[etech]

Hi all,

I have been working on my old Hickok 600A and it is working fairly well. All the voltage tests are within specs and I have pulled and cleaned the english pot and recalibrated it. I have also run the mutual conductance check and it seems within spec on the 3000 micromho range. All seems well.

Next, I would like to verify the calibration on the 6000 micrmho range. So, does anyone have a 6L6 calibration tube that I might be able borrow? If you would be so kind, I would pay first class mail shipping both ways.

Not only would I like to verify the calibration on my newly tuned up 600A, but  I also have five 6L6 metal tubes that I purchased years ago for a buck a piece. The gm readings I am getting on these tubes range from 4800 to 6300 micromhos. Once my 600A is accurately calibrated with the test tube, I will test all of these metal 6L6's and put aside and mark one of them that measures close to the test tube for use as my own test tube.

I would appreciate any help.

Thanks,

Ed

[etech]

Hi all,

Since 67 people have viewed this, I figured there might be some interest, so here is what I did. Since no one was either willing or able to loan me a calibration tube, I went ahead and purchased a pair of NOS 6L6GA tubes for my 38-690, which I needed anyway. These new tubes were Sylvania with a code date of 1945. They had been tested by the seller at 44/5500 and 46/5750 micromhos respectively, on a recently calibrated TV-7 tube tester.

That was good enough for me. I went ahead and used these tubes to calibrate my Hickok 600A. After which, without touching any of the controls, I measured the gm of all of my 6L6 tubes. You can see them with the results in the pics below.

[Image: http://i135.photobucket.com/albums/q136/...jpghttp://][Image: http://i135.photobucket.com/albums/q136/...s6nzdl.jpg]
[Image: http://i135.photobucket.com/albums/q136/...njhfhc.jpg]
[Image: http://i135.photobucket.com/albums/q136/...53wetk.jpg]

Besides all of my good glass bottle 6L6G's, I have five 6L6 metal tubes, which I will probably never use in radios, that I can use as calibrated test tubes. So, if anyone needs to borrow one to get their Hickok tube tester verified as "in the ballpark", I would be happy to loan you one for round trip first class postal service shipping. This would probably be a couple bucks each way.

Best regards,

Ed

[Eliot Ness]

I don't think many of us own calibration tubes, so your offer to lend a tube is appreciated.

What did those tubes measure on your 600A before you calibrated it?  The reason I ask is the TV-7 does not measure directly in GM so the readings have to be converted.  I'm not sure if I'd want to do more than a minor tweak on a 600A going on a converted TV-7 reading.

[etech]

I don't think many of us own calibration tubes, so your offer to lend a tube is appreciated.

What did those tubes measure on your 600A before you calibrated it?  The reason I ask is the TV-7 does not measure directly in GM so the readings have to be converted.  I'm not sure if I'd want to do more than a minor tweak on a 600A going on a converted TV-7 reading.

Hi John, Actually, from what I have read, a properly calibrated TV-7 is pretty accurate. They are calibrated at 125 micromhos per point. So, a calibration tube with a gm of 5000 micromhos, which is the Hickok charted average for a NOS 6L6, would read 40 on the TV-7.

I have a pretty good degree of confidence in my newly calibrated 600A. All of my 6L6 tubes read about 400 points higher, to the point of being abnormally high, before I re-calibrated the machine. Now, most of my tubes range from about 4300 to 5750, which to me puts them right where I would expect them. I have two lower rated 6L6G's which I know to be weak, which were in my 38-690, and they read 2800 and 3000 respectively. These two tubes also read in the questionable range on my Precision 10-54 emissions tube tester.

But to me, the most important thing is to have a tube which will be used as a calibration tube when I wish to read gm accurately on the 600A. As you probably know, any of these Hickoks (600, 600A, 800, 800A, etc.) with the English control with the dots on them which are used in setting the gm scale are known to be inaccurate. Frequently, the gm reading will vary 200 points or more, just by the direction you have approached the dot from. At least with some sort of calibration tube you can set the english/shunt control for the proper reading before you test your tube. Or, I could buy a better tester, lol!

