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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Jan Thu 18, 2018 8:07 pm 
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Location: Tucson, AZ
The settings for the 6SN7 is just for convenience as to giving someone a common reference point to set the selectors. You could just as easily use any other tube settings and have the proper connections for the filaments, cathode, grid and plate. The screen is not used, and the bias would always be set to 0. There may be some other tube setting that could damage the calibrator or tube tester because of the connections, one reason why I suggest just using the 6SN7 setting. In my 539B my signal voltages were slightly off, so I added a 10 turn variable resistor to set the level for the grid signal dividing chain of resistors and had to change a few of the other resistors that were not in spec. This was many tears ago, the final result was all signal voltages were within 3%.

It should make no difference if you had a 10,000 Gm or 15,000 Gm calibrator setting, on something like the 539B/C they all use the same signal voltage of 0.25V. There is nothing to adjust if you are getting the correct reading on the 0-15,000 range, then the other ranges should be within something say 5%. Starting to tweak each range introduces other errors. You also may run into the linearity of your meter, which is a common problem with the older meters. On mine I did adjust the flux bar to get the proper full range meter deflection, on other tube tester meters I needed to add magnetism.


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Jan Tue 23, 2018 10:46 pm 
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How do you add magnetism??

Jim


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Jan Tue 23, 2018 11:14 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
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Quote:
How do you add magnetism??

Ask any woman.........

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Jan Wed 24, 2018 10:05 pm 
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Joined: Aug Wed 31, 2011 11:23 pm
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Location: Tucson, AZ
In one case I replaced the magnet with a newer one of the same width and then used the flux bar to adjust the meter sensitivity. In another case I added another small Neodymium magnetic, you need to "carefully" position it so it adds magnetic flux to the existing magnetic armature.


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 5:57 pm 
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Joined: Sep Tue 22, 2009 3:26 am
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Location: Riverside, CA
I built the calibrator and I used it to calibrate a Hickok 800. Using the calibrator as the standard, I found the red marks on the shunt dial were a little off, so I put new marks on it to show each GM setting per the calibrator (I used the midscale reading as the standard for each range). However, when I tested a 6L6 bogey tube with a known GM (tested on an RCA WT-100A and Triplett 3444). It now reads way too high (8200 vs 5000). All voltages are correct. Why the massive discrepancy and obvious inaccuracy?

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 6:55 pm 
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My first suspect would be the bogey tube ... how does anyone really know what the absolute reading on one of those is, who made it, when, how, and if it is still the same as when it was first characterized? this question is at the core of the whole tube tester accuracy debate, upon which volumes of discussion have been written here and in many other forums.

mksj will probably weigh in on this as well

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 9:09 pm 
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I own the RCA WT-100A and Triplett 3444 testers and they both agree fairly closely on the value of the bogey tube at 5000. The bogey tube is an RCA metal can version. I have never seen a 6L6 test at 8200 gm as the Hickok 800 now does after calibrating it with the calibrator.

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 9:14 pm 
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I suppose the next logical question is ... was the bogey tube in question used as the calibration reference for the WT-100A and the 3444 ?

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 9:21 pm 
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No. I used one from Roger Kennedy of AllTubeTesters. Besides, no 6L6 should test at 8200, especially a used tube. The top value in the RCA Tube Handbook is 6000.

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 10:10 pm 
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I tried another 6L6 of known value and it also tested high (5800 vs 4400). I then tested a 6SN7 of known value and it was very close to correct. Why would a pentode throw it off so much compared to a triode?

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 10:20 pm 
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Guessing loading effect on the power supply due to filament current draw. Also there are differences between Hickok and others I have not had time to look up yours specifically....... wild guess.. I'm at work, wlll research some more later. Mksj I am certain will weigh in eventuallhy

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 10:33 pm 
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If you download the following file, slide #7 gives a comparison of the results of the same 6L6 on multiple tube testers. The range of the Gm readings is anywhere from 4750-8550 and percentage 44-114. Basically a Bogey tube is only good for one tube tester and the question is what/how one determines "calibration" for that model. I have a 6L6 bogey tube from Rodger Kennedy, at the time it was tested on an Amplitrex at1000, so for most other tube testers they couldn't replicate the test conditions. I am able to replicate the test conditions on my AVO and uTracer 3+, the results were quite close given the same test conditions. https://www.sarsradio.com/index_htm_fil ... 021917.pdf

