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 Post subject: 6SN7 tests strong on Emmission tester and weak on GM tester?
PostPosted: Nov Wed 29, 2017 3:35 am 
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Joined: Apr Sat 30, 2011 2:14 am
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Location: Mississauga, Ontario
Seasons greeting to all,
I kind of know the answer to this but really was hoping to hear it explained so I get that "uh ha!" Moment.
I have a Sencore TC-162 emission tester and a B&K 747 gm tester, both recently calibrated. The Sencore tests this 6SN7 tube at 85% and the B&K puts it at 48%.
I've seen this type of result before but never such a difference.
So, is this tube good for my amplifier or is it better in the scrap bin?
I have a few of these tubes so not an issue, it's more of an educational thing for me and wondering which tester I should trust.
Thanks for any insight.


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 3:44 am 
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Other than testing for gas or leakage, best test is in the equipment...

If there isn't a noticable difference in, uhhh, a "good" tube use it...


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 3:57 am 
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Location: Maryland 20709, USA
The only proper test is in the actual equipment that uses the tube.

You cannot validly compare results from different tester models from the same company.

Tube testers are good for finding shorts or open filaments, or for matching pairs as in p-p amps.

- Leigh

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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 4:06 am 
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Location: Syracuse NY
An emission tester checks to see how much current flows from the cathode to the grid. I have a Sencore TC-136. When I'm testing horizontal output tubes, I get the same results with or without the plate lead connected. Yes, the lead is good because when I test a tube with a grid cap, it fails if it's not connected.
Some other emission testers allow you to disconnect individual pins and look for a difference.

A Gm tester checks the ability of a tube to amplify (Gm).

I use a curve tracer, which shows me the Gm over a wide range of plate voltages and grid bias. Since I buy big boxes of loose tubes, I see lots of bad ones. In general, tubes with very low emission on the curve tracer will show up as bad on an emission tester. I have also seen tubes with good emission, but with low Gm. The curve tracer also finds tubes which exhibit a loss of Gm slightly above the published current.

What does it all mean? An emission tester is fine for troubleshooting. It can tell you if a tube is shorted (this should be the first test), burned out, or weak.
A Gm tester gives a more complete picture of the tube's condition.
A curve tracer is more useful to those of us who like to dig deeper, but overkill for repair work.

Leigh brings up a very good point - tube testers do not agree with each other!

Regardless of which method is used, a tube tester is a very useful tool for weeding out defective tubes, but the equipment it's used in is the final authority.

Bob

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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 7:01 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10972
Location: Mpls, Minnesota
That is a lot of difference between the testers.
1. Bad tube, try a different tube and see if the results are the same
2. Tube testers need repair
3. Incorrect setup information

Dave


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 7:08 am 
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easyrider8 wrote:
That is a lot of difference between the testers.
True, but remember...

Tube testers are not calibrated at the factory, they're "adjusted".

This involved using a reference tube that had been tested on some tester, with values noted.
It might have been the same model as the production units, or a different one.

The idea was to make the new tester read the same on that tube as the original readings.
All new testers were _presumably_ adjusted for the same reading.

But in a large production department, there would be several reference tubes in use.
But tubes age, line voltage varies, bla bla bla...

- Leigh

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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 7:14 am 
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Quote:
So, is this tube good for my amplifier or is it better in the scrap bin?

As 35Z5 said, try it and see. A "weak" tube might serve very well as a low level audio amplifier but be unusable as an audio output tube (some wire recorders did this) or vertical oscillator. And even if it doesn't match a "good" tube, it might still be good enough. It all depends on the application.

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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 9:39 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Mpls, Minnesota
Leigh, Hickok did not use a "calibration tube" at the factory, they used a constant current source. I went through their Cleveland factory in the late 50's courtesy of the factory rep.

Dave


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 11:13 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4879
Location: 253 Blanche St. Plymouth, MI USA
Basically, an emission tester is a very simple tester. It will catch most "bad" tubes, which can be weak or have shorts. It will "pass" some bad tubes as its not really operating the tube as an amplifier, but rather as a rectifier, with all the elements tied together except the cathode and filament.
A Mutual Conductance tester is better by far. It will catch MORE bad tubes than the Emission tester. Still wont catch some bad tubes (noisy tubes, intermittent, etc).
Also, no one tester will test all tubes from 1920 to 1970. You need at least two, an often more. I own 7 testers, and I use different ones for various purposes.
Mark Oppat


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Wed 29, 2017 2:16 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1541
Location: Lafayette, CO
l have seen "weak" or marginal output tubes really boom. It's all subjective.....testers tell general condition. Just another tool in troubleshooting. Craig


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Thu 30, 2017 1:50 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
Read the manuals and get to know your equipment. The B&K 747 uses the transconductance method to test tubes but nowhere does it say that it actually measures transconductance. The only scale that matters is the "Bad-?-Good" one. The 0-100 scale is arbitrary and only used for relative comparisons; it is not percentage of actual transconductance or anything else. The only things you can calibrate on it are the shorts/leakage, grid emission, grid voltage, and meter zero; there is no transconductance calibration. Actually there is one other thing: you have to make certain the exact original type pilot light bulbs are installed as one of them is used for a ballast resistor in the grid voltage circuit.

