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Old 6th January 2011, 04:08 PM   #1
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Default produce 80Vdc with a 480-277 1 phase power transformer

I need about 80V at about 7 amps to convert a 3 channel tube organ amp to solid state. I'm not building a split supply amp, I don't like replacing speakers that they haven't made them that way for 40 years. I've made enough mistakes over the years the output capacitors in my ST120 and the output transformers in my ST70 have saved my *** several time.
Has anybody used a 480VAC to 277 VAC line frequency power transformer to cut 120VAC down to 70 VAC? I can get a 500 VA unit for about $200 plus freight. link Acme Single Phase Transformer Sales, Acme Transformers on Sale! part number GP12-500s.
I calculate 7.2 amps at 70 volts if you hook it up 480-277 and run it on 120 VAC.
I understand old ST120's show up on E-bay but I don't have a pay-pal account, I don't have computer enough to even look at their listings, and I don't like having the freight being to-be-determined depending on which UPS store kicks back the most to the vendor.
I bought a non-functioning PV1.3K for $60, which has split 80V supply, but I don't think the power transformer would like it if I only ran one side of the center tap out to run three chip amps or something. I'm sure not repairing it then hooking those 10 output transistors to my Hammond speakers and dare them to fry them. Also it is too big to fit in the organ case without a lot of sawing down.
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Last edited by indianajo; 6th January 2011 at 04:16 PM.
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Old 7th January 2011, 01:01 AM   #2
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What exactly to you want this transformer to do? Is it a 120V 60Hz to 70V step down with isolation or will an autotransformer do?
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Old 7th January 2011, 01:41 AM   #3
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Thanks for listening. Yes, for hum control it should be a real transformer, not an auto-, or a low noise switcher supply. Adjustable auto-trans howl anyway when the carbon brush arcs, not good for audio. I use an auto-t to cook dormant tube equipment at low voltage. I live in the 60 Hz western hemisphere. I'm going to produce 3 ST120 clone audio channels with series output capacitors at about 40 VDC, PS is single rail 72 VDC regulated by BJT's from something, possibly 80VDC. The organ has 8 transistor preamps driven now by 100 VDC produced by regulating down from 380 VDC with a resistor. I'm not changing the preamps, I need 80-100VDC at milliamps for them. I do hope to get rid of the dual 5AR4 tube power chassis. Because the tube sockets are congenitally loose (all over the internet about this model being crackly) and because the tube socket grounding rings are not available anywhere, it is really easier to just transistorize. But I really don't want to spend $200 on a power supply transformer. This new 480-277 VAC 1 phase transformer is a possible, but I would have to find a salvage one to make my budget. I'm also thinking of stacking up 3 each 32 VDC @ 4 A wall transformer ($1 each) salvaged from HP printers, to make 90VDC at 4A but it would make a mess on my power chassis, and not quite enough current for one 40W bass channel and 2 12W mid-range channels, all at 8 ohms.
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Last edited by indianajo; 7th January 2011 at 01:49 AM.
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Old 7th January 2011, 02:45 AM   #4
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Late update. I found a 750VA transformer with 380-415 VAC input, and 240 VAC @ 60 Hz output. If I put 120 VAC 60 Hz on the 380V input, I calculate 69 VAC @ 10 A, out. $65 with freight. Please, someone tell me why this is a stupid idea before tomorrow.
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Last edited by indianajo; 7th January 2011 at 02:51 AM.
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Old 7th January 2011, 02:52 AM   #5
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I would stay away from a 480/277 (415/240 in my country) transformer, the regulation will be very poor when run at 1/4 voltage. Common and cheap are 24VAC control transformers from industrial equipment. Maybe 2 x 24 + 1 x 12, But try the industrial surplus places first. I managed to pick up a 52V 25A transformer last year and I am currently using a scrounged 56V CT transformer in a subwoofer amplifier so there are transformers out there. A 48V fork truck charger or telco 48V psu might have a suitable transformer.