The other issue is earlier Hickok 800 tube testers have a signal level of 5.0V which is too high to use the calibration standard, you will get a lower Gm reading than expected and thus by correcting your shunt dial it will result in all other tubes reading higher than expected. The maximum signal should be under 2.5V (preferably 2.0V or less) , under 5V (preferably 4.5V or less) is only for the 1500Gm level and that is pushing it. The bias must be at 0V. When I tested the standard I used signal voltages of 0.10 and 0.25V and the measured Gm on all three switch settings (1500, 3000, 6000), were better than 1% on all readings. I did not test it at the higher signal voltages, but the readings decreased as the bias voltage went past 2V (overloaded). SO a combination of a higher signal voltage and not having the bias at 0 will probably cause you to have lower Gn readings using the calibration standard than expected.


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Tue 06, 2018 11:03 pm 
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mksj wrote:
Basically a Bogey tube is only good for one tube tester and the question is what/how one determines "calibration" for that model.

I have been preaching that on this forum for 15 years.

Dave


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Wed 07, 2018 2:15 am 
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Joined: Sep Tue 22, 2009 3:26 am
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Location: Riverside, CA
Excellent presentation! I've calibrated a number of tube testers over the years and I know from experience how much their readings can vary- even the same tester with the same tube on a different day (especially ones that use the shunt to set the ranges like the Hickok 800).

For calibration, I use bogey tubes and/or the old 10k resistor with 50 volts AC and have generally good results. However, this calibrator seems to be the best method yet if I can understand why there is such a great discrepancy between triodes and pentodes and can somehow compensate for it. Incidentally, when I used the calibrator on the Hickok 800, the grid bias was set to 0 and the signal voltage was measured at 2.5 volts.

One last thought is that there may be problems with the shunt pots on that individual tester because the recalibrated settings on the shunt control are far from standard, even if I use a bogey tube as the standard for calibration.

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Wed 07, 2018 5:03 am 
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Location: Columbus Ohio
I've given up on the "perfect" calibration because these units are 50+ yrs old, I recap them, check all the resistors I can get to then calibrate it.
Start adding up all the variables with resistors that check within tolerance, what if it is now high side vs original low side, it may not come up
in a calibration, but may show up in a particular test, contacts which are pretty thin copper, plated? worn down? Carbon tracks with some wear here
and there. The more complicated the tester, more there is to drift. Its not practical to completely rebuild one, probably wouldn't work well at if you did.
I say to repair it, recap, do calibration to verify voltages, and try it. Tubes on a 539c and a 600a present different reading. ie:
Hickok 539b/c, a 6L6 should show min 3300
Hickok 533a, 600a, 605a 6L6 should show min 5000

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Wed 07, 2018 10:23 am 
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Forgive me if this is an oversimplification of this topic, but imho, a calibrator such as the one proposed in the paper in question, similar to the so-called "bogey tube" .. would not necessarily work for more than one model of tester no matter what you try to interpret from the results. Tube testers were all built for the purpose of having a given tube produce a known reading based on the specific tube tester with it's specific roll chart. Generally, I'd think it would be fair to expect a known 'bogey' or the calibrator in this case to give predictable results when used WITH THE TUBE TESTER FOR WHICH THEY WERE MADE. Taking this calibrator, and expecting it to produce reliable results on another model tester, is probably folly. One difference between a calibrator and a bogey tube is that the bogey tube can change over time. The calibrator could be expected to maintain it's accuracy indefinitely.

I've idly dreamed of a "calibrated standard" that would work across all makes and models of tube testers. I do not believe such can ever be realized. You'd have to compensate for each manufacturer's method of electronically comparing a given tube type to the roll chart data for THAT tube (or calibrator). Tube testers "of the day" were built to be used in a specific way, differing for each manufacturer. I think the closest we can come to this "standard" is using something like the calibrator proposed here to calibrate your 539, and then testing a few different tubes. Take THOSE results and you can use that specific tube on some other tester to get a rough idea of that tester's calibration accuracy. There's no way to make it precise. Nor, really, is there any need to do so. It seems the intent for many people is to be able to match tubes.... that did, and always will, require a curve tracer to really do it properly. To restate for the thousand'th time, a tube tested on tester X is never going to give the same results on tester Y.

