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 Post subject: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Tue 23, 2021 1:56 pm 
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Hi there!

I recently acquired an old Precise 111 that I have restored changing capacitors and resistors, a couple of failing slide switches and gave it a good clean. It calibrated successfully and get quite sensible Gm data from it with a notable exception: 6BQ5 (aka EL84). If I follow the instructions in the roll chart (or the pdfs found in the web for that matter) I get ridiculously low values for tubes I know are OK. The Precise 111 data states a "minimum" Gm value of 2900 umhos and I never get values over 1000 no matter how many of those tubes I try, and I've got quite a lot of them!

I'm suspecting the BIAS value to be wrong. Regarding the charts it should be 9V (that is, -9VDC) but I think this must be a typo. Most of the other tubes nearly match the dial position of the BIAS pot when it is adjusted for the given value in thg Gm test with the proposed position of that same control in the Em test, but not in this case. It is a major discrepancy. The indicated position in the Em test for this tube is "11". This position corresponds to 0.9V BIAS in the Gm test. I thought it was a typo in the roll chart (9V vs. .9V) but if I select the equivalent .9V BIAS volts in the Gm test then the results are the other way 'round: they're too big!

I don't think it's a calibration error because I did the calibration very carefully and repeated the (lengthy) process again just in case and, besides, other tubes, including other pentodes, read fine.


Anyone faced this problem?


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Wed 24, 2021 9:47 am 
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Well, I've been searching the web thoroughly on this topic and have found that I'm not the only one with this problem:

https://philcoradio.com/phorum/showthre ... #pid113126

Still I would like to have the correct settings for this tube... :roll:


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Thu 25, 2021 1:14 am 
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I have not had my 111 out in years but was thinking recently about it. I will pull it out and see what results I get. Will be a few days. What results do you get when doing the Em test?

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"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Thu 25, 2021 9:44 am 
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I get values surrounding 1000umhos when a good tube, regarding the charts, must be at least three times that value.


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Thu 25, 2021 7:11 pm 
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More on this: it seems it's happening not only to me but to other tube testers as well:

https://www.youtube.com/watch?v=0T8BJqvoLF4

Must be true that, after all, the Precise 111 is a Hickok in disguise :roll:


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Fri 26, 2021 1:10 am 
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Interesting about the Hickok 600.

I did pull out my Precise 111 and I got the same results as you on the 6BQ5/EL84. Good tubes test around 1000. I re-calibrated the 111, it had drifted slightly from when I calibrated it about 10 years ago, and got the same results testing the 6BQ5s.

I then tested the 9 6BQ5 tubes that I have ( 1 NOS) on my Heathkit TT-1A and I got a range from new to below the reject point. I then tested them all on my RCA WT-100A Lab tube analyzer, using the RCA tube manual for operating point, and recorded all the Gm readings. The readings correlated to what I saw on the TT-1A.

The Precise 111 lists the RP (Reject Point) at 2900 in the Gm mode of operation. The agreed standard that I have seen for reject point is 65% of new but in the Precise manual when they discuss developing settings for unlisted tubes they state 60% (40% loss). Using that information I calculated that a new tube should read around 4830 on the Precise (2900/.6 = 4833). I then mapped out my tested tubes on the 111 scale converting the readings from actual Gm to the Precise scale, and came up with a bias setting of 6v. That setting will identify tubes at the RP and show others as acceptable or not.

Try setting your bias to 6V and see if your readings make sense. Maybe someone transposed a 6 into a 9 when they printed the chart?

Also, as per the manual, be sure you are setting the Line and Bias with the tube installed and the test switch set to HOLD.

Curious, are you running this off of a step down transformer or did Precise make a 220V version?

Regards,

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Tim

"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Fri 26, 2021 8:50 am 
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Thanks a lot Tim for this and your clarifying email too (you're TV MAN, right?). I'll check my EL84/6BQ5 tubes later with your suggestions and will post here.

Regarding the 220V option, no, my Precise 111 is a stock 110V american unit and I power it from a variac. For some reason, my Precise 111 does not work well with the nominal 110V. If I select that voltage I have to trip the LINE pot up to the top, making some tubes' line adjustments impossible. I even tried to calibrate it with a 110V supply to no avail. The sweet spot is 120V. Higher voltages (125V-127V) make the LINE pot to work a lot and gets too hot. I finally decided to calibrate it at 120V and all seems to work well.


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Fri 26, 2021 1:16 pm 
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I'm at the Precise 111 right now. I've been testing a bunch of 6BQ5/EL85 with that BIAS setting of 6 and the readings are still a bit on the lower side (3000-3200umhos). It seems that 5V makes more sense: I have tested some known to be good NOS EL84 (Miniwatt), newer EL84 (Ei and Groove Tubes "Premium") and some NOS 6BQ5 (Pinnacle) and they all measure quite equally around 4200umhos with those 5V for BIAS.

