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Old 5th Jan 2016, 11:41 am   #1
D Cassidy
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Default Homebrew R-C Bridge

Hello and Good Morning.
A few years ago I was given some items from a local hobbyist that he had been given from a deceased Radio Amateur.
Over the holiday period I decided to sift through some of the items. I came across a neat little R-C bridge. It uses a VR130 ex government valve and just a few other components. The valve is an oscillator to drive phones. It originally was built in November 1947 but must have been modified later on for dry batteries. It came with a D cell dated 2005 which still was supplying 1.36v the two PP3 batteries were leaking.
I replaced the PP3's and connected phones, within seconds the oscillator was running even though the 2v filament valve is under-run. I've checked the bridge with various resistors and capacitors and it seems spot on. Not bad for being homemade in 1947 and sitting redundant for a good few years.
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Old 5th Jan 2016, 5:51 pm   #2
newlite4
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Default Re: Homebrew R-C Bridge

That's just lovely! It would make an ideal valve starter project for a newcomer to the hobby.
Neil
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Old 5th Jan 2016, 6:31 pm   #3
greenstar
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Default Re: Homebrew R-C Bridge

Could it be this (with thanks to American Radio History and their search facility)?
Although the date is a puzzle, the article is December 1947 - unless your's was the prototype!
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Old 5th Jan 2016, 6:39 pm   #4
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Default Re: Homebrew R-C Bridge

Superb example of KISS (keep it simple stupid), I think you could make it an RLC bridge by adding a few inductors.
 
Old 5th Jan 2016, 7:35 pm   #5
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Default Re: Homebrew R-C Bridge

While you obviously can add 'reference' inductors, you can run in to two problems. One is that the inductance may depend on the current through them. The second is that they may magnetically couple to the component you are testing.

In general it is better to balance an R-L pair against a C-R pair, so you can actually use reference capacitors to measure inductance. A book like 'Measurements in Radio Engineering' (Terman) should explain this and also show how to add another variable resistor to be used to null out the resistance of the component under test.
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Old 5th Jan 2016, 11:21 pm   #6
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Default Re: Homebrew R-C Bridge

Quote:
Originally Posted by greenstar View Post
Could it be this
It certainly looks like it even if the magazine article has a serious fault which the homegrown version hasn't according to the schematic in posting #1.

Aah, but which one ?
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Old 6th Jan 2016, 9:40 am   #7
D Cassidy
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Default Re: Homebrew R-C Bridge

Hello.
How very interesting. The bridge I have isn't missing the grid resistor as it is in the PW article.
I believe that these items I was given came from a Mr Jackson, on the table a date in November 1947 pre-dates this to the PW article.
Neil, I agree with you wholeheartedly, the bridge is an excellent little project for someone's first attempt using valves.
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Old 6th Jan 2016, 10:02 am   #8
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Default Re: Homebrew R-C Bridge

Bridges have been around a long time and with the components available in any era there may be completely independently made things which turn out to be quite similar. Or Mr Jackson could be the author of that article?

David
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