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Can't find a tube checker for a 407A




 
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October 22, 2018, 07:43:33 PM *
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Author Topic: Can't find a tube checker for a 407A  (Read 530 times)
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k7mdo
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« on: September 18, 2018, 10:16:08 AM »

A friend handed me a bunch of 407A tubes, mostly appear new in the box.  He asked me to check them but I've had no success.

My TV-7DU lists the tube and does show about 1/2 the prescribed meter reflection but never anywhere near the specified "good" reading.

My Hickhok 600a does not list the tube as an "a" but does list "407" without the suffix.  Attempted test as a 407 with no success.

My SECO lists the tube and on testing it finds one half of the tube as "good" but shows no deflection for the other half.

I've not ever run across the tube before, nor any other 20 volt filament tube for that matter.

Ideas?.



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wb1ead
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« Reply #1 on: September 19, 2018, 11:43:36 AM »

Hi Tom..frustrating for sure I'm guessing..let me point you on another "avenue"..here at my test bench is a Sylvania capable of ALL testing including gain/transconduct..often a "new" to me tube comes along..1st thing I do if it isn't on the roll chart or additional charts I have is to look up the specs in several manuals here and ALSO the tube basing..important is the fil voltage plus ALL other tubes that fall under this basing..for instance as an example if I had the 4 number 12v equivalent of the std 6146 I would use the settings for the 6146 and set the fil voltage for 12.6v..it does work out..I've done it..now the basing for the 407A is 407A..however if you compare this diagram to the 8CJ basing then it certainly appears the same..what tubes are in this family of basing?..the 2C51 and
the 5670..they both use 6.3v for the fil setting..if I had to check a 407A here I would use the 5670 settings I now have and advance the fil voltage to 19v..(don't have 20v)..I'm not saying this is the way but it might give you a "clue" YMMV..this ought to allow you to test both sides equally if ur tube tester covers the two tubes I mentioned..if you think I'm outta my mind..OK..but it is at least some hint on how ya might get ur answer...73 de DAVE
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AMer livin in "Moose Country"
k7mdo
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« Reply #2 on: September 20, 2018, 12:26:46 AM »

What you say makes all the sense and I will give it a go tomorrow....  Can't hurt to try.

I was only somewhat surprised that the TV-7DU didn't even seem to work.

Will report results.

73, Tom
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wb1ead
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« Reply #3 on: September 20, 2018, 01:33:15 PM »

Hi Tom..yup give it a go..not sure on ur TV-7DU unless the tube you tried to check cudda been a dud or one side gonzo??..here BTW are two more from the 8CJ family..6386 and 7861..b/4 you do check them please take a moment and compare the two basings 407A and 8CJ just so you will see the almost identical diagrams of these two..it'll assure you I think..the only oddball thing on the 407A is that one tube manual lists the fil voltage as 20v AND 40v..??..I'm thinking misprint or maybe two together in a series string??
                           Anyhoo the other 6 I have say 20v filament/heater..let us know if and how it works and if ur tester is like mine in any respect at least the test switch to check gain is fused and spring loaded..to eliminate AH OH!!   73 de DAVE
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k7mdo
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« Reply #4 on: September 21, 2018, 11:23:11 PM »

Well, yes, the tube was checkable  as other similar based tubes but without a figure of merit the only thing I could say was that of about the ten on hand, none checked significantly better or worse than any other...  Thus I think we can assume all are good, but that is it... 

That was good enough for the fellow that asked me to check them...

In my history of collecting tubes and filament transformers i can't recall a 20 volt filament before and I wonder if in the TV7 checker the 20 volt setting has ever been used?  It did seem like the meter reflection came closer to the acceptable by turning up the filament setting to a little higher voltage!  Next time I have the cover off i will check it's accuracy...  Should have done it first but got lazy.  Too many projects.

Thanks for the ideas, 73,Tom
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #5 on: September 22, 2018, 09:50:29 AM »

Hi Tom..yup give it a go..not sure on ur TV-7DU unless the tube you tried to check cudda been a dud or one side gonzo??..here BTW are two more from the 8CJ family..6386 and 7861..b/4 you do check them please take a moment and compare the two basings 407A and 8CJ just so you will see the almost identical diagrams of these two..it'll assure you I think..the only oddball thing on the 407A is that one tube manual lists the fil voltage as 20v AND 40v..??..I'm thinking misprint or maybe two together in a series string??
                           Anyhoo the other 6 I have say 20v filament/heater..let us know if and how it works and if ur tester is like mine in any respect at least the test switch to check gain is fused and spring loaded..to eliminate AH OH!!   73 de DAVE

The filament connections are a little odd. When you power the filament between pin 1 and 9, it needs 40 Volts. When you tie 1 and 9 together and power between these pins and pin 5 (CT of the filament), you need 20 Volts
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kb4qaa
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« Reply #6 on: September 22, 2018, 03:41:12 PM »

Won't using the CT method help eliminate AC hum, just like in a transformer?  Seems like a advantageous method.
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #7 on: September 22, 2018, 08:41:14 PM »

The CT is more to be able to use the tube at 20 or 40 volts. The main application of this tube was a negative resistance amplifier to amplify telephone lines bi-directional. The filaments could be powered by the 24 or 48 Volts system battery of the telephone central or repeater station. Many thousands of these negative resistance amplifiers were in use in the years 50. So hum wasn't a problem, they were powered by DC
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