Couriously I've got a pair of 8BQ5 (same as 6BQ5 but with 8V for heater instead of 6.3V) that someone marked as "matched" with 7500umhos each. It is funny that to get that somewhat high reading in the 111 I have to set the BIAS at 0.9V, which was my original suspicion :D With a BIAS setting of 5 both measure around 4300umhos. Note that the 111 has no 8V heater setting but 7.5V which is what I selected.

I think that, at least for my 111, 5V for BIAS makes mores sense. If you look closely, with the chart font used in the roll chart and later listings, an overprinted 5 may well look like a 9. By the way, the roll chart in my 111 doesn't even have the 6BQ5 listed :roll: I had to look in one of those newer pdf charts in BAMA.


Last edited by Calambres on Feb Sat 27, 2021 9:50 am, edited 1 time in total.

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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Fri 26, 2021 11:49 pm 
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Found my notes, here's your answer:

Attachment:
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IMG_4131a.jpg [ 595.52 KiB | Viewed 253 times ]

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IMG_4133a.jpg [ 305.34 KiB | Viewed 253 times ]


Just set the heater to 6v. The 8BQ5 is the same as a 6BQ5 exc heater. I double verified this and get good readings. The test shows a new JJ tube, also tried a couple used tubes. RCA manual shows 11,000 GM, so this is reasonable. I was using the 6v setting on bias until I found a this. It runs along with the error on the 50L6. Use the 25L6 settings exc. 50v heater. Hope this helps. Forgot to update my restoration thread.

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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Sat 27, 2021 2:57 am 
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Calambres:

We have not been at 110v in the U.S. for a very long time. When the Precise 111 was made, 117V was the prominent and moving toward 120. Now, many areas are at 125V so it makes perfect sense for you to use 120V

I understand that testers are going to be different but I think that setting the bias where you like your new tubes to be may possibly cause weak tubes to read above the reject point. I always aim to get the RP correct and let the good and worse tubes fall where they may. Remember that the value for a new tube is an average of many and not a hard number that every new tube attains. I believe that the purpose of a tube tester is to find tubes that may no longer work well in products, not to display the relative "goodness" of good tubes that will work fine and may read differently. So a bias somewhere between 5 and 6 will definitely be an improvement over 9 anyway.

I am not TV MAN but I see he has joined the conversation.

TV MAN:
So did you also conclude that a bias voltage of 6 is reasonable for the 6BQ5 if the RP is 2900?

I see that the 8BQ5 lists the RP at 5700 which may make more sense than the 2900 listed for the 6BQ5. If 5700 is the RP then a new tube average would be (5700/.6) or 9500 on a Precise, not the 11,300 given by the spec sheet. (.6 instead of .65 because the Precise manual indicates that they use 60% not 65% as the RP) I will test some of my tubes using those settings and see what my results are.

Regards.

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Tim

"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Sat 27, 2021 9:36 am 
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Tim wrote:
We have not been at 110v in the U.S. for a very long time. When the Precise 111 was made, 117V was the prominent and moving toward 120. Now, many areas are at 125V so it makes perfect sense for you to use 120V

When I was a kid (and now I'm 61) 220V was the most common grid voltage in the bigger cities with lots of 125V still in many places, being rapidly superseded by those 220V. In the '80s, 125V was quite unusual. Nowadays the nominal voltage here in Spain is 230V but at home I use to have more on the vicinity of 238V or even 240V. The variac is of great help in this case.

Tim wrote:
I am not TV MAN but I see he has joined the conversation.

Two "Tims" talking about the same topic and hence my mistake :mrgreen:

I'll try the proposed settings and will post here the results. Thanks!


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 Post subject:
PostPosted: Feb Sun 28, 2021 11:37 am 
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I've been testing a bit. Using the settings for the 8BQ5, corrected for heater, proposed by TV MAN (BIAS=2V, RP=5700umhos):

Code:
8BQ5 G.E.        8250 umhos
8BQ5 G.E.        8500 umhos   both 8BQ5 labeled as 7500 umhos. I need to set BIAS to 2.5 to get that.
6BQ5 Pinnacle    5700 umhos
6BQ5 Pinnacle    5200 umhos   -> Under RP specs but measures almost full scale in the Em test.
EL84  G.T.       7100 umhos
EL84  Miniwatt   7100 umhos
EL84  Miniwatt   7800 umhos
EL84  Ei         8200 umhos
EL84  Ei         7500 umhos


The datasheet for the 6BQ5/EL84 indicate as "Class A1 typical operation" for 250V plate voltage:

g1 (control grid) voltage = -7.3V
transconductance = 11300 umhos

I don't think the Precise 111 can deliver those 250V plate volts :roll:

[EDIT: typo]


Last edited by Calambres on Apr Wed 14, 2021 6:55 pm, edited 3 times in total.

