I bought a bunch of these to use as CCS.
Without prior screening for matches, I took 2 units and got it connected from a 270VDC,
1Kohm grid stopper and 1Kohm trimpot (2 legs shorted together) from G-K,
fed it to a Sovtek 6922.
The 6922 Cathode is Green LED wired to ground, developing about 2V on both sides of the cathode.
I tuned the trimpot so that anode voltage on both sides are 100VDC output, and found that the trimpot got pretty warm to touch after 5 mins. I guess for long term reliability I'd better replace it with a permanent resistor.
Now, once powered down, I measured the resistance of both the trimpot and got 272 ohms and 239 ohms respectively.
Is that how badly matched the 10M45S can be within a batch of purchase?
Without prior screening for matches, I took 2 units and got it connected from a 270VDC,
1Kohm grid stopper and 1Kohm trimpot (2 legs shorted together) from G-K,
fed it to a Sovtek 6922.
The 6922 Cathode is Green LED wired to ground, developing about 2V on both sides of the cathode.
I tuned the trimpot so that anode voltage on both sides are 100VDC output, and found that the trimpot got pretty warm to touch after 5 mins. I guess for long term reliability I'd better replace it with a permanent resistor.
Now, once powered down, I measured the resistance of both the trimpot and got 272 ohms and 239 ohms respectively.
Is that how badly matched the 10M45S can be within a batch of purchase?
Just out of curiosity (since you're blaming the semiconductors): the two halves of your tube are perfectly matched ?
That's a valid point and I haven't checked; just that my Sovtek 6922 are hardly used and I just took them out from my storage bins to test out. They are not NOS stuffs so the assumption of relatively good matching on both triodes are high.
NOS means "unused old stock" rather than "perfectly matched" 🙂 And the former doesn't mean anything as far as matching is concerned - most dual tubes will be grossly mismatched (by a difference of 10+%), new, old, doesn't matter. Using fixed bias (or a LED) only makes this more apparent. Not that it matters much in audio where one section is generally in each channel and difference of 10-20% in audio volume is negligible ...
Unless your tubes were perfectly matched your testing method is invalid. If you want to match your current sources for whatever reason, use fixed resistors in your jig and leave the tubes out. If it was me I would be more concerned with getting similar THD and voltage swing rather than complicate my life by trying to match parameters that might ultimately be irrelevant 😉
Unless your tubes were perfectly matched your testing method is invalid. If you want to match your current sources for whatever reason, use fixed resistors in your jig and leave the tubes out. If it was me I would be more concerned with getting similar THD and voltage swing rather than complicate my life by trying to match parameters that might ultimately be irrelevant 😉
It is important to measure the actual current through the 6922 tube. It can be measured through the cathode resistor, ie, V/R.
Then, you can tell if the current is right or there are difference in the tube. I usually use the same resistor for the 10M45 assuming the IC is more precise and stable.
Johnny
Then, you can tell if the current is right or there are difference in the tube. I usually use the same resistor for the 10M45 assuming the IC is more precise and stable.
Johnny
The data sheet only gives one spec related to this. It says for Rset=300 ohms, the current will be between 7 and 15 mA with a typical value of 10 mA. Obviously, this is a very wide tolerance.
Why does anyone assume bogey parts bought in a batch are well matched? You'd need to make a jig and sort them for transconductance or current if you want a really close match. (or make the CCS adjustable)
An ideal CCS should deliver about the same current to the load regardless of the load's effective resistance once the voltage across the device is some level above its minimum compliance voltage. Ideally it should not and does not depend on the parameters of the active device in its load circuit. (This is far from an ideal device, but again above the minimum required voltage it should be pretty predictable...)
These appear to be basically high voltage mosfets, have you looked at the data sheet? You will see that the output current with a bogey resistor between source and gate can vary by -30% to +50% of design nominal..
Each device needs to be trimmed for the specific operating current you desire.
An ideal CCS should deliver about the same current to the load regardless of the load's effective resistance once the voltage across the device is some level above its minimum compliance voltage. Ideally it should not and does not depend on the parameters of the active device in its load circuit. (This is far from an ideal device, but again above the minimum required voltage it should be pretty predictable...)