Best regards,

Ed

[Eliot Ness]

Sounds like you have it nailed down pretty good Ed. 

I have a TV-7 and a Precision 10-12, but I do most of my testing on the 10-12.  For some reason I like the Precision a little better even though it is basically an emission tester.

[morzh]

Ed


As I remember a discussion about calibration tubes here, folks here were not very fond of them, so very few probably have them. Then again, a tube tester, unless used for very specific purpose, needs only so much accuracy.

[codefox1]

IMHO the bogey 6L6 was just another urban myth. although it was a good general workhouse barometer.

[etech]

IMHO the bogey 6L6 was just another urban myth. although it was a good general workhouse barometer.

Well maybe. But, take a look at this article from the Tube Sound website. He asserts that the 6L6 "bogey" tube was actually used at the factory as the last step in the calibration of the Hickok 6000 tube testers. Whether you agree or not, it is definitely an interesting read.

http://tubesound.com/2010/04/28/hickok-6...libration/

Here is the statement I was referring to:

"Hickok did not calibrate the red dots on your Shunt control using this test, nor does the factory calibration document make any mention of this test. The factory used a calibration tube — a real tube — a 6L6 of known value — and marked the Shunt dial according to the micromhos reading of that tube.  (Using a calibration tube to mark the micromhos ranges is Step 9 of the Hickok 6000 factory calibration instructions.  The calibration tube was the LAST step of the calibration procedure.)"

I think we can debate the value of a calibration or "bogey" tube at will, but I believe Hickok did use them in the calibration of some of its models, specifically those which used the dots on the shunt or english control to set the gm meter range.

And my offer still stands, if anyone has an official "bogey" or calibration tube and wants to send it here for testing, I will pay the shipping both ways. Or, if anyone with such a tube wants me to send one of my "calibration" tubes to them to compare against their test tube, I will be happy to do so.

Best regards,

Ed

[TA Forbes]

An interesting thread! 

The most important thing about a "calibration tube" is the instrument that the tube was tested on. Any calibration tube should be tested on a laboratory-quality tester in order to be as sure as possible about the value of the tube. Examples of laboratory type testers would be: Triplett 3444, Weston 686, RCA WT 100A, and Hickok 700. Testers like the Hickok 539 and Precise 111 are very good benchtop testers.  

BUT herein lies the problem with calibration tubes: 1. The testing method used by the laboratory tester may not be the same as the method used by your tester; 2. Calibration tubes measured on any of the testers using "bias/english" pots with no associated meter to read what you are dialing in mandate that the operator will place these pots in the exact same place when they are verifying the tester calibration; 3. All tubes perform differently depending on the tester method utilized. Some testers use a 4 or 5 KHz grid signal, while most others use 60 Hz. Grid bias, plate, screen, and even cathode potentials differ depending on tester components and methods used.  

Ed's calibration tubes ARE a good idea for his personal tester, as they can indicate if cal is on or off. That is, assuming that the bias/english (shunt) pots are in the exact same place. 

It is important to remember that most of us have "bench top" testers. They are accurate to a degree, but this should be considered only with respect to tubes tested on your own tester. On most Hickok testers, the NOS value for your 6L6 is vic 5200uMho, while NOS for Weston and Precise benchtop models is 6000 uMho. 

Comparing the results to other testers will often lead to some interesting results. In his book Allan Douglas made some interesting comparative studies of uMho testers and the "Dynamic Conductance" testers like the Eico, Jackson, and Precision models.  Although I LIKE these testers, the results proved that different testers will sometimes yield VERY different results. 

