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Old 26th March 2006, 06:13 AM   #1
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Default step-up travel transformer for small preamp?

I have seen those small 50 watt 120v to 240v transformers floating around, and I have often wondered if that sould be used to power the B+ of a small preamp. 240v*10mA=less than 3 watts, so I dont see why power-wise it would not work. I don't know if they are current-isolated from the mains, but I was hoping someone here might have some more info.
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Old 26th March 2006, 06:15 AM   #2
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Something like this:
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Old 26th March 2006, 10:16 AM   #3
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They are usually autotransformers so not suitable.
Check for continuity between the 110V tap and the 240V tap.

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Old 26th March 2006, 10:17 AM   #4
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Is that a conventional power xfmr? I used to use something that small for a bias supply.

I guess it would be OK for class A duty. The steady current requirement (especially if used for a class A preamp) would draw little problem from the potentially bad load regulation characteristic of the xfmr. I would probably want a substantial filter capacitance and only ask a few mA from it.
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Old 26th March 2006, 06:04 PM   #5
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Quote:
Originally posted by lndm
Is that a conventional power xfmr? I used to use something that small for a bias supply.

I guess it would be OK for class A duty. The steady current requirement (especially if used for a class A preamp) would draw little problem from the potentially bad load regulation characteristic of the xfmr. I would probably want a substantial filter capacitance and only ask a few mA from it.
That is what I figured. I am thinking of experimenting with one running a single 12ax7 tube for the preamp and a 12au7 output buffer, so I would not be drawing more than half a dozen milliamps.

I have seen a few advertised as "isolated" so I suppose they would work.. maybe I'll get a few and see which are autotransformers and which ARE actually isolated.

I usually like to run regulated voltage in my preamps anyway, so I am not overly concerned with poor tranny regulation.
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Old 27th March 2006, 12:40 AM   #6
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I have found that the cheapest way to get 240VAC is a Triad N-68X. $11.20 at Mouser and it is good for 50 watts. It has 2 115 volt primaries and 1 115 volt secondary. Turn it around (use the secondary as a primary) and you have a 230 volt center tapped secondary. with a FWB rectifier I get almost 300 volts DC at over 100 mA. They also have a 6.3 VAC 3 amp filament transformer (41FD030) for $6.98.

So you have the power supply iron for $18. With the Edcor $18 output transformers I have all of the iron for a good sounding SE UL amp for $54!
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Old 27th March 2006, 05:53 AM   #7
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Quote:
Originally posted by tubelab.com
I have found that the cheapest way to get 240VAC is a Triad N-68X. $11.20 at Mouser and it is good for 50 watts. It has 2 115 volt primaries and 1 115 volt secondary. Turn it around (use the secondary as a primary) and you have a 230 volt center tapped secondary. with a FWB rectifier I get almost 300 volts DC at over 100 mA. They also have a 6.3 VAC 3 amp filament transformer (41FD030) for $6.98.

So you have the power supply iron for $18. With the Edcor $18 output transformers I have all of the iron for a good sounding SE UL amp for $54!
that's a great tip. Thanx. I have to do a Dale resistor order @ Mouser anyway, so I may as well toss one of those on.
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