These appear to be basically high voltage mosfets, have you looked at the data sheet? You will see that the output current with a bogey resistor between source and gate can vary by -30% to +50% of design nominal..
Each device needs to be trimmed for the specific operating current you desire.
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Note in this case you trimmed the current to result in 100V of plate voltage so in effect you do not know the actual operating current - you need to measure it. You will find the current in the two sections of your 6922 probably doesn't quite match due to differences in the transconductance of the two triodes. In addition add the uncertainty of the transconductance of the CCS itself and you have no idea what the current is.
You can make it easily settable by inserting a 1 ohm resistor in series with your bias leds or you can substitute matched precision resistors (or use the same resistor) for the 6922 and set for the voltage drop required for the desired current. Choose a resistor value that won't cause excessive dissipation in the CCS however. Were it me I would add the 1 ohm resistors which would make it easy to check over time for drift.
Use sufficiently large resistors in the CCS to prevent heating from causing drift in the set current. Adequate heatsinking of the devices themselves is a must.
If you're curious, measure the exact voltage across each trimpot and knowing the R you can calculate the standing current in each side.
But I don't know how important Vgs matching could possibly be in ordinary CCS circuits. If you had a commercial build and needed a certain current range with a fixed value resistor, maybe you would need to know the spread and do some selection.
One thing I've noticed is that 10M45 seem to deviate from the published curve for I vs. Rset, needing smaller Rset values than indicated on the datasheet as the desired current I is increased.
PC mount trimpots are rated at about 1/4W dissipation, for the whole element. You are using 1/4 the resistance so need to derate to 1/4 power (about 0.06W), and your Rset at ~12mA should be dissipating about 0.03W. I use trimpots for Rset up to about 10-15mA and found that they are reliable. It's better to place the adjustment in the middle or towaerd the upper end of the full value for more dissipation headroom, e.g. for 12mA use a 500 ohm trimpot.
But I don't know how important Vgs matching could possibly be in ordinary CCS circuits. If you had a commercial build and needed a certain current range with a fixed value resistor, maybe you would need to know the spread and do some selection.
One thing I've noticed is that 10M45 seem to deviate from the published curve for I vs. Rset, needing smaller Rset values than indicated on the datasheet as the desired current I is increased.
PC mount trimpots are rated at about 1/4W dissipation, for the whole element. You are using 1/4 the resistance so need to derate to 1/4 power (about 0.06W), and your Rset at ~12mA should be dissipating about 0.03W. I use trimpots for Rset up to about 10-15mA and found that they are reliable. It's better to place the adjustment in the middle or towaerd the upper end of the full value for more dissipation headroom, e.g. for 12mA use a 500 ohm trimpot.
slight variations in the voltage across the green LEDs will also contribute to variations in bias. You may be (in part) compensating for this difference by changing your current source value to force the plate voltages to the same value.
Lots and lots of variables.
Lots and lots of variables.
Your 10M45S could be perfectly matched and give the results you are seeing unless the triodes are matched too.
While true the difference from this source should be miniscule as long as the LEDs are the same type (manufacturer, vintage, and PN). I've measured fairly large numbers (25 pieces or more) of a limited number of different kinds of LEDs and within types I tested the variation in forward voltage was typically less than 20mV at the recommended design current. Finding ones that match to small fractions of a % is not too difficult with a sufficient quantity of parts. How well they match over a wide range of currents and temperatures is something I have not evaluated, and come to think of it probably should..
These chips were bought 30pcs at once and they come in a tube full, so the relationship to the manufacturing datecodes and hence getting the devices that are from the same wafer lot is highly likely.
I was not pleased with the large resistor tolerance needed to get both sides of the triode to match in their operation points, that's all.
The green cathode LED showed a difference of 0.05v between both sides and the output is almost spot on at 100v on both sides. I would say both sides of the triode are fed almost exactly the same current and are running at almost the same operating point. If the tube is ill matched on both sides I shouldn't be getting these spot on values on cathode and anode voltages do you agree?