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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Sun 28, 2021 7:59 pm 
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Correct, it cannot supply 250V, so, as most testers, will not test under the actual conditions in the tube manual. It tests at a different operating point but should reflect the condition of the tube. The average Gm for a new 6BQ5 is 11,300 and if you figure the usefulness is gone at 60% (the figure Precision seems to use), that would put the RP at 6780 (11,300*.6). Instead, Precision lists the RP of the 8BQ5 at 5700 which would mean an average new tube would read 9500 (5700/.6) on the 111. Of course the plate voltage and bias are causing measuring at a different operating point. You cannot go by the Tube Manual numbers to interpret the 111 scale readings.

I also noticed that testing my 6BQ5 tubes with low GM for emission that they showed very good emission. Hence the difference between emission and Gm testers.

I started to test some tubes using the 8BQ5 data and my 111 started acting erratic so now I have to troubleshoot that and see what is going on. :(

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"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Sun 28, 2021 9:03 pm 
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Tim wrote:
I started to test some tubes using the 8BQ5 data and my 111 started acting erratic so now I have to troubleshoot that and see what is going on. :(

I experienced something similar. For some tubes the needle vibrates a lot and, before someone mentions it, yes: I put a new 100uF lytic cap through the panel lugs. With other tubes, the needle was rock solid... tube oscillation?
Some other tubes gave different readings with different Gm scales (20K, 8K, 6K) and thought the 111 somehow lost the calibration I did a few days ago but with other tubes, the readings were consistent through the various scales :?

Also, some tubes reduce the reading when I grab them with my hand, others don't. Those EL84 and relatives are weird!


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Feb Sun 28, 2021 9:50 pm 
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Fixed it. Bad solder joint on the Gm/Em switch.

I do not experience any of the meter vibration that you mention. I have the stock 100mfd cap on the meter. I have seen that though on other testers when the meter cap was open.

I used the 8BQ5 settings at 6V filament and all my 6BQ5 tubes gave reasonable results. Good tubes tested good and bad tubes tested bad. All results were comparable to my RCA-WT-100A, Hickok TV-3B/U and Heathkit TT-1A. I made a note on the roll chart. Good catch TV Man.

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Tim

"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Mar Mon 01, 2021 2:26 am 
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I experienced the wacky meter syndrome on my 111m, where it would bounce, vibrate and do weird things so I found that adding a 4700uF 10v cap across the 100uF meter cap solved most of the problem. It no longer slams, bangs or dances. I did not have this issue on my earlier 111. I also noted that they ARE sensitive to line voltage fluctuations, to the point that if the electric stove was on, my readings would change slightly each time the burner cycled on or off.

In trying the bias differences, I tested 10-12 different tubes and got a usable setting before I discovered the 8BQ5 settings. I am sure there will be some other mistakes in the settings, but as others mentioned, this follows my Hickok 600a and Jackson testers concerning good/bad. A "weak" tube can work fine in most, but not all circuits, same as a good testing tube may not work at all in said circuit. Oscillators are notorious for this. And, yeah, emissions tests often show a weaker tube as good. I noted the 111m does use higher voltages than some other testers, 234 VDC on the G (plate) GM setting, and 300 VAC on the E (EM plate) setting (FYI), which is one of the reasons the 111 is my "go to" tester. It does take a little longer to set the controls v.s. the Hickok, but I feel it is more realistic. It will "light up" the output tube under test, much like in an actual circuit, but has the gentler settings for the small signal tubes.

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 Post subject: Re: Testing 6BQ5/EL84 in a Precise 111 tube tester.
PostPosted: Mar Mon 01, 2021 9:33 am 
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Tim wrote:
Fixed it. Bad solder joint on the Gm/Em switch.

I too had an issue with that very switch. It was indeed a solder problem but not in the form of a bad solder joint or cold solder but excessive quantity of solder. One of the terminals was originally soldered with a lot of tin and a blob flowed down towards the upper contact spade, preventing it to move freely and thus making false contacts continuously. I removed the excess with a solder wick and the problem disappeared but sure it buggered me a lot until I discovered the fault.

Tim wrote:
I do not experience any of the meter vibration that you mention. I have the stock 100mfd cap on the meter. I have seen that though on other testers when the meter cap was open.

Well, perhaps I was not clear in my previous post. It is not that the needle vibrates up and down wildly. It is a very fast and low excursion vibration that makes the needle look blurry. Happens with some of the 6/8BQ5-EL84 tubes I tried but with others it was rock solid. This is why I wondered whether it may be some kind of tube oscillation.

Tim wrote:
Good catch TV Man.

Sure!. It's great you have a later model with the updated roll chart. I couldn't find settings for the 8BQ5 in mine's roll chart (nor for the 6BQ5 for that matter) or in the supplemental BAMA lists so I had to make my own supplemental data listing 8)


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