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 Sometimes Tube Testers Lie

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FrankB
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FrankB

Join date : 2010-11-22

Sometimes Tube Testers Lie Empty
PostSubject: Sometimes Tube Testers Lie   Sometimes Tube Testers Lie Icon_minitimeMarch 5th 2019, 10:59 pm

Yes, a tube tester, no matter how sophisticated can lie.
 They only test a tubes operation at certain fixed parameters, not in varying ones like in a set.
 Case in point:
 Mopar 1949 Car Radio.
 Total rebuild. Unable to get osc. to work.
7A4 tested just fine, as did 5 others I tested and tried.
I cleaned the pins on the tube and the socket both. no joy.
 Finally tried one more, a used one, and it worked like gangbusters.
 These were all original brand tubes too. NOS.

So this goes to prove that even though a tube tests just fine in a tester, it may not work in a set.

This was a really common problem in the early days of TV. A set in a big town like New york that had transmitters close by would work with almost dead (And even open fil!) RF amp tubes.
 If the customer lived like 65 -100 miles from the town, even a small drop in the gain of the tube could result in poor or no reception. Yes, the tubes tested good, but had lost a bit of gain. This can make a serious impact if you like to DX radio & TV signals.

The real test it using a tube in a set to see if it works to your satisfaction.

 Now I like to use a tube tester to check for opens, shorts, grid emission, etc, and to sort tubes. Don't continue to test a tube if it shows a short. Embarassed  I lost a B&K 707 tube tester that way when I wasn't paying close attention.

In early B&W and Color sets, it was not uncommon to find a 6SN7 that suddenly wouldn't oscillate properly. Typically the vert. & horiz. osc. sections both used them, (and later on 6CG7/FQ7 tubes). Often swapping the vert. & horiz. osc tubes would restore perfect operation. Obviously some parameter in the tube changed and at the different freq. involved, it just wouldn't work anymore. at one freq. but worked fine at a different freq.

 (Yes, the proper repair was to replace the tube with a new one, but some folks back in the 50's & '60 were on hard times and even a couple of dollars was important. Remember, bread was .10 a loaf then, and gasoline was .19 a gallon. I remember making service calls for $5.00 to $7.50 to the house back then, and charging from $1.50-$3.50 an hour in that time period.)

The real test it if it works.
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chas
Senior Member 75+ Posts
Senior Member 75+ Posts
chas

Join date : 2017-04-09

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PostSubject: Re: Sometimes Tube Testers Lie   Sometimes Tube Testers Lie Icon_minitimeMarch 6th 2019, 10:36 am

Oscillator/mixer tubes, the pentagrid converter usually require two tests one as a triode and the other as the mixer. The grid/plate of the oscillator slowly gets contaminated and fails as a "plate". This also occurs in the notable 1L6 and FWIR the 6A8 in some of the AA5's. Some tester will do both tests, often passing the oscillator portion. The tube testers filament voltage can come into question.

Often said, the best test is the receiver itself...well dunno sets vary, even an identical set could have circuit tolerances that one tube would work in one and not the other...

YMMV

Chas
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FrankB
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FrankB

Join date : 2010-11-22

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PostSubject: Re: Sometimes Tube Testers Lie   Sometimes Tube Testers Lie Icon_minitimeMarch 6th 2019, 5:15 pm

Chas,
 You are dead on.
I had forgotten about the 6A8 tubes being a pain too.
 You are correct that 2 identical sets can have tolerances differing, and tubes working in one and not the other.
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Cliff Jones
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Cliff Jones

Join date : 2010-11-22

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PostSubject: Re: Sometimes Tube Testers Lie   Sometimes Tube Testers Lie Icon_minitimeMarch 6th 2019, 6:45 pm

Most tube-testers also cannot test under real conditions, but give phony signals (not RF just 60 & 120 Hz) although I remember one or two actually had oscillators, lower voltages for most dynamic and some Mutual conductance types.

The only way would to be measure everything in circuit. That's why I would love to have a TV-2 at least it can measure screen and grid and plate currents and also voltages. But at those prices? NO WAY

_________________
I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
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Rod Clay
Technician
Technician
Rod Clay

Join date : 2018-08-01

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PostSubject: Re: Sometimes Tube Testers Lie   Sometimes Tube Testers Lie Icon_minitimeMarch 11th 2019, 10:20 pm

I used to see a lot of TV-2 tube testers for sale at hamfests that went begging but probably not anymore. At the time, all those meters looked intimidating. I went with a TV-7D and a TV-10. I preferred the TV-10 after a while. I found a very good roll chart for it featuring communicatons and transmitting type tubes. 

At one time I was reconditioning National NC-200 receivers for myself and other hams that like to operate AM Phone. It is a good set for that purpose. National revised the HFO circuit in the NC-200 over that of the earlier 1930s sets with the aim of improving stability and warm-up characteristics. They went with the proven high C oscillator circuit with some temperature compensation added. I found in practice, at least some 50-60 years later, that the choice of 6J5 oscillator tube became a critical one. Many 6J5 tubes refused to oscillate on the lower end of some of the bands. The amount of capacitance in the tank circuit was quite high on the lower bands. So I tried various 6J5 tubes to find one that would work over the whole range. One set I was working on for friend was particularly bad in this regard. I finally found one very ancient looking 6J5 that would work. After delivering the set, I found out later the new owner, not liking the looks of this tube, replaced it with a nicer looking NOS JAN 6J5 tube. I never said a word about it to him.
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Cliff Jones
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Cliff Jones

Join date : 2010-11-22

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PostSubject: Re: Sometimes Tube Testers Lie   Sometimes Tube Testers Lie Icon_minitimeMarch 12th 2019, 1:46 pm

The TV-2 has a very hard learning curve. It can do more though, than any other of the TV series.
It takes more time to set up also. More of an engineers tool.
But other than the prices asked for this model (upwards of $2000) I would love to Have one!

_________________
I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
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PostSubject: Re: Sometimes Tube Testers Lie   Sometimes Tube Testers Lie Icon_minitime

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