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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Thu 15, 2018 3:18 pm 
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I just finished running an experiment to evaluate the accuracy of the simulations for the Hickok 539B tester. First, I trimmed all the resistors in the transconductance measuring circuit of my 539B as accurately as I could using an ohmeter spec'ed to have 0.3 % accuracy, except for R42 and R37 which were changed from 500 ohms to 512 ohms. So all but these two resistors were as close as possible to exactly what Hickok specified they should be for this set and that were used for the simulation. The new values for R42 and R37 were also used in the simulations. The only actual calibration was to change the MAIN METER sensitivity to 181.8 mv. as measured on a meter spec'ed at 0.7 % accuracy.

The resistors in the Calibration Box that were selected according to the values from the simulation were also trimmed using the 0.3 % ohmeter. So if the simulation is correct, the transconductance readings actually obtained from the tester when using the Calibration Box should be the same as the transconductance values selected for the Calibration Box and that were used to calculate the values of the resistors in it.

Since the actual MAIN METER on my tester had already had to be replace with another less sensitive meter that needed about 375 mv. for Full Scale deflection, I already had a simple op amp voltage amp circuit installed which boosted the 181.8 mv. to about 2.6 volts (more than enough to drive most any meter) with the analog meter connected to an adjustable output voltage divider. Since I have already experimentally verified that the replacement analog meter is only linear to about 3 %, I added a second adjustable voltage divider to allow 181.8 mv. on the input to also drive a 200 mv. Digital Panel Meter at 100 mv. so it actually reads % of full scale deflection. This is spec'd at 0.5% accuracy, and is what was used for the comparisons.

The actual 539B readings compared to the readings expected from the Calibration Box (percentage difference between the two) were as follows for ranges A: through F: -0.5%, -0.5%, -0.05%, -0.4%, -0.4%, and -0.2%. They were in agreement to better than 99.5% for all 6 ranges, and so the simulations would seem to be highly accurate. These results also validate the concepts behind the design of the Calibration Box.

Regards,

MIke


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Thu 15, 2018 4:03 pm 
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Location: Sunbury, Ohio 43074
Great work, Mike. Now if they only made tubes accurate to 0.5% :-D (hey, someone had to say it) :lol:

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Thu 15, 2018 4:24 pm 
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Joined: Sep Tue 22, 2009 3:26 am
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Location: Riverside, CA
That's excellent. However, how would a tube with a fairly heavy filament draw affect the other voltages in the tester? I've found that a 6L6 tube can change the other voltages vs when no tube is plugged in. Would that have a bearing on the accuracy given that the voltages would no longer be ideal? In other words, would real-world testing alter the accuracy of a tube tester set by the calibrator and can that be accounted for?

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Thu 15, 2018 5:25 pm 
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Location: Sunbury, Ohio 43074
Quote:
I've found that a 6L6 tube can change the other voltages vs when no tube is plugged in

?? ... you mean when the 6L6 is plugged in? If you mean the drain on the power transformer due to the heavy filament, the tube tester chart already takes that into consideration. The ever raging debate on whether to adjust the line set control before, or after you plug in the tube, or both, can of course have some effect on this also. If I am not mistaken, and Mike will certainly chime in here, if you follow the Hickok directions exactly, the chart numbers will be correct for the tube under test.

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Thu 15, 2018 6:45 pm 
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Joined: Aug Wed 31, 2011 11:23 pm
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Location: Tucson, AZ
There are several factors here and it depend on the Hickok model, some specify in the operating instructions setting the line set after the tube is plugged in (not under test), other models specify to set the line set and bias with the tube under testing (539(B/C, 580/A). Only a few Hickok testers have a bias meter to check the voltage under test. My understanding of the Hickok circuit is that in a crude way it is self compensating, in that the voltage sag under test would also change the bias and other operating voltages to give a similar reading. Although the concepts of trying to achieve better accuracy with the Hickok tube testers is informative, just a small deviation in the line and bias voltage can have a significant effect on the Gm readings, and at best the analogue meters are maybe a few percent accurate and if you can see where the needle is. Having gone to digital readouts on the bias and line set helps to see the actual voltages for what it is worth.


