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Old 7th October 2012, 04:25 PM   #11
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For a tube - here is the only gm decay curve I can find - I've posted it only to show the shape of the curve.

For a 6sn7, the life expectancy would be more in the order of 10,000 hours.
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Old 7th October 2012, 05:14 PM   #12
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Uunderhill View Post
.
However, here is an idea that suddenly dawned on me after looking at a datasheet
Build the class A reference circuit as shown in the datasheet - apply a signal generator - and look at the output on a scope.
Then check the gain of a new reference tube versus the tube that is being tested.
http://www.radiostation.ru/tubes/6SN7.pdf

Would this work ?
.
Yes. Your idea is better. In fact testing the tube in the intended circuit is the best way to test a tube. In fact even if yo own a "real" tester this way is still the best.

But even with the above you may still reject some tubes that would work in some circuits. Tubes inside a local feedback loop or cathode followers can work even if they test poorly in the class A amplifier. In fact I've swapped 12AX7 for 12AT7 and don't see any difference in performance in cases there the tube is on a local NFB loop. That said, most of the time the tube, or at least one section of it is NOT in such a circuit.

As for balance in applications like phase splitters. If you care a lot, design in a "balance" adjustment.
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Old 8th October 2012, 03:07 PM   #13
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Originally Posted by ChrisA View Post
Yes, testing the tube in the intended circuit is the best way to test a tube. In fact even if you own a "real" tester this way is still the best.

But even with the above you may still reject some tubes that would work in some circuits. Tubes inside a local feedback loop or cathode followers can work even if they test poorly in the class A amplifier. In fact I've swapped 12AX7 for 12AT7 and don't see any difference in performance in cases there the tube is on a local NFB loop. That said, most of the time the tube, or at least one section of it is NOT in such a circuit.
Thanks for the guidance.
For a 6sn7, here is the class A resistance coupled amp as shown in the data sheet.

I'll need to give things a bit more thought before asking anymore questions.
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Last edited by Uunderhill; 8th October 2012 at 03:09 PM.
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Old 8th October 2012, 04:37 PM   #14
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Uunderhill View Post
For a tube - here is the only gm decay curve I can find - I've posted it only to show the shape of the curve.

For a 6sn7, the life expectancy would be more in the order of 10,000 hours.
The practical problem with tube testing for life expectancy is this:

Lets agree the when you are at 80% the tube is "on it's way out". But the 80% figure starts from whatever the tube measured when it was new. Do you know what it measured new? Tubes can vary by 20% out of the box. So you might have a brand new tube that measures 80%

So the only way to use this curve is to start a measurement program where you test each tube when new and then again every few hundred hours and look at the SHAPE of the curve and use relative numbers

In case I'm not clear. Lets say you want to measure a block of ice to see what percent of the ice has melted. So it weighs 3 pounds. Now you tell me what fraction of the ice is gone. Hard to do without knowing the starting point

One more thing. Your class-A test circuit is very sensitive test. But some circuits in amps are not. For example unity gain cathode follower might show zero change as the tube ages until one day it just stops working. This is why tube-swapping sometimes fixes a problem. A nearly dead tube would work fine in that spot. So what is the definition of "lifetime"? How many hours it will work in a class A common cathode amp or how many hours it will work in your specific use case?

Your best bet if you are worried about tubes is you buy a new tube and verify it works. Then keep that as your "standard" and if ever you suspect a tube of that type has failed swap it out for your standard and see if that fixes it.

These preamp tubes will last for years and tears, decades maybe.

If you want to make them last even longer then control the heater voltage. I typically place a big sand stone power resister in series with the heaters to bring the voltage down to about 5% under 6.3 volts, or about 6.0 volts. Doing this dramatically lengthens the life. the specs say you can go even lower but I leave room for low AC mains voltage. May tests show the tubes work fine at 5.5 volts. THey tend to be a little bit over engineered. the 6.0V setting is conservative and works. I use something like a 0.1 to 0.68 ohm resister. It does not take much to drop 0.3 volts
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Old 9th October 2012, 11:49 PM   #15
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Originally Posted by ChrisA View Post
If you want to make them last even longer then control the heater voltage. I typically place a big sand stone power resister in series with the heaters to bring the voltage down to about 5% under 6.3 volts, or about 6.0 volts. Doing this dramatically lengthens the life. the specs say you can go even lower but I leave room for low AC mains voltage. May tests show the tubes work fine at 5.5 volts. THey tend to be a little bit over engineered. the 6.0V setting is conservative and works. I use something like a 0.1 to 0.68 ohm resister. It does not take much to drop 0.3 volts
Yes - I was going to mention this, but was going to discuss one item at a time.

