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Old 15th April 2013, 07:08 PM   #1
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Default What happens when Output transformer max voltage (slightly) exceeded?

I'm planning a GM70 build, and am now looking at output transformers. The B+ voltage would be at 1000V (maybe 1050) from 800VAC.

I was thinking of getting a James JS6123, but I found a thread where the manufacturer says its not appropriate for 813 tube, even though its rated at 1000V max. In fact, they even said its only appropriate for 300B and similar tubes. Then I contacted Edcor and asked for a 1200V OT, but they said they cannot do > 1000V .

Of course there's ElectraPrint, Hammond 1628SEA, and the James 6135 which can do GM70 type tubes. And I will go for one of those for safety's sake.

But then there's Pete Millett's 813 , which uses a Tango XE20 which is also rated at 1000V and he uses it at least 800V, close to spec.

So for my enlightenment, what will happen if a 1000V OT is used at 1050V?
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Old 15th April 2013, 07:24 PM   #2
Gilgy is offline Gilgy  United Kingdom
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What happens to any component beyond its design parameters? It becomes unreliable and/or dangerous. In a transformer run right on the edge of its specification I wouldn't expect the winding insulation to last very long.
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Old 15th April 2013, 07:31 PM   #3
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Quote:
Originally Posted by Gilgy View Post
I wouldn't expect the winding insulation to last very long.
Why? would it melt the insulation or break it down in some way?
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Old 15th April 2013, 07:51 PM   #4
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If a transformer is designed for 1kV operation voltage, then the safety margin is presumably 50...100%. I would say that 1050 V for a transformer specified for 1000 V is fully OK.
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Old 15th April 2013, 08:14 PM   #5
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So the max voltage rating is not "absolute maximum", such as in semiconductors or tubes, which will kill the device if exceeded?
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Old 15th April 2013, 09:15 PM   #6
DF96 is offline DF96  England
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Apply 51V to a 50V transistor and it won't necessarily kill the device. It might shorten its life. Same for a transformer. It will probably work fine. If it fails within a few days or months you have no warranty.
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Old 15th April 2013, 09:40 PM   #7
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Quote:
Originally Posted by walangalam View Post
Why? would it melt the insulation or break it down in some way?
Overload is acceptable only for a few seconds, if it is on a steady situation a catrastrophe may occour.
It will do a short-circuit on output tubes and speakers voice coils and maybe a fire in the room.

transformer fire - YouTube.
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Old 15th April 2013, 10:19 PM   #8
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If you are going to go down to 5% differences then perhaps you should make an effort to estimate the minimum voltage across your tube from characteristic curves, and regulation of your power supply.
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Old 15th April 2013, 10:20 PM   #9
JMFahey is offline JMFahey  Argentina
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Quote:
The B+ voltage would be at 1000V (maybe 1050) from 800VAC.
Sorry but no, it will be around 1130V, way over Max.
Quote:
its rated at 1000V max.
Ok, max is MAX , not "operating voltage" where there may be say, a 10% or 20% safety margin built in.
In fact it's good practice to operate at least 10%, preferrably 20% below Max.
Quote:
Pete Millett's 813 , which uses a Tango XE20 which is also rated at 1000V and he uses it at least 800V, close to spec.
"200V "below" max and 200V "above" are both mathematically "close to spec", but are not the same.

Example:
These guys are 4 yards "to the left" of the edge. Close.

Mathematically it would be the same if they were 4 yards "to the right", huh? Still close

Click the image to open in full size.
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Old 15th April 2013, 10:29 PM   #10
VaNarn is offline VaNarn  Australia
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The insulation of the transformer has to be capable of withstanding inductive spikes that may occur as a result of tube failure,clipping and mains fluctuations. L/C resonances can also be triggered which can cause excessive peak voltages.
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