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Wed 07, 2018 4:26 pm 
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You make a good point there Barry about the majority want to match tubes. If you are matching tubes, calibration doesn't matter.
If your tester is out of calibration by say 3-10%, you can still match tubes, the reading may be off, but you can still compare 2 tubes on that tester.
It would affect measuring single tubes accurately, but you could still see an amp factor.
I spent about 5 hrs calibrating the 539c I just sold using the modified Daniel Schoo method, and looking up all the corrections and opinions.
The final outcome? a minor adjustment on a dc circuit. The tubes I measured before and after, I didn't notice a difference in the readings.
But I started with a pristine 539c that was well cared for in its 1 owner life.

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Wed 07, 2018 5:21 pm 
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Location: Riverside, CA
Indeed that is true. I also know that most tube testers like the Hickok 800 that I'm working on were made primarily to see if a tube is good or bad, not measure its transconductance for resale or matching purposes. However, that doesn't change the desire of people who want to have a "calibrated" tube tester, so my goal in this is to find a convenient, relatively accurate way of determining accuracy and setting the calibration, even if it's not precise like on the Hickok 800.

Also, glue_ru's comment that some tube testers like the 533A, etc. have a reject point of 5000 for the 6L6 is not correct. The manuals for those testers say that the gm reading on the chart is the average test value for each type of tube, not the reject point. The charts don't specify a reject point. It appears that they intended for you to use the English function to determine if it's good or bad. For the 6L6, the 5,000 gm value as printed on the chart aligns with the RCA tube manual as an average value for a 6L6 and that value was what I was shooting for in calibrating the Hickok 800 based on the bogey tube I have.

mksj mentions that a signal voltage higher than 2v could affect the calibrator adversely. Do you think it would be advantageous to add a switch and resistor in series with the grid connection to drop that voltage for testers with grid signals higher than 2v?

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Feb Wed 07, 2018 7:03 pm 
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Location: Tucson, AZ
I have not done anything with the calibrator nor tube testers for many years, so it is pretty much just a tool to verify if your tube tester is working and to periodically verify the calibration has not changed. There are other means of verifying the Gm ranges, but just more complicated. Gm changes/accuracy is probably more an issue with changes in the 83 tube aging and to a lesser degree the 5Y3. On most Hickok's there is no adjustment for this, and for the most part very limited "calibration" adjustments. Yes the 83 tube can last a long time, but I didn't want to deal with the warm up period and tube balance issues that I experienced with a few 83's. So I have used the calibrator in both my 539B and 580A testers and it has proven to be very consistent, both of these allow a few more "calibration" tweaks. The 580A can be a can of worms, without significant modifications. I am sure the calibration standard can be modified further to allow use with higher input signal voltages, as mentioned the 1500 range does accommodate a higher signal level because it adds and input voltage divider. But the purpose of it is not as a universal check for all Hickok testers, I think if you test a known good tube and you get a good reading on most Hickok's that's about it. The general purpose was just to check shorts, gas and good/bad.

I did a pretty extensive test on various tubes using my uTracer 3+, AVO-CT160 and my Hickok 580A. The latter two have been modified to allow more precise voltage readings, the 580A allows unfiltered bias and balanced voltage taps, and the CT-160 uses SS rectifiers. Tubes where tested at their book values per their spec sheets/AVO data listings. Overall both Gm and plate current were surprisingly similar at all the test points. This has also been my experience when using tube testing jigs and comparing the results with tube testers operating at the same values. So it is possible to have comparable readings across different tube testers, providing the operating points are the same and held constant. Chasing anything less than say 10% is a bit futile, even testing the same tube at different times can vary quite a bit. That being said, I often use a 6SN7 as a calibrator tube and it is very consistent. It can be setup to test a wide range of bias and plate voltage conditions. In the attached file I calibrated the 6SN7 tube in a test jig, then on my 539B with almost identical results at the same operating points. When doing a comparison between the test jig and my uTracer, the same was true at 250V on the plate and sweeping the bias voltage.


Attachments:
Calibration Tube 6SN7.pdf [55.98 KiB]
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