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Old 21st February 2014, 10:02 PM   #1
zman01 is online now zman01  Bangladesh
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Default Noob question on power transformer primary input voltage

Many of the available power transformers come with a 120/240 volt primary input voltage rating. The country where I live in has mains voltage that usually ranges from 220-230 volts.

If I opt for a a 240v input rated power transformer, how would that affect the voltages in a tube amplifier? My limited understanding tells me that B+ would be slightly lower. Any other key voltages that might get affected?

Thanks in advance.
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Old 22nd February 2014, 12:07 AM   #2
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Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
Have you actually tested your wall Current?....the voltage you are actually getting...right now?
Yes if your wall current is at, say 224 VAC out of the wall & you use a transformer that is rated with 240VAC specifications, you will get a slightly lower output on the secondary.
At one point I tested the wall voltages, once at 222VAC, another time I got 217VAC....
Power companies are expected to have some degree of regulation, say plus or minus 5--8%. Some transformers will have 220,230, and 240 VAC primaries, selectable by re-wiring. selecting differing input wires.
Do the math.............What target voltages are you trying for?(B+) What form of rectification? By simply changing to a different rectifyer tube you can alter the output voltages for your B+ quite easily.
Look up Duncans power Supply simulator....

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Old 22nd February 2014, 01:58 AM   #3
laplace is offline laplace  Australia
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220V is 8.3% lower than 240V so your transformer secondary voltage will be 8.3% lower too.

I second the suggestion to (carefully!) measure your mains, but do it a few times in a 24-hour period to observe the magnitude of daily variations. At a previous house I lived in, we frequently browned out to 190V (long, overloaded power lines in the country, neighbor running huge refrigeration plant) despite being in a nominally 240V country. Moved to the suburbs and now I get a rock solid 243V.

I use my Smart UPS to monitor the line voltage; I get 24-hour graphs from it. They can be quite revealing of your and your neighbour's power consumption patterns.
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Last edited by laplace; 22nd February 2014 at 02:01 AM.
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Old 22nd February 2014, 02:42 PM   #4
fpitas is offline fpitas  United States
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Besides measuring your mains (a good idea in any event), you can design around a lower B+ voltage, and perhaps compensate in the power supply design as noted above; but the filaments are sometimes a bit pickier. Going to a DC constant current source, or regulated DC voltage for the filaments would be an option.
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Old 23rd February 2014, 09:51 PM   #5
zman01 is online now zman01  Bangladesh
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Thanks all for your inputs.

Mains is ~220-225v when I measured IIRC. I can also run from an online UPS which delivers constant 230v.

I was more worried about the impact on filaments - but as fpitas has pointed out a DC CCS or regulated DC voltage design would take care of that.
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Old 23rd February 2014, 10:36 PM   #6
zman01 is online now zman01  Bangladesh
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BTW, I have posted this question in another thread, but what is the role of a bias voltage tap on a power transformer? If the amp design does not require it but the chosen PT has a bias voltage tap, is it OK to keep it unused?
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Old 23rd February 2014, 10:45 PM   #7
DF96 is offline DF96  England
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By 'tap' do you actually mean tap or winding? Anyway, generally you can simply leave unconnected anything you don't wish to use. Just make sure it can't short to anything else.

8% low on heaters should be OK, but is getting a bit near the normal low limit of -10%. It doesn't leave much room for mains variation.
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