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Old 22nd April 2008, 01:37 AM   #1
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Default modified sine wave power inverter

Hello guys,

I was just reading that thread about the battery powered tube amp off ebay; and I figured I'd mention this...

I have this Black and Decker 200 watt power inverter (12 VDC to 115 VAC). Now, any amp I plug into it; the actual amp it's self (I think the transformer) makes a really audible and scary sounding buzz. My solid state amps weren't as bad; but when I plugged it into my 6V6/5Y3 rig it made a buzzing so loud I didn't even wait for the filaments to start glowing before I turned it off..

Now, the manual for this inverter says it makes a "modified sine wave." In the manual it shows a small graph comparing a sine wave to a modified sine wave; and it looks like your everyday square/pulse wave. Is it safe to run tubes off of this?

Edit: If anybody actually wants to "hear" it I can get a decent recording no problem..
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Old 22nd April 2008, 01:44 AM   #2
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the power trannies won't like it because of the high harmonic content. If you can get past the noise (mostly due to mechanical tx construction...the laminations vibrating), there will be more power tx losses and likely some noise from heater to cathode coupling. I wouldn't use it for those reasons alone, but it may be safe to run if the power tranny doesn't overheat. This also depends on the 'pulse' width, which is created by the unit so that the RMS voltage is 120VAC, but the peak may not be 170V like a pure sinewave... so this might affect the rectified B+ output voltage.
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Old 23rd April 2008, 12:43 AM   #3
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Default so, no

so basically, no?
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Old 23rd April 2008, 05:55 AM   #4
fizzard is offline fizzard  Canada
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Unless you want to lowpass the square wave into a 60Hz sine. Those inverters are really only good for resistive loads like heaters. You can get some pure sine wave inverters.
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