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-   -   Effect of high mains voltage? (http://www.diyaudio.com/forums/tubes-valves/204948-effect-high-mains-voltage.html)

rntlee 20th January 2012 07:28 PM

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:
http://www.diyaudio.com/forums/attac...s-lifetime.gif
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?

Alexontherocks 20th January 2012 07:45 PM

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.

DF96 20th January 2012 07:52 PM

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.

Yvesm 20th January 2012 07:56 PM

Interesting to see that under voltage is much more objectionable . . .

DF96 20th January 2012 08:14 PM

A cool cathode has less protection from ion bombardment.

aardvarkash10 20th January 2012 08:55 PM

is your line voltage measurement accurate? Loaded or unloaded? It may be that you don't have a problem at all...

FullRangeMan 20th January 2012 09:08 PM

Quote:

Originally Posted by rntlee (Post 2871248)
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:
http://www.diyaudio.com/forums/attac...s-lifetime.gif
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

simon7000 20th January 2012 09:15 PM

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.

Speedskater 20th January 2012 11:11 PM

More on buck/boost transformers:

Bucking Xfmrs

rntlee 20th January 2012 11:59 PM

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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