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Old 21st February 2014, 09:02 PM   #1
zman01 is offline 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 21st February 2014, 11:07 PM   #2
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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, 12:58 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 01:01 AM.
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Old 22nd February 2014, 01:42 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, 08:51 PM   #5
zman01 is offline 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, 09:36 PM   #6
zman01 is offline 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, 09:45 PM   #7
DF96 is online now 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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