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Old 2nd June 2013, 04:39 PM   #1
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Hey Anyone,

There used to be a link here to an article "Rating Unknown Power Transformers."
But it seems to have disappeared. I want to use an Eico ST70 power transformer in Poinz's EL34 MM but using 6550 instead of EL34. I want to get closer to the 35 watt capability of the Eico OPT's. the original tube compliment of the ST70 totals about 300ma. I would need closer to 400ma for the outputs. (I will run the 6GK5's from a different power supply.)

From what I remember of the article the parameters necessary are much the same as the transformer data entered in PSUDII. My question is this, if you design a power supply using PSUD if you are overtaxing the B+ windings will it know? And will it tell you?

Kevin
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Old 2nd June 2013, 05:00 PM   #2
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Is it what you are referring to?
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Old 2nd June 2013, 05:42 PM   #3
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As Little Cheeser says, thank you! thank you! thank you!

If possible, could the DIY Audio gods change the title of this thread to "Rating Unknown Power Transformers" so anyone can find it?
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Old 2nd June 2013, 05:52 PM   #4
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Quote:
If only two high-voltage leads are found, check to see if the centre-tap is grounded internally.
From para4.Was this common?
Is it done with modern EI transformers?
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Old 2nd June 2013, 07:04 PM   #5
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Andrew, I can't tell you about modern transformers. But I do know that some manufacturers put fuses between the center tap and ground inside the transformer.
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Old 2nd June 2013, 07:06 PM   #6
AndrewT is offline AndrewT  Scotland
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I have heard of internal Thermal fuse in the primary.
But not from centre tap to ground.
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Old 3rd June 2013, 01:55 AM   #7
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Quote:
Originally Posted by mr2racer View Post

My question is this, if you design a power supply using PSUD if you are overtaxing the B+ windings will it know? And will it tell you?

Kevin
It won't tell you explicitly as far as I know, however the B+ voltage will continue to drop as you increase the current draw. After measuring the unloaded voltage, you can assume that the voltage regulation is typically 5-10%, giving you a reasonable voltage window.


I've measured a few unknown transformers based on the article, and it will get you somewhat close. Measure the DCR of the secondary and look at the graph.
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Old 3rd June 2013, 03:49 AM   #8
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Thanks Boywonder,

I checked the Eico per the article instructions and it should be good for about 250ma. The original tube compliment totals about 265ma. So I'm in the right neighborhood. I plan to have Heyboer build me another power transformer so I can build a pair of 6550C MM monoblocks. They'll have 438 volts of B+ at 94ma per 6550. With 188ma for the outputs and 24ma for the CCS 6GK5's, at 212ma the transformer will have some nice headroom. The amp should produce a little over 30 watts. With the Z out of 4k the 16 ohm becomes 8 and the 8 ohm becomes 4. Thanks for the help so far!

Kevin
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