For those of us who own benchtop uMho testers, a frequent issue is voltage sag when testing high current tubes like 2A3 and 6L6. Testers like the Precise 111 and Weston 978 utilized two power transformers vice one to mitigate this. Another way to lessen this is (with Hickok-circuit type testers) replace the 83 tube (or 5U4 in Precise 111) with a solid state replacement. This frees up ~3A current draw for both examples.    

[keith49vj3]

Check out this tread that will tell you how to make a solid state GM standard that will work on a Hickok as well as Triplett testers. calibration standard


Keith

[morzh]

To me for diagnostic purposes if a tester within 10-15% of the true value that's probably enough. It will tell you a weak from medium from strong.
I got 6L6 from a guy who sold them, my 600A gave me the reading exactly the same that was written on the tube.
To me it only means the tube is good and the tester is good enough.

Some things I like exact, some - just good enough.

[TV MAN]

I have to agree with Mike. A tube tester will give you a good idea of the condition of a tube, shorts, emission gM and such. But the real test is in the circuit. Oft a tube will not work in one circuit, even though it tests perfect on the tester, yet work fine in another. With most types of tubes we use no longer being produced, don't toss anything that may read weak, only if it is shorted, dead or verified gassy. Output tubes are least sensitive, and if you don't want room shaking bass, a weak tube can serve for years. Been using a weak 47 in the old 70 for years. Sounds fine so far.

I have had a few experiences with tube testers, but by no means a pro at it. I have both a Hickok 600a, and Precise 111. They don't read the same. I tried once to calibrate the 111 to match the 600a. Didn't work. They simply read different. The values are on the charts. If I recall, the 600a read higher than the 111. Calibration seemed ok on the 600a, and I had painstakingly calibrated the Precise. Same NOS tubes checked accurately on both according to each ones tube chart. The RCA tube manual can give you fits to when you are looking at the gM of a tube. Seldom will the tube being tested match the RCA manual, even when new. Something else that comes to mind is the variations in line voltage. I cant accurately test a tube on the 600a when my wife is cooking supper on our electric stove. Each time an element cycles, the reading on the tube tester also changes!

As Mike mentioned, you don't need a laboratory tube tester for bench radio work. Even a simple emissions tester will give you an idea of the condition of a tube and check for shorts. They don't put a load on the tube like the gM testers tho. I prefer the 600a over the 111, mainly for the ease of setup, but if I really want to test a tube, check for noise, heater current or halfway match tubes, then I go to the Precise 111. Mine does have solid state rectifier for the 83. I have also read up on the bogey tube. It really isn't a bad idea to have one, whether it is one you made or from some supplier. It gives you a quick reference to check your calibration, but there are other ways to calibrate the 600a, which is how I checked mine, and left it alone.

[Steve McDonald]

I don't use calibration tubes. I made a a 5mA. transformer I use as a current source. 5 mA. corresponds to 1000 mhos on your meter. The most important part of calibration is to make sure the tested tubes are tested at the proper voltage.

[Phlogiston]

So late to this but - -

The value of testing any power tube in a tube tester is limited to the shorts test and maybe the noise test - grid emissions.

I have restored a lot of tube type hi-fi gear, kept most of it, and I can tell you that a tube that tests good on the 600A or a TV-7 or most tube testers can run poorly in a amplifier or even red-plate. They just do not come close enough to the real operating environment to give an accurate test.

Most of the amps I have use 7868 tubes , a 6L6© variant. So I wanted to test them accurately. I had a good quality mono amp that was set up to run them near their max spec. It was converted into a test rig that can be used to measure the plate current, bias and the interaction between a push pull pair (comparison of plate currents/on known bias).

I also built one of these for 6BQ5s.

You can, and should do this test in the device that the tubes are to be used in - lacking a test fixture such as this. Some modifications may be required.

   

[Phlogiston]

I don't use calibration tubes. I made a a 5mA. transformer I use as a current source. 5 mA. corresponds to 1000 mhos on your meter. The most important part of calibration is to make sure the tested tubes are tested at the proper voltage.

This is a good calibration procedure (too).