The current draw for a 6dj8 is listed at 385 mA.
When I was tinkering with a 6dj8, using a voltage regulated supply for the heater,
When first switched on, the current jumped up past 700mA for the first second - yikes !
People have discussed current regulation for the heater elements,
but converting existing equipment is not practical.

However, for voltage regulated heaters, I'm wondering if a thermistor should be connected in series with the heater elements.
Maybe something like a CL-30 thermistor ?
.
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Old 10th October 2012, 10:05 AM   #16
DF96 is offline DF96  England
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Higher heater current at switch-on is to be expected. The valve is designed to cope with this. A series thermistor is only needed for long series heater chain (e.g. in AC/DC radios and TVs), so that the smaller valves don't overheat while the large valves are still warming up. To reduce the problem, some valves were made with heaters which warmed up at a known rate - sometimes indicated by an A suffix on the valve name. This problem does not arise with parallel voltage fed heaters.
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Old 10th October 2012, 05:57 PM   #17
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Uunderhill View Post
...
However, for voltage regulated heaters, I'm wondering if a thermistor should be connected in series with the heater elements.
Maybe something like a CL-30 thermistor ?
.
Don't worry about it. All heaters have low resistance when cold draw more power then use less current when hot. The resistance depends on the temperature of the heater element. Light bulbs are the same way.

Why is it safe? Because it is self limiting, the heater is never over heated by the high current because just before it starts to over-heat the current goes down. It is a kind of feedback temperature regulation. The hotter the tube heater gets the less current it draws. So you get a current regulated system with just a transformer and nothing more.

What I typically do is place a 0.1 to 0.68 ohm resister in series with the transformer. Many transforms have to high of a heater voltage when used on 120VAC mains. I like to drop it to 6.0 volts with 120V input You can calculate using ohms law but that never works partly because resisters only come in a few standard values and the effect is not linear (tube heaters do not follow ohms law) I just try a few resisters, then wait 10 minutes and measure voltage

The series resisters do hold back the turn-on serge a little. But as I said this hardly matters

Your thermister idea could only work if you found a NTG thermister with the exact required specs, I doubt you'd find one. It would need very fast time constant and the EXACT resistance when hot (a 0.1 ohm error matters) And as I said above, this is not a problem you need to fix.

Back to tube testing: The only reason to have a tube tester is i you are in the business of selling used tubes and you want to assure your customers the tube is not dead. For testing your own tubes, a simple test circuit is easy to rig, you just place it in the amplifier. Back in the "day" in the 1950's the real use of tube testers was to sell tubes. You would show the customer that the tube tests "low" and then sell her (Almost always female) a new tube. Not that the low tube was really all that bad. The repair techs would simply swap in a known good tube and not bother with the meter.

Last edited by ChrisA; 10th October 2012 at 06:08 PM.
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Old 11th October 2012, 04:41 PM   #18
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Just curious, what about "balancing" pairs of tubes, how is that done?
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Old 14th October 2012, 07:41 PM   #19
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I would like to have some method of measuring the state of a vintage twin triode.

Referring back to the Class A resistance coupled amp from the data sheet,
this is a common cathode amp with the cathode resistor bypassed.

I think the equation for Av is correct. Certainly, from an AC point of view, Rp and Rs are in parallel.

However, the problem from measuring gm from this circuit, is the wide range in a tube's plate resistance.

For example, my RCA data book says for a 6sn7, rp min and max is 6.7K to 7.7K ohms.

Unfortunately, rp dominates the other resistor values.

How about using a new Sovtek tube in this circuit and then comparing it to the vintage tube in question ?
.
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Last edited by Uunderhill; 14th October 2012 at 07:49 PM.
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Old 14th October 2012, 08:57 PM   #20
DF96 is offline DF96  England
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Quote:
Originally Posted by Uunderhill
However, the problem from measuring gm from this circuit, is the wide range in a tube's plate resistance.
gm and plate resistance are related: Rp = mu / gm

Measure any two of these and you can calculate the third. You could measure Rp by varying Rs in your circuit and seeing how gain varies. Then, knowing Rp, you can calculate gm from your results.
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