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Old 15th January 2007, 11:14 PM   #1
TGRANT is offline TGRANT  United States
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Default Lowering voltage output of transformer

I came across a 400-0-400 transformer that has a 6.3 and 5 volt winding as well. The secondary is a bit high for what I want to do - I plan to put about 250-280v at a few miliamps on the plates (amp still in the design phase).

I want to drop the voltage down a bit. If I drop about 70 volts with the rectifier tube (GY3), Iíll still have more than I want. Is there an easy way to do this (read: cheap, reliable, low part count)? I thought of using a classic voltage divider with some high power resistors (say in a 1:8 ratio) after the rectifier tube but before the RL section. Is this a bad way to do it? I havenít seen any schematics that do this, so my guess is that thereís a problem with it that Iím not seeing.

Any suggestions on a good way to do this? Iíve read about using different cap and choke values to drop voltage, but I donít know how to calculate these yet. Iíd like to use the transformer since it was cheap.

Any schematics out there? Iíve searched the forum and the net but canít put my hands on them.

Thanks
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Old 16th January 2007, 12:25 AM   #2
nhuwar is offline nhuwar  United States
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Well easy way use a dropping resistor on the secondary.

But if you want a more controlled approach buy a variac of ebay it works like a rheostat but it's a transfomer.

The common one you find is rated for 1 kva usually 120 to 140 volt out you get one of these on your primary and your good as gold.

Nick
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Old 16th January 2007, 01:00 AM   #3
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There have been extensive posts on this subject, with a number of good suggestions. I would recommend performing a few searches with buck, boost, transformer, turns, etc.

Essentially, the simplest solutions are adding turns with unused windings, choke input, and resistor dropping.

With what you are looking for, it sounds like dropping across a resistor is impractical.
It also produces poor regulation.
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Old 16th January 2007, 01:09 AM   #4
kevinkr is offline kevinkr  United States
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Choke input would be my recommendation with the lossiest tube rectifier you can find.. The 5Y3 comes to mind as one possible candidate, another possibility would be the 5R4, lightly loaded though it might not drop as much as hoped.

Download PSUDII from Duncan amps and figure it out.. With choke input be sure that you shunt sufficient load current to ground through a power resistor such that the output stays in regulation while the tubes in project warm up..

Bear in mind that the 5Y3 should not be operated into a capacitor input filter at plate voltages in excess of 350Vrms, the limit for choke input is 500Vrms.
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Old 16th January 2007, 11:01 AM   #5
TGRANT is offline TGRANT  United States
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Thanks to all who replied.

Of course, now I see that Moe, Larry and Curley would be proud of me. I think I made the problem harder than it should have been. I downloaded PSUDII from Duncan amps and realized that if I use a choke input followed by an RC filter - since I need two different voltages Iíll use two, and I got exactly what I need.
By the way, the rectifier tube I was planning on using is the 5Y3 which can drop 30-40 volts. My quiescent current is between 10-20 mA, so Iíll be within the tubes parameters. The PSUDII is a nice little program.

Since this is my first tube project, I want to use what I have on hand if possible rather than purchase a bunch of new stuff. If I get better, Iíll go that route and aim for perfection.

Thanks again.
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Old 16th January 2007, 12:56 PM   #6
kevinkr is offline kevinkr  United States
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Sounds like a perfectly reasonable approach to me.. I'm always scouring the countryside looking for any old transformers and chokes - I then sit on them until the ideal project appears on the horizon. (Sadly the old hamfests which used to be great source of cheap iron have been decimated by eBay, and most of the iron there is not as cheap.)

Yes PSUDII is a helpful program.

Most of all have fun with your first tube project!
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Old 17th January 2007, 08:17 AM   #7
Merlinb is online now Merlinb  United Kingdom
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Incidentally, because choke input filters need massive and very specific chokes, a couple of cheaper methods are:

-Zener diode(s) between centre tap and ground.
-Use a small 100V transformer as an auto-transformer on the secondary , so it doesn't affect the heater voltage as a variac would.
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Old 17th January 2007, 08:27 AM   #8
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There's also the option of trading the transfomer with someone who has a better candidate.

Not as simple, but did you consider a tube regulator in the B+ ?
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Old 17th January 2007, 10:29 AM   #9
TGRANT is offline TGRANT  United States
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I didnít consider a tube regulator - mostly because I donít know how to design one yet. I cobbled together a supply using PSUDII. If I interpreted the data from PSUDII correctly, using the 5y3, a 10H choke and two RC networks (2.7K, 3.3K and a load of 15K), the simulator kicked out values near to what I need. I picked the 10H choke because Iíve seen that in other designs - Iím not sure how to calculate that value yet.

By the way, I had a little trouble figuring out how to interpret the data on the table in PSUDII - Iím assuming the RMS value of the resistors is the voltage drop, which I subtracted from the input. The row entitled VD1, which I think is the voltage drop across the tube, had some weird numbers that donít make a lot of sense to me. The mean voltage was -370 and the RMS was 572. For example, using the values above in the simulator, the table showed mean values of VD = -370, VL = 0.731, Vr1 = 76, Vr2 68 and Vr3 = 221. I did this: 400-370 = 30v, which Iím thinking is the diode drop, then adding the rest of the values to that gives me 395V - close enough to the 400V I started with. Is that the correct way to calculate the values or am I out there?
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