end

Have you seen the paper by O.H. Schade with his equivalent curves? They actually do have a little of the tetrode-triode switchover on the left end but not so soft on the knees at the bottoms... Maybe you're going more into the +ve grid region for more of the curves?
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Last edited by Michael Koster; 9th April 2012 at 10:08 PM.
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Old 9th April 2012, 10:50 PM   #167
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Hi Michael,

The load resistor is just for current limiting the plate. It sets the slope of the tips in the plots. In this case it is around 1.75K.

Yes, I am quoting the divider ratio as percent feedback. The values of the pot are (grid fixed at 10k)

2.4% 400K
3.2% 300K
9%. 100K
17%. 50K

All traces have a x=50 v/div. y=20 ma/div

Leftmost curve: yes you are correct the zero curve gets squashed by the positive grid current. I moved the bias down to eliminate it.

I have some 6l6, I will hook one up to see if I get similar curves to the schade ones.
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Old 10th April 2012, 01:02 AM   #168
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Default 6l6GC with Parallel feedback

I traced some curves for a 6L6GC. For this one I changed the grid resistor to 100K to achieve the divider ratio easier.

I ran the screen at 250V. Can't determine what voltage Schade used, may have been 300V.

My tracer can only do 10V max between steps, Schade used 20V per step.

Overall, I think my traces are not that far off from the Schade ones.

I did not play with the negative bias this time, so the cutoff is visible. Last picture has a higher series resistor that chops the cutoff visibility off.

The pot resistor values are:

10% : 900K
20% : 400K
30%: 230K

Last edited by avincenty; 10th April 2012 at 01:12 AM. Reason: added divider values
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Old 10th April 2012, 02:17 PM   #169
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Quote:
Originally Posted by avincenty View Post
I traced some curves for a 6L6GC. For this one I changed the grid resistor to 100K to achieve the divider ratio easier.

I ran the screen at 250V. Can't determine what voltage Schade used, may have been 300V.

My tracer can only do 10V max between steps, Schade used 20V per step.

Overall, I think my traces are not that far off from the Schade ones.

I did not play with the negative bias this time, so the cutoff is visible. Last picture has a higher series resistor that chops the cutoff visibility off.

The pot resistor values are:

10% : 900K
20% : 400K
30%: 230K
Yes, you are getting the same result as Schade, and it's working the way it does in circuit. You could draw a load line.

You've built a nice instrument, congratulations!

Michael
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Old 10th April 2012, 02:28 PM   #170
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by mogliaa View Post
After a lot of work (more than I thought it was going to take), managed to bring the valve curve tracer to life.

Never thought I'd ended up wiring and soldering so many cables! Anyway, it gave me some headaches with the phase adjusting RC stage. It worked brilliantly on the breadboard, but now it seems not to be sufficient level of correction. I used same Pot=100K (10T) and C=50nF. When I drive down (or up?) the pot all the way through can't move the retracing down to the bottom of the curves, so probably need to increase R but adding a series resistor or perhaps reducing the capacitance? I guess need to try and see what happens...

Other minor hiccup was the neon lamp. It's a classic russian IN-1, and wired to the +80V with a 30K resistor I think to enable 0.5mA for an operating voltage of 65V as read on the specs. It worked for some time but then it went dead. Any idea what can be?

Well, couldn't resist the afternoon and had some red wine whilst playing with the tester....

Cheers,
Ale
If greater phase correction is needed, shall I try adding an RC stage before the 100k and 50nF Phase correction circuit?

Any ideas?
Thanks
Ale
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