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Old 29th Sep 2022, 8:06 am   #1
Diabolical Artificer
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Default 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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Old 29th Sep 2022, 9:03 am   #2
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Default 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..
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Old 30th Sep 2022, 1:56 pm   #3
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Default Re: Output transformer simulation using resistors etc.

Quote:
You could use a choke, paralleled by a resistor of the load impedance you want to load the valve with.
But then we'd need more HT again as the R would have a big V drop? I'm guessing the choke would then allow for a higher voltage swing because of the inductance? I guess we'd need a high inductance choke, though thinking about maybe not as we don't need to test at all frequencies.

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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Old 30th Sep 2022, 4:52 pm   #4
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Default 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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Old 30th Sep 2022, 5:13 pm   #5
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Default 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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Old 30th Sep 2022, 5:43 pm   #6
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Default Re: Output transformer simulation using resistors etc.

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Originally Posted by Diabolical Artificer View Post
But then we'd need more HT again as the R would have a big V drop? I'm guessing the choke would then allow for a higher voltage swing because of the inductance? I guess we'd need a high inductance choke, though thinking about maybe not as we don't need to test at all frequencies.
No, you'd use the HT voltage you'd expect. The quiescent anode voltage would be equal to the power supply voltage (strictly, it would be a bit less owing to the choke's DC resistance, but it would be a few volts - easily made good). The parallel resistance would have no effect.

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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Old 1st Oct 2022, 7:07 am   #7
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Default Re: Output transformer simulation using resistors etc.

Quote:
I wonder how a purely resistive load could dynamically simulate an ultra-linear circuit
I'm not bothered about that, just a straight forward power test.

Quote:
Why would you want to test without the transformer?
Because I haven't got one. Removing one from a set isn't an option, as I've no set and by set I presume you mean a radio which would have a small 1-3w OPT, not much use in this case. Re RS the choice of OPT's is very limited and I have no cash to spare at present.

Quote:
No, you'd use the HT voltage you'd expect. The quiescent anode voltage would be equal to the power supply voltage (strictly, it would be a bit less owing to the choke's DC resistance, but it would be a few volts - easily made good). The parallel resistance would have no effect.

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!
Thanks, I'll find or wind a suitable choke and have a play.

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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Old 1st Oct 2022, 7:57 am   #8
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Default 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
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Old 1st Oct 2022, 9:11 am   #9
John M0GLN
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Default Re: Output transformer simulation using resistors etc.

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Originally Posted by Diabolical Artificer View Post
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?
I'm sure this subject was covered in the last few years but I can't find the thread,TFL, Andy.
There was a simple circuit on here a few years ago for testing KT66's to make sure they didn't have thermal run away, it didn't need an output transformer and I built it and I ran a KT66 for a few hours.
I can't find the thread and don't know if it would be of use to you anyway.

John
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Old 1st Oct 2022, 9:25 am   #10
kalee20
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Default Re: Output transformer simulation using resistors etc.

Quote:
Originally Posted by Ed_Dinning View Post
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
Why not just put the R across the primary though, then you don't have to consider turns ratios, winding resistances, imperfect coupling factors...

It'll be a different value of R of course, but then you do have a free hand!
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