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Old 25th September 2008, 07:38 AM   #1
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Default high AC mains problem

my home mains measured 239VAC instead of the supposed 230VAC.
as a result, my transformer secondaries for 0-5vac measured 5.9vac instead.

can i still use it as filament supply for the 5AR4?
is there an easy way to drop a little of this voltage?
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Old 25th September 2008, 11:05 AM   #2
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This is well within 5% which is fully acceptable IMHO.
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Old 25th September 2008, 11:23 AM   #3
Mooly is offline Mooly  United Kingdom
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Is that voltage off load ( the secondaries that is ) . A -/+5 % change in mains voltage will change the secondary proportionally. So 230 -/+ 5% is 218.5 to 241.5 and the secondary if it were 5 volt is 4.75 to 5.25 volts.
The 5 volts will be at some rated current, draw less and the voltage rises.
5.9 volts is 18 % over.
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Old 25th September 2008, 12:04 PM   #4
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As a temporary quick fix, you can use the APC Models LE600I and LE1200Iu voltage regulators. They are cheap and will give you time to find a permanent solution. I have a similar problem with voltage going to 127V. Still trying to find an affordable long term solution. I am interested in the APC H10 and H15 products, but unsure he they are audiophile grade.

surfing the 2nd harmonic ... @@@@
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Old 25th September 2008, 12:23 PM   #5
timpert is offline timpert  Netherlands
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The 5.9V you measured, was that without a load? If so, the voltage will surely drop to about the right voltage once the tube filament is connected to it.

Don't worry too much about these things though. In my part of the world, the mains specification is 230 +/- 10 V. Up to a few decades ago it was 220 +/- 20 V, and tubes have no problems with these variations. The heater voltage is typically specified as this-or-that voltage +/- 10%. Unless you're building some very accurate measurement gear, you'll be fine.
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Old 25th September 2008, 03:26 PM   #6
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The measurement is done without load.

Looks like i should measure again with load before deciding on a fix.
I read that a quick fix would be to stick a small R resistor to each of the heater legs. Correct?

My other problem is the 280-0-280 winding is putting out 364-0-364 on my multimeter without load.
Its giving me a bit of a headache as I do not know what values to input PSUD.
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Old 25th September 2008, 03:39 PM   #7
Mooly is offline Mooly  United Kingdom
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You can only determine a value when it's all running and the heaters are at correct temperature. Whatever, the value will be small.
Again your HV supply needs loading first.
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Old 25th September 2008, 04:11 PM   #8
kevinkr is offline kevinkr  United States
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Your transformer's primary and secondaries all have appreciable resistance and unloaded the voltages you measure will be appreciably higher. Fully loaded at the nominal line voltage the transformer should meet its specification. This design approach allows for smaller cores, and less copper - smaller gauge wire, but makes getting the desired voltages a bit more tricky. (Generally they run both hotter and noisier than more conservatively designed transformers as well.)

IMO filament voltages on US and European types should be within +/- 5% of nominal for best performance and life.. A lot of people bandy about the +/- 10% number which seems to be more common with Russian and Eastern European made tubes.

For PSUD measure the dcr of your secondary winding and the unloaded voltage and use those values in the transformer model dialog.

Your line voltage is within the +/- 5% criteria if the transformer is properly loaded and you may not need to do anything about it. Worst case some series resistance will fix the problem.

Should you still be concerned about line voltage issues you can build a boost/buck box using a suitably rated filament transformer. You connect the primary across the incoming ac mains and the secondary in series with the load, you can configure to boost or buck by swapping the polarity of one individual winding. You can also do things with a 12.6VCT transformer that are quite interesting by selecting both polarity and which tap you are using. You can get -12.6V, -6.3V, 0V, +6.3V, +12.6V relative to your nominal line voltage. Larger or smaller voltage steps are possible by selecting different transformers, and if you want to get really fancy you can cascade one or more different voltage transformers to get even more adjustment range. Simple configurations are generally cheaper and much quieter (mechanically) than a variac, once you go beyond 2 transformers and about 5 - 6A surplus variacs start to look more attractive.. In the case of two transformers connect both primaries across the incoming line and cascade the secondaries - you are just starting to get into tap changer territory here.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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