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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Sat 02, 2017 1:31 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3010
Location: Florida
For a tube tester using the basic Hickok bridge circuit you can "calibrate for GM" by setting up a current source and then recording the corresponding "Sensitivity" knob setting for a particular meter reading. I did this for my B&K 707 and recorded the settings for "100" meter scale readings that correspond to 2000, 3000, 4000, 5000, and 10,000 uMhos.

For instance, a knob setting of 93.5 gives a reading of "100" for 2,000 umhos, so 50 is 1000, 75 is 1500, and 120 is 2400 umhos, The other ranges are set up similarly.

My 707 knob settings are 93.5 for 2,000; 82.5 for 3,000; 72.5 for 4,000; 65 for 5,000; and 49 for 10,000 uhmos ; at a meter reading of 100. Different testers will have unique settings.

RRM


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Sat 02, 2017 3:34 am 
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Joined: Apr Sat 30, 2011 2:14 am
Posts: 578
Location: Mississauga, Ontario
That's great information to have!
I also have a 707, I'm going to test this out.
If you can tell me how to set this up I'll experiment with this on the 747.
I wonder how close it would be to the 707?
Thanks for pointing this out.


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Mon 04, 2017 2:54 am 
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Joined: Sep Wed 03, 2008 1:24 am
Posts: 791
The late Alan Douglas, in his book on tube testers, published the results of a test comparing readings on an emissions tube tester and one that measures Gm. I believe the emissions tester was the Eico 666.

Anyway, there was very little correlation between the strength of the emission reading and the Gm. Some tested low on both, some tested high on both, while others were all over the place.

Different tube testers use different operating points, and often have their own pass/fail criteria based on testing some sample tubes at the factory.


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Mon 04, 2017 4:44 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 9402
Location: Powell River BC Canada
There is certain aspects of tubes that manufacturers are keenly aware of, especially
when faults get linked to using their brand. Then they tweak them.

One such was the bugaboo when using 10 megohm gridleaks in the triode part
of dual detector triode tubes. I'll try and find how a British tube maker fixed a distortion
problem with their tubes .

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Mon 04, 2017 4:05 pm 
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Joined: Jul Sun 09, 2006 3:11 am
Posts: 5098
Location: Aurora, CO
I have a similar situation when testing a Cunningham CX345. On my Triplett 3480 emission tester it tests 70% emission which by experience I know is a good reading for a 45. On my Hickok 600A using the good/bad scale, it's barely good. Using the 2000 scale, it's about 1100. Just for grins I broke out the 752. It tests 500 where 575 is minimum. The tube works fine but probably won't be a candidate for Ebay.

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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Mon 04, 2017 6:35 pm 
Member

Joined: Apr Sat 30, 2011 2:14 am
Posts: 578
Location: Mississauga, Ontario
That kind of makes sense to me now.
The 45 is a tube I'm familiar with and I know that even tubes that test low in conductance perform just fine in radios.
There's a lot of life in those tubes!


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 Post subject: Re: 6SN7 tests strong on Emmission tester and weak on GM tes
PostPosted: Dec Tue 05, 2017 2:04 am 
Member

Joined: Sep Thu 23, 2010 6:37 am
Posts: 9402
Location: Powell River BC Canada
Among testers here are 747 as well as a TV-7 B/U and 1-177B.

Perhaps using the Alan Douglas technique of a current sense resistor
in series with the plate, and an a vector test socket, p-p grid volts,
and amps/volt data could be gathered. From this trans conductance
comparisons could me made.

The issue is this: Sure, you know what the 747 and military testers
report for a given tube.

Then a further measurement of p-p grid voltage vs p-p plate-cathode
voltage could be easily made using the same setup.


This would work well for tubes with very high plate currents.

However, getting a meaningful value of amps/volt for a 6SN7
would be difficult unless the sense resistor was large. In that
case the plate conditions would be changed.

Few have lab grade resistors and meters with multi digit precision
in the micro volt ranges.

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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