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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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29th Sep 2022, 9:06 am | #1 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Output transformer simulation using resistors etc.
If I want to test a power valve like a 6L6 or EL34 in a mock OP stage without an OPT how can I go about it? An OPT has a small primary winding resistance so a 100r or similar resistor would do there, but how to represent the impedance - a capacitor? Or would one use a choke say, in series with a 4-6k ish resistor to ground?
I'm sure this subject was covered in the last few years but I can't find the thread,TFL, Andy.
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29th Sep 2022, 10:03 am | #2 |
Dekatron
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Re: Output transformer simulation using resistors etc.
You could use a choke, paralleled by a resistor of the load impedance you want to load the valve with.
Or - better still as an accurate simulation - just use a beefy resistor. Down side is you'll need to use a far higher HT voltage, to compensate for the DC voltage drop in a pure resistor. So if you're testing a 6L6 with 300V on the anode, 70mA anode current, and 3.6kΩ load impedance, you'll need a 552V test supply so that the quiescent anode voltage is the 300V that you want. But the valve won't know any different from the choke/resistor, or the actual circuit with a (good) OPT.. |
30th Sep 2022, 2:56 pm | #3 | |
Dekatron
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Re: Output transformer simulation using resistors etc.
Quote:
I know I can work a lot of a valves working parameters out from a load line but often a datasheet doesn't have a graph for certain screen grid voltages or if one wants to use a valve designed for non audio use like big TV beam tetrodes a lot of info isn't on the datasheet, hence the question. Andy.
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30th Sep 2022, 5:52 pm | #4 |
Dekatron
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Re: Output transformer simulation using resistors etc.
I wonder how a purely resistive load could dynamically simulate an ultra-linear circuit beloved of the audiophile types who never let beefy valves loose to deliver their full capabilities.
You need something inductive if you want to be able to swing the anode voltage significantly below the screen voltage. And be aware that some of the more powerful tetrode valves can both source and sink screen current when working to their full abilities!!
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30th Sep 2022, 6:13 pm | #5 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
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Re: Output transformer simulation using resistors etc.
Why would you want to test without the transformer? Could you not salvage one from a scrap set, or use a generic one from the likes of RS? Not only would this provide an easier solution, but you could also hand a scope on the output of the transformer which would allow you to see distortion.
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30th Sep 2022, 6:43 pm | #6 | |
Dekatron
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Re: Output transformer simulation using resistors etc.
Quote:
Frequency? Just use a frequency such that the choke's reactance is 10 - 20 times the parallel load resistance. You've got a free hand there, up to a few hundred kilohertz if you want! Ultra linear... Choke would have to be tapped, which makes things complicated. But that wasn't part of Andy's initial remit! |
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1st Oct 2022, 8:07 am | #7 | |||
Dekatron
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Location: Sleaford, Lincs. UK.
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Re: Output transformer simulation using resistors etc.
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To clarify I'm after testing some horizontal/vertical output valves like the 6BQ6GTB to find the best primary impedance so I can then buy or wind an OPT. As mentioned because these valves weren't designed for audio use their datasheets don't have all relevant info, by simulating an OPT I can find the best operating point. I did have a 25W OPT with tapped primary's of 10k to 4k, like an idiot I sold it some time ago. Thanks all, Andy.
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1st Oct 2022, 8:57 am | #8 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: Output transformer simulation using resistors etc.
Hi Andy, why not just use any suitable mains transformer you have. Then calculate the required power R value to go on the sec of your choice to present the valve with it's required Z
Using a CT sec winding as the anode winding you can simulate PP stages Ed |
1st Oct 2022, 10:11 am | #9 | |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
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Re: Output transformer simulation using resistors etc.
Quote:
I can't find the thread and don't know if it would be of use to you anyway. John |
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1st Oct 2022, 10:25 am | #10 | |
Dekatron
Join Date: Feb 2007
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Re: Output transformer simulation using resistors etc.
Quote:
It'll be a different value of R of course, but then you do have a free hand! |
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