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Old 23rd May 2019, 12:44 am   #1
radiograham
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Question Using a GDO

I have a Qmax gdo which i use every now and again in the usual way to check resonant frequency of a LC circuit,recently i have been winding some coils for a 625 line tv if strip and wanted to check them.In this IF strip as i understand it there are no parallel caps to tune the coils,this depends on the stray capacity of the valves.I get that but wanted to ask is it still possible to use a gdo to check the coils.The circuit is from the Mullard design for a 625 line experimental tv which was published in about 1960 using ef80s.I have built this before using the coil details from the book with good results,this time i am using smaller diameter coil formers hence the above query. Regards Graham.
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Old 23rd May 2019, 3:09 pm   #2
Bazz4CQJ
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Default Re: Using a GDO

I cannot think of any reason why that method should not work. However, what might be a useful exercise would be to wind the coils, but before putting them in-circuit, stick one or two fixed capacitors across them (e.g. 5 or 10pF) and use the GDO to see where they resonate, and that will also tell you their inductance. Then when you put them in the circuit, you have some reference points. IIRC, the IF is about 35MHz?

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Old 23rd May 2019, 8:18 pm   #3
Ed_Dinning
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Default Re: Using a GDO

Hi Graham, if you can see if you can find a Q meter; the Advance T2 is usually a good and reasonably priced buy. It goes up to 100MHz and will tell you all you need to know about your coils.

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Old 25th May 2019, 12:18 am   #4
radiograham
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Question Re: Using a GDO

Quote:
Originally Posted by Bazz4CQJ View Post
I cannot think of any reason why that method should not work. However, what might be a useful exercise would be to wind the coils, but before putting them in-circuit, stick one or two fixed capacitors across them (e.g. 5 or 10pF) and use the GDO to see where they resonate, and that will also tell you their inductance. Then when you put them in the circuit, you have some reference points. IIRC, the IF is about 35MHz?

B
Hi Yes this is what i did with my coils,i just wondered if the stray capacitys in the circuit are there when the circuit is on and if i could use the gdo to then check them .The if in this case is the usual 39.5mhz. Graham.
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Old 25th May 2019, 1:57 am   #5
Bazz4CQJ
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Default Re: Using a GDO

Mmmm, I have never thought about how the capacitance in a valve RF circuit changes between being powered on or off, but I'm sure someone on here does!

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Old 25th May 2019, 10:10 am   #6
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Default Re: Using a GDO

I've never heard of any specific requirement for the valve to be heated when doing GDO-measurements - not even in situations where in normal-use the grid will be driven hard so significant grid-current flows (class-C RF amps).
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