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Old 3rd Jan 2020, 2:28 pm   #81
G0HZU_JMR
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Default Re: "Burning - in" Double Triodes

Thinking about this a bit more, the grid current makes sense because the cathode is emitting clouds of electrons and some will hit the grid and this will create the negative voltage here with a 1Meg grid resistor. It should be affected by changes in anode voltage (and current) but not significantly. Maybe I should have done my testing with a 100k grid resistor rather than 1Meg as this would hold the valve closer to a true Vg=0V.


I could try testing the other half of the valve but I suspect I'm going to be exploring differences in the build quality of Chinese 12AU7 valves rather than revealing anything useful about the 'burn in' characteristics of a classic 12AU7 valve.

I'm tempted to grab one of the 12AU7s out of my old Tek 585 scope to see what the grid leak current is like and also the anode current in this test. This scope is older than me and I used it a lot in the 1980s and it is probably all original in terms of the valves. The valves in this scope will have seen a lot of use over several decades so I think it is all nicely 'run in' by now
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Old 3rd Jan 2020, 2:36 pm   #82
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Default Re: "Burning - in" Double Triodes

Quote:
Originally Posted by G0HZU_JMR View Post
Thinking about this a bit more, the grid current makes sense because the cathode is emitting clouds of electrons and some will hit the grid and this will create the negative voltage here with a 1Meg grid resistor ...
This is true. A quick and easy way of checking whether a valve has significant air in it (enough to ruin cathode emission but not enough to whiten the getter) or if something else has knobbled the cathode is to power the heater only and see whether the control grid goes a fraction of a volt negative relative to the cathode. If it does then the air content is probably fine. If it doesn't then the valve is probably dead.

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Old 3rd Jan 2020, 2:49 pm   #83
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Default Re: "Burning - in" Double Triodes

Grid current in graph form:

http://www.r-type.org/articles/art-171.htm

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Old 3rd Jan 2020, 4:39 pm   #84
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Default Re: "Burning - in" Double Triodes

Quote:
A quick and easy way of checking whether a valve has significant air in it (enough to ruin cathode emission but not enough to whiten the getter) or if something else has knobbled the cathode is to power the heater only and see whether the control grid goes a fraction of a volt negative relative to the cathode.
I tried this and logged it from a cold start after allowing a long time for the valve to cool to room temperature. See the graph below of GridV vs time in seconds. I'm not sure why there is noise on the result, maybe there is mains hum pickup on the test leads. However, it shows how the grid voltage varies over about 35 seconds after the heaters starts up with no HT voltage at the anode and a 1Meg grid resistor.


Quote:
Grid current in graph form:

http://www.r-type.org/articles/art-171.htm
Thanks! Some very useful info there.

I tried a few quick tests of anode voltage and current vs grid voltage with a 1Meg resistor at the grid. I tried using my boost converter PSU and it actually managed to deliver about 24mA at 150V. Above this the little PSU got quite inefficient and I was worried it might fail. The datasheet doesn't go much higher here anyway as the Pdiss in the valve gets quite high. I only let it run at 150V and 24mA for a couple of seconds.


AnodeV Anode mA Grid V

7.5 0.09 -0.53
10 0.17 -0.518
15 0.45 -0.491
20 0.87 -0.469
30 1.93 -0.437
40 3.23 -0.413
50 4.58 -0.403
60 6.11 -0.385
70 7.66 -0.37
80 9.37 -0.365
100 13.1 -0.358
120 17.2 -0.331
150 24.2 -0.33
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Old 4th Jan 2020, 7:18 pm   #85
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Default Re: "Burning - in" Double Triodes

I did some more tests yesterday. I tested the 2MHz crystal oscillator circuit and powered the circuit numerous times over the space of an hour or so. At the end of this I retested the anode current at Vg=0V and 60V HT and it had dropped from 6.13mA to 6.07mA.

Not much of a change but then I soak tested the Vg=0V test for about 5 hours and if anything the current crept up to 6.08mA at the end of this test. So there was no drift at all for 5 hours.