Comparing two tube testers that operate on different principles is like comparing a tire pressure gauge to a water meter. Yes they both respond to pressure but if they ever agree it would only be by coincidence. If the 6SN7 has no leakage, gas, or shorts, and good emission, then it is possible the lower Gm results are due to its plate resistance being at the high end of the tolerance range. (Plate resistance is not measured by ordinary tube testers and was subject to variation in manufacture as much as 40%). Interesting thing is, a tube with above average plate resistance will actually produce more voltage gain in-circuit even though it tests lower on a transconductance tester.

Don't chuck the tube. Put it in the equipment and measure the grid, plate, and cathode voltages with an accurate VTVM. Do they match the voltages on the schematic reasonably closely? If so, the tube is fine.

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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Thu 30, 2017 2:21 pm 
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Joined: Apr Sat 30, 2011 2:14 am
Posts: 579
Location: Mississauga, Ontario
Thanks for all the replies and suggestions.
Kind of what I already thought but the different explanations were quite informative.
I've this exact situation before but never such a wide gap. I've never seen it go the other way though.
I rechecked calibration and they were both spot-on.
I further tested the results on 2 other testers, a B&K 606 which agreed with the Sencore TC-162 and a Hickok 800 which agreed with the B&K 747.
Just one ofvthosethings, I guess?
Thanks to all for the education


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Thu 30, 2017 2:49 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7212
Location: Long Island
It actually makes sense that a tube with weak emission will also usually have low transconductance. Transconductance has to be measured at some definite plate voltage and load current which are normally determined by the specs for each tube type. So if a particular tube has low cathode emission, plate current will be low and the transcoductance reading will be low too.

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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Thu 30, 2017 3:44 pm 
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Joined: Apr Sat 30, 2011 2:14 am
Posts: 579
Location: Mississauga, Ontario
One would think so but I have the opposite condition,
Strong emissions and weak transconductance.


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Thu 30, 2017 8:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4879
Location: 253 Blanche St. Plymouth, MI USA
My late dear pal Chris Haedt became a tube tester expert but died in 2005 I think it was. He was doing a fantastic job of exploring why tube testers dont agree and also why some tube types can vary so much on even the same tester when they dont vary as much on others. In particular he was focusing on "filamentary triodes" like the 2A3, 6A3, 45, 50 and others.
What he found mainly was that Hickok and others did not drive the tubes strong enough to make the test reliable. Think of it like riding a bike too slow, its wobbly and could sometimes give very odd results. Those tubes were given strong signals in the circuits they were used. He had a LOT of data he was developing when he passed suddenly. At the time we tried to find one person to take over the biz but found no one and had to part out his stuff. One tube seller (Mike Marx at SND Tubes sales) got most of the research files. I read some of it but it was mostly WAY over my head in data.
I believe if you do an search for author "Chris H" here on ARF you will find many of his posts still exist and are very informative.
Mark Oppat
Antique Audio


Last edited by oldradioparts on Dec Fri 01, 2017 12:00 am, edited 1 time in total.

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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Thu 30, 2017 8:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4879
Location: 253 Blanche St. Plymouth, MI USA
HEre is an old post that really explains testers well

http://antiqueradios.com/forums//viewto ... 8&t=193114

Mark Oppat


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Nov Thu 30, 2017 9:12 pm 
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Location: Maryland 20709, USA
oldradioparts wrote:
HEre is an old post that really explains testers well
/viewtopic.php?f=8&t=193114
Mark Oppat
Hi Mark,

Most of the photos in that thread are no longer accessible due to the PhotoBucket nonsense.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Fri 01, 2017 3:17 am 
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Location: ZIP 23831 South of Richmond, VA 25 miles down the pike.
On your B & K 747 what does your 48% equate to in µmhos?
Bill J.


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Fri 01, 2017 9:41 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
The 6SN7 has a Gm of 3000 at 90 volts with a plate current of 10 mA


The 6SN7 has a Gm of 2600 at 250 volts with a plate current of 9 mA




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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Fri 01, 2017 7:42 pm 
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Joined: Apr Sat 30, 2011 2:14 am
Posts: 579
Location: Mississauga, Ontario
NumberMaj 1 wrote:
On your B & K 747 what does your 48% equate to in µmhos?
Bill J.


Bill,
Although the 747 is a GM tester there is no translation from their scale to actual GM numbers, unlike the TV7 and TV2 military testers which have a published correlation.
Mauro


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