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Old 20th January 2012, 07:28 PM   #1
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Default Effect of high mains voltage?

Hi,
I have recently received an MS-20L and it appears to be built for a mains supply of 115 VAC. I've measured my mains voltage at 126 VAC, so I checked the heater voltage at the socket and it's at 6.9 VAC. I searched through some old threads and found this graph:
Click the image to open in full size.
Which shows that at 10% higher than rated heater voltage, tube life decreases by 10%...so I'm debating whether remediation is worth the effort...

I can get a variac for $100, but is that the way to go?
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Old 20th January 2012, 07:45 PM   #2
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rather than getting a variac which I would rather use for developing projects try to source a custom (or prebuilt) transformer which will give you the desired voltage drop for 115VAC use.
An isolation transformer might be what you are looking for. It costs a fraction of an ordinary transformer (isolated windings) and will still support large power demands.
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Old 20th January 2012, 07:52 PM   #3
DF96 is offline DF96  England
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A 10% change on a number (valve life) you only know to about one significant figure (at most) is not really a change at all. If you are worried either use a resistor to drop the heater voltage a bit, or a buck transformer on the mains supply.
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Old 20th January 2012, 07:56 PM   #4
Yvesm is offline Yvesm  France
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Interesting to see that under voltage is much more objectionable . . .
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Old 20th January 2012, 08:14 PM   #5
DF96 is offline DF96  England
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A cool cathode has less protection from ion bombardment.
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Old 20th January 2012, 08:55 PM   #6
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is your line voltage measurement accurate? Loaded or unloaded? It may be that you don't have a problem at all...
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Old 20th January 2012, 09:08 PM   #7
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Quote:
Originally Posted by rntlee View Post
Hi,
I have recently received an MS-20L and it appears to be built for a mains supply of 115 VAC. I've measured my mains voltage at 126 VAC, so I checked the heater voltage at the socket and it's at 6.9 VAC. I searched through some old threads and found this graph:
Click the image to open in full size.
Which shows that at 10% higher than rated heater voltage, tube life decreases by 10%...so I'm debating whether remediation is worth the effort...

I can get a variac for $100, but is that the way to go?
I will agree with a previous poster, I do not like Variacs or external transformers, cause the new avaliable power to the amp, will be the only the max Variac power, which is smaller than a electric plant or building big transformer.
I would prefer re-winding the amp transformer to the right tension of the house.
Good luck
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Old 20th January 2012, 09:15 PM   #8
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Try a 12 volt transformer in series with the line to buck the voltage. Place the buck transformer primary after the bucking secondary and you should be dead on. A 4 to 6 amp unit should do fine.
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Old 20th January 2012, 11:11 PM   #9
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More on buck/boost transformers:

Bucking Xfmrs
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Old 20th January 2012, 11:59 PM   #10
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Would something like this work?

Acme T-1-81048 Buck Boost Transformer AT1467 - 120x240 Primary 12/24 Secondary - 0.1 kVA - 60Hz - 1 Phase - NEMA 3R Encapsulated
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