So maybe the oscillator circuit causes accelerated ageing. The anode current will be switching on and off like a 2MHz (pulsed) electron gun when the crystal oscillator is running because the large sinewave at the grid will be turning the valve fully on at the peaks of the grid sinewave and it will be off for a fair chunk of the negative half of the sinewave. The grid current will vary across an RF cycle and the anode current will be gated on and off at a 2MHz rate.

I'm surprised to see this much drift vs time in the Vg=0V current but then I've never tested a valve like this before. I wonder how much less it would have drifted if I had not run it as an oscillator and had just left it running a couple of days at 6.7mA anode current at Vg and Vk=0V and 60V HT?
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Old 5th Jan 2020, 6:46 am   #86
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Default Re: "Burning - in" Double Triodes

I guess that you have been working with just one half of the valve so far? It might be interesting to take a look at the other half at some time and see how it compares, especially with respect to "burn-in".

I wonder if the modern Chinese valves might be made in cleaner conditions than were in use back in the 50-60's, and that alone could improve the consistency of the product?

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Old 5th Jan 2020, 1:19 pm   #87
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Default Re: "Burning - in" Double Triodes

Quote:
Originally Posted by Bazz4CQJ View Post
... I wonder if the modern Chinese valves might be made in cleaner conditions than were in use back in the 50-60's, and that alone could improve the consistency of the product?
I'm afraid the few pictures of the insides of Chinese and Russian plants that I've seen suggest not. Of course these plants have been running since the 50's-60's (sometimes earlier) so they've worked out by now what matters and what doesn't. I was surprised the first time I saw the films of the Mullard Blackburn plant that none of the valve assembly women was wearing gloves or face masks. I guess they knew that the metal innards would be RF heated to orange hot during the pump-out so a bit of finger grease wasn't going to be left behind. And maybe they only held the glass parts by the outside ? That just leaves the micas.

Cheers,

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Old 5th Jan 2020, 3:12 pm   #88
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Default Re: "Burning - in" Double Triodes

Quote:
I guess that you have been working with just one half of the valve so far? It might be interesting to take a look at the other half at some time and see how it compares, especially with respect to "burn-in".
Unfortunately, I goofed a bit here because I have always run the other half with the 6.3V heater voltage and a grounded cathode and a 1Meg grid resistor... but no HT. This is how it is configured in the Marconi TF144H sig gen when the calibrator is not in use.

So I'm not sure what the impact of that will be in terms of ageing. However, I connected 60.0V HT to this half about 15 minutes ago and the anode current was 6.10mA. This wavered up and down by 0.1mA for the first few minutes but is now stable at 6.10mA.

I also tried measuring the sensitivity to the 6.3V DC heater voltage.

6.2V DC 6.086mA
6.3V DC 6.112mA
6.4V DC 6.138mA

I just rechecked and the current is down to 6.088mA at a 6.3V heater. I'll try and leave it running for a few hours today.
The grid voltage was -0.397V with the 1Meg grid resistor to ground.

I did wonder if a couple of 12AU7 valves could be configured to emulate the Wenzel (solid state) pantry AM transmitter. I'm not sure how well it would work. Ultimately, I think I'm going to try making a guitar preamp to see what it sounds like. I do have a couple of old electric guitars and a practice amp although I haven't played them for years.
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Last edited by G0HZU_JMR; 5th Jan 2020 at 3:26 pm.
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Old 8th Jan 2020, 7:02 pm   #89
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Default Re: "Burning - in" Double Triodes

I ended up running this side of the valve for three very long sessions each day/evening. So it has now had about another 20 hours' run time. I found that it settled at 6.15mA within an hour or so and it has remained very close to 6.15mA ever since.

So it seems very stable. This side of the valve has never been run as a 2MHz oscillator and I suppose I could try configuring it as an oscillator to see if the ageing rate accelerates. But this won't be for a few days as I'm back at work now and I have several work related jobs to look at over the next few days.
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Old 15th Jan 2020, 12:44 pm   #90
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Default Re: "Burning - in" Double Triodes

I suppose if it's that stable after an hour and remains so for 20 hours, there might not be much more change. If that's right, the vacuum is probably quite good and the initial burn in at the factory was long enough to activate the cathode fully.
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