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Thu 15, 2018 7:21 pm 
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on a 539c, you adjust line twice, with tube in warming up and again after pressing P4
Refer to page 4, step 3 of the Hickok manual.
--------------------------------------------- copy/paste -------------------------
3. The line adjustment control rheostat in the 539C Tester is connected with a small A.C. voltmeter as a
constant calibration indicator which is normally always in circuit. The small A.C. voltmeter may also
be used to register 60 cycles A.C. line voltage fed to the set by operating the test button P7 designated
"LINE TEST" in the lower right part of the control panel. Readjust after pressing the P4 Test
Button.
----------------------------------------------------------------------------------

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Fri 16, 2018 6:35 am 
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Location: Tucson, AZ
Per the FORWARD in the 539B manual "voltage adjustments are made while the tube being tested is delivering its rated load"

As I indicated, the instructions for the Hickok 539B/C indicates to "readjust" both the "LINE TEST" and the "BIAS VOLTS" after P4 is pressed, i.e. with the tube under Gm testing. It is assumed you have plugged in your tube, turned on your tube tester and brought up the rheostat to the line voltage set point and then allowed the tube to come to operating temperature before proceeding to the testing process, only at the Gm test are both voltages readjusted. This differs between tube testers so one should not assume otherwise unless specified in the instructions. Many Hickok tube testers indicate setting the line test (i.e. readjusting it) with the tube inserted, but not during Gm testing, example would be the 752A tube tester. It also indicates to set the "Line Test" (i.e. readjust) after the tube reaches operating temperature. The 800A just states "Make final adjustments after the tube being tested is placed in the socket". So they are all readjusted, but most are not under Gm testing conditions as I indicated. Each manual and version can vary as to the test conditions and what is being measured, often it is just the reject point for a tube, typical Hickok. Once again, the numbers mean little, as the conditions are specific to each model and to some degree differ significantly between versions of the same model. Whatever.


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Fri 16, 2018 7:06 am 
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Location: Riverside, CA
I guess I wasn't clear on my comment on the voltages. I know you can monitor and adjust the line and grid bias voltages as needed. However, you have no control over or way to monitor or account for any screen and plate voltage changes induced by different filament loads. If Hickok accounted for all of that for each tube, then that is not an issue.

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Fri 16, 2018 2:31 pm 
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Regarding the math behind the operation of Hickok, the math in the patent assumes that Gm is a constant, and may have other flaws as noted. I'm envisioning a model where it is dependent on another parameter (we all know the strong dependence of Gm on plate current). The voltages are not DC so what I'm thinking is that during some given part of the cycle, lets say near zero, Gm is near zero and near the peak, Gm is likewise at its peak. I now have to take an integral (not necessarily an average) in conjunction with a transfer function (Child-Langmuir?).

Mike - did your spice model do something like this? In this preliminary model, I'd have to confine the treatment to a triode, I think, to make the analysis workable


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Fri 16, 2018 6:36 pm 
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Joined: Mar Thu 06, 2014 6:01 pm
Posts: 128
Hello all,

I will try to give my take on some of the topics that have come up. For some of them, I don't know the answers and I can only speculate.

First, the 539B transconductance measurement is surprisingly sensitive to the line voltage and I would think that the models that lack the line voltage adjustment are probably not very accurate since they use pretty much the same circuit. There are several things which could contribute to this, but I suspect the major effect is that of the line voltage changes and their effect on the control grid test voltage. I have a couple of the lower end Hickoks, but I have not had the time to look into this yet. In any case, it is very important to make sure the line voltage is properly adjusted before taking any actual measurements or when checking any of the calibration measurements or adjustments (at least on the models where this option is available).

As to how well the line voltage adjustment compensates for current draw from the main transformer, theoretically, it SHOULD work pretty well, especially on those models like the 539 that have separate filament transformers. (That should pretty effectively de-couple filament current effects from the rest). I will try to take a look at this using the actual tester and will post the results when I get time to investigate it.