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Without really knowing how well matched those LEDs are I would suspect the 50mV difference in forward voltage could indicate a significant disparity in currents through them or not. I think it would be very worthwhile to stick a small resistor in series with the cathode or a plate depending on the available resistor values. Until you actually measure the current there are too many unknown variables present in these two circuits to know for sure just how well matched they are.. In terms of actual circuit performance it is probably not very important, in terms of removing guess work and uncertainty it is quite worthwhile to do the measurement.
Closely matched devices would have to be selected during die test or during package retest/grading. Full parametric testing is often performed again after packaging to assure that internal stress on the die during packaging has not changed things significantly. Closely matching devices or devices within a specific parametric range may have already been selected out for specific customers, so what you are probably getting are the devices left after selection- those from either end of the normal parametric distribution. Spent a number of years in the semiconductor ATE industry, but by no means have the whole picture.. (And unused knowledge evaporates quickly.)
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The green cathode LED showed a difference of 0.05v between both sides and the output is almost spot on at 100v on both sides. I would say both sides of the triode are fed almost exactly the same current and are running at almost the same operating point. If the tube is ill matched on both sides I shouldn't be getting these spot on values on cathode and anode voltages do you agree?
Well, the anodes are spot on because you adjusted the Rset of each CCS and made them so.
The difference in LED voltage, 50mV, corresponds to about 5mA current difference for most LEDs I test; i.e. 10 ohms dynamic resistance at about 10mA standing current. This assuming the LEDs are perfectly matched, which they won't be if my samples are indicative. So the LED voltage is probably an unreliable indicator.
If you want to know the currents, you can easily measure the voltage across each Rset trimpot, divide by the corresponding measured resistance of each trimpot, and then you'll know for sure.
Well, the anodes are spot on because you adjusted the Rset of each CCS and made them so.
The difference in LED voltage, 50mV, corresponds to about 5mA current difference for most LEDs I test; i.e. 10 ohms dynamic resistance at about 10mA standing current. This assuming the LEDs are perfectly matched, which they won't be if my samples are indicative. So the LED voltage is probably an unreliable indicator.
If you want to know the currents, you can easily measure the voltage across each Rset trimpot, divide by the corresponding measured resistance of each trimpot, and then you'll know for sure.
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How do you perfectly match an LED for cathode application?
I was under the impression that measuring the fwd voltage using a diode meter is sufficient to match them.
To best match them, you need to measure the voltage drop at the current you intend to run them at.
Once you do this you can adjust your current sources for the same current using sense resistors.
This leaves the minor point that anode voltages will not be the same for two triodes unless they are matched at this bias point as well. Don't worry about that. They should be close enough. (within +/-10V).
Once you do this you can adjust your current sources for the same current using sense resistors.
This leaves the minor point that anode voltages will not be the same for two triodes unless they are matched at this bias point as well. Don't worry about that. They should be close enough. (within +/-10V).
50mV difference in grid-cathode voltage is probably of no consequence to a 6922; i.e. there is no need to match LEDs. If the LEDs are this far apart, all other things being equal, you would see less than 2V offset at the plates.
My point was that the current through the 2 sections is probably not the same in order to reach 100V anode-cathode, in spite of the grid voltage being closely matched. It's more likely to me that the triode sections are mismatched and the 10M45s are pretty close, but a measurement would confirm or deny this suspicion.
FWIW, my standard practice is to match or set anode voltage by circuit goal and let the current be what it is according to the tube as long as it's within reasonable range. If the 2 sections are matched within 20% in terms of current it should be plenty close for most uses. If you need it closer than this a distortion analyzer would be a good tool.
Measuring across the Rset of the CCS will tell how closely matched the tube sections are vs. mismatch between CCS parts. I for one would like to know after all this discussion...
My point was that the current through the 2 sections is probably not the same in order to reach 100V anode-cathode, in spite of the grid voltage being closely matched. It's more likely to me that the triode sections are mismatched and the 10M45s are pretty close, but a measurement would confirm or deny this suspicion.
FWIW, my standard practice is to match or set anode voltage by circuit goal and let the current be what it is according to the tube as long as it's within reasonable range. If the 2 sections are matched within 20% in terms of current it should be plenty close for most uses. If you need it closer than this a distortion analyzer would be a good tool.
Measuring across the Rset of the CCS will tell how closely matched the tube sections are vs. mismatch between CCS parts. I for one would like to know after all this discussion...
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