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Old 22nd February 2007, 02:03 AM   #1
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Default need a 120vct transformer... where to look?

I am building a small guitar amplifier and need a transformer. It needs to be 120vct @ ~150ma.

As you probably guessed, its another line operated design, but I am improving on it. Aside from using a power transformer, using a FWB will give me a secondary of about 158vdc, which is better for a 50c5. That eliminates the need for a 35xx tube for the rectifier, and allows me to make a much cleaner power supply. I would like to run the 50c5 in series with a 12xx tube, and wire them from one side to the centre tap on the transformer.

This keep parts count to a minimum, and allows a better power suppply design. Since I have a lot of these tubes around, I could make a lot of amplifiers for some people that I know that are looking for the kay/harmony/silvertone sound without the dangers. I reallise there are better ways to do this, but this is what I want to do.

I don't know where to begin to look for a transformer other than tubesandmore and tubedepot, so I was wondering if any of you know of a good place I could order them from.

Also, I was wondering if using one half of the secondary to run the power hog heaters if it would mess anything up /unbalance the power supply?


I gues if I had to I could just use a low wattage iso tranny an throw a big power resistor in series with the tubes, but I don't really want to do that if I can get the right voltage utilizing the transformer centre tap. I seem to remember at least one or two of those old designs that did just this very thing.
Thanks much, ~Tsd88
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Old 22nd February 2007, 04:10 AM   #2
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Default Found your transformer

Mouser Electronics www.mouser.com has a Triad N51X, Mouser Part number 553-N51X for $12.00. Its rated at 300mA. Good luck with your project.
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Old 22nd February 2007, 04:26 AM   #3
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Default If you must go center tapped

Angela Instruments has some Hammond transformers 167G120 120ct FL volts, 127.00 NL volts, 0.5 amps. EACH $17.00
www.angela.com
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Old 22nd February 2007, 09:26 AM   #4
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In the UK, we force our builders to use 110V tools powered from a centre-tapped supply in order to keep the voltage of each phase to 55V (making it less dangerous than 240V when they accidentally damage the cable and it touches scaffolding). I imagine that such transformers are made in sufficient quantities that they are reasonably cheap.
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Old 22nd February 2007, 12:31 PM   #5
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Default Powering 50V +12V tube in series

If you choose the Hammond transformer you could tie the anode of a 1N4004 diode to the two secondary legs, as attached.
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Old 22nd February 2007, 10:21 PM   #6
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Hey thanks for the quick replys!

Wouldnt your drawing be for a 120-0-120 transformer? I guess I could use an iso transformer with a big resistor for the heaters. Remember I'm also using it as b+.

I guess my question is , is it ok to use a 120vct transformer like this ( see photo )

Would this cause an imbalance in the supply, or would it be ok as long as I allowed for enough current that it did not sag under load of the heaters?
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Old 23rd February 2007, 11:32 AM   #7
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Default All depends what you are looking for....

The circuit I drew provides about 84 VDC as shown, with a 150 ohm dropping resistor to provide current limiting. You can still use the same transformer for your B+ see attached.
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Old 25th February 2007, 08:27 PM   #8
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ohh I see now! Its the same as mine basically, except you rectified thefiliments and added a current limiter. I asume the limiter is to keep one side of the transformer from drawing too much curent, and thus being loaded down so the voltage sags?

Also, would it be ok to just run them off AC? 82v seems a bit high for 62v worth of heaters.
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Old 25th February 2007, 08:52 PM   #9
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So called control transformers on ePay, probably from Square-D
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Old 26th February 2007, 12:24 AM   #10
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Default One more time

Quote:
Originally posted by ThSpeakerDude88
ohh I see now! Its the same as mine basically, except you rectified thefiliments and added a current limiter. I asume the limiter is to keep one side of the transformer from drawing too much curent, and thus being loaded down so the voltage sags?

Also, would it be ok to just run them off AC? 82v seems a bit high for 62v worth of heaters.

By using the 2 diodes at the top to rectify the 120 VCT you get 84.8V VDC if fed into a capacitor. Since the filiments of a 50C5 draws 0.15 Amps The equivelent resistance would be 333 ohms, the 12 Volt tube would be an 80 ohm eqivalent and the 150 ohm resistor would drop about 22.5V. The 120V on the bridge rectifier should yield 168 VDC on the output of the brige / cap.
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