Alternatively replace the 5AR4 with silicon diodes and put either a choke, resistor or series pass regulator in line to keep the original voltage.
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Old 7th January 2011, 03:36 PM   #6
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Thanks for your input. If I maintain the 27 tubes I have 25 tube sockets to change, and 25 socket grounding rings to manufacture out of sheet brass. Reseating the tubes due to vibration is a real nuisance. I have had the brass for a year and replacement sockets and haven't successfully made one grounding ring. I really need to make a stamping die, and I am not a machinist and do not have a lathe, mill, or drill press. Also 5 tubes are 7199's and one is an EF86, which are no longer available. As I currently have about 68 W power output and the 380-240 transformer is rated 750 VA, maybe the poor regulation-current loss won't hurt me much. Been reading the "can I use 220 VAC transformer at 110VAC" thread, think perhaps the responses there are pertinent. Maybe the poor current source ability will keep me from blowing the Hammond speakers at 200W/channel.
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Last edited by indianajo; 7th January 2011 at 03:40 PM.
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Old 8th January 2011, 05:23 AM   #7
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Now I see the problem. The thread you mentioned is pretty much on topic. Some alternatives are a pair of 300W 110V to 35-0-35 V toriodal transformers, they are really easy to add turns to if you get one which is say 10V less than what is needed.
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Old 8th January 2011, 09:15 PM   #8
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Thanks for responding. Not really sure which toroids are okay for 60 hz and which are only for hf inverter drive, as I proposed using a 2:1 toroid out of a blown PCAT switcher PS front end (pre-rectifier off the 240-120 switch) and several people pointed out it had to be a hash suppressor, not a 240-120 vAC 60 hz transformer, and totally unsuitable for 60Hz because it didn't have any iron in it. So, ?? Anyway, checked apexelectronic.com in LA before ordering, and they had 115-95 VAC E-frame transformers, also 115VAC-73VAC and 115VAC-67 VAC, and because the volt amps weren't specified, I bought two of each for total $45 because I might have to parallel a couple for the power amp and use the 95 one alone for the 100 VDC preamp power. They were all about 6 time wider and taller than a quarter coin, whatever that means. I hope it means about 150 VA or so, but we will see.
I was just out in the garage a minute (it's 19 deg F outside, good electronics weather) fumbling around deciding whether to package these power supplies in a weird wedge shaped steel box dead 1500 W electric heater , or in an elegant 2.5"x10'x14" aluminum casting box from an ishida scale. I'm leaning to the nasty looking heater box because of the iron. These transformers are not encased in steel like the ones in my Dynaco amps and the ones on the organ itself, so probably I need all the iron I can get to keep the hum out of the amps and preamps 2.5 feet away. I've got a lot of really nice looking stainless motor drive boxes with heat sinks, nice for the amps but useless for keeping the RF hum in. The hammond 400V@1A transformer is so iron enclosed that it also has copper sheet wrapped around the E laminations, cool but unreproduceable sixties technology. Thanks again.
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File Type: gif apex-TR16-W-67VCT.gif (63.3 KB, 50 views)
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Last edited by indianajo; 8th January 2011 at 09:44 PM.
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Old 9th January 2011, 10:55 AM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by indianajo View Post
I need about 80V at about 7 amps
is that +80Vdc and is that continuous 7Adc that the circuit requires?
If the 7A is a peak and the average is much much lower, then a 300VA to 500VA transformer can do the job.
But, if the 7Adc is continuous, then the transformer must be >14Aac
a 115:30-0-30Vac transformer will give about 80Vdc but it could be anywhere in the range 300VA to 1500VA depending on the continuous DC current that you require.

How did you arrive at 7A?
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Old 9th January 2011, 08:01 PM   #10
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The Dynakit ST120 produces 72VDC regulated with a shutdown feature at 6.5 A, which I have reproduced and tested to trip point with an adjustable 10 ohm 250W resistor load before the amplifier stages. If I were going to use one transformer for the organ, i would like it to be lightly loaded to continue to produce the 100 VDC for the preamps while producing at the mininum 40W for the bass and 12W each for the two 8" mid-range speakers, the same as the current H182 tube amps. With the dual or triple small salvage transformer model, I can run the 97V transformer independently for the preamp voltage, and provide 1,2,or 3 each 67VAC or 73VAC transformers for the 3 stereo 120 copy channels I'm going to build.
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