MarkD,

As to the math behind the Hickok circuit, it looks to me like it would probably involve solving at least two simultaneous differential equations since at any instant you would have to account simultaneously for the current charging up the capacitor through the resistor in the "active" leg of the "bridge" and for the current discharging the capacitor through the resistor in the other leg of the "bridge" with a waveform that is time dependent (varying with time). If you could get a (mathematical) solution for the current through (or voltage across) the capacitor, you could just integrate it over a half cycle.

I also have also wondered about the fact that the 1/2 cycle waveform that is actually used goes from 0 plate current to values significantly larger that the average current and thus the transconductance of the tube will vary to at least some extent over the 1/2 cycle. I have been working on a way to test for this and it is nearing completion too, but I don't have an answer yet. I did do one experiment with the curve tracer that leads me to think that maybe it will all average out in such a way that this Hickok method may turn out to be pretty accurate after all, but this is VERY preliminary.

The sim that I am using merely simulates the circuit itself that measures the difference in the current changes in each half cycle of the plate circuit and is fairly simple, but seems to do a pretty good job. I was (pleasantly) surprised at the experimental results. I would have been happy if it had only been 2%.

Regards,

Mike


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Mar Sun 18, 2018 3:50 pm 
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Joined: Mar Thu 06, 2014 6:01 pm
Posts: 128
Another question answered (another experiment!):

This time I used the calibration box on each range before and after putting a 4 ohm resistor across the filament pins with 6.3 volts. The 6L6 pulls 0.9 amps and the 4 ohm resistor pulls almost double that. In each case, the line voltage was adjusted, if needed, right before the measurement. The results (% differences ) for ranges A: through F: were: 1%, 0.1%, 0.3%, 0.5%, 0.15%, and 0.4%, all 1% or less. (The filament voltage dropped by about 0.5 volts, but that didn't affect these results since that was on a separate transformer in this tester, 539B/C). So as was expected, the line voltage adjustment appears to work well (not so much for the filament voltage, though).

I also monitored the voltage from the 5 VAC winding of the main transformer that is used to generate the control grid test voltage, with and without the 4 ohm resistor and with the same results. As long as the line voltage adjustment was made just before taking a reading, the control grid test voltage stayed "right on". The actual % differences were 0.4%, 0.3%, 0.76%, 0.5%, 0.1%, and 0.5%.

Mike


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Jul Tue 24, 2018 2:44 pm 
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Joined: Dec Thu 31, 2015 3:07 pm
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Can Mike please supply figure 15 from his web page posted with Roper?
Mike Can you please elaborate on the issues with the circuits to be avoided for the solid state 5y3 replacement?


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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Jul Tue 24, 2018 2:59 pm 
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Location: Littleton, MA
pclay wrote:
Can Mike please supply figure 15 from his web page posted with Roper?
Mike Can you please elaborate on the issues with the circuits to be avoided for the solid state 5y3 replacement?

I believe I have that figure along with some text changes, but updating Mike's monograph hasn't made it to the top of the to-do list yet. I'll try to find the couple of hours I need when I get back from vacation next month.

BTW, my site is "Byan-Roper". Roper is my wife's family name, Byan is mine. I've never managed to convince Sue to post anything on her part of the site, though.

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 Post subject: Re: Updated Hickok 539 Calibration document
PostPosted: Jul Thu 26, 2018 2:56 am 
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Posts: 128
pclay,

I took a quick look at the calibrations instructions and, frankly, I can't remember what was supposed to go there. I need to redo portions of the instructions to reflect newer discoveries and some better ways of doing things, but I just haven't had the time yet.

I decided to revive my "junker" set to have another test bed and to investigate the problem of replacing the analog meters with digital ones (the junker was missing quite a lot of pieces, including all the meters, but the transformer was still good.)

This has turned out to be MUCH more time consuming than expected, but has been "illuminating". For example, it appears that the Main Meter can be replaced accurately with a 200 mv. digital panel meter with an even simpler circuit than the op amp based one. I also have a better op amp circuit than the one in the calibrating instructions paper, and which could be used for almost any meter, either digital or analog. This has the advantage that you could theoretically use just about any meter as a replacement for the original main meter analog one, possibly a less costly or more accurate (or, at least, an available) one.

I have every intention of getting to this project, but there may some delay, as the family vacation is looming on the horizon, and which will delay it some more, I'm sure.

Regards,

Mike


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