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Calculate Total Power Xformer Current


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9/7/2000 11:59 PM
Daver
Calculate Total Power Xformer Current
I know this has probably been discussed before, but....... How do you correctly calculate the total current needed to be supplied by a power transformer? I thought all 12A*7 types used 3mA ( same as heater current) and you'd use max plate current for the output tubes. Near as I can figure, for, let's say a Super Reverb, it might take 200mA. Specs for replacement power trannys that I've seen list 140mA. Can anyone give me the formulas and the straight skinny on this? Thanks!  
Daver
 
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9/8/2000 12:13 AM
eric

go to www.duncanamps.com and just do a search for your tubes. Then just write down the mA's used for each tube. BTW, the heaters for a 12ax7 is more like 30mA not 3mA. You need to remember to look for the correct voltages. The heaters use 6.3vac on you tubes, so you need a good deal of current on that secondary (its in the multiple amp range for that one) if you want a tube rectifier, the most common ones use 5vac (if you want to build an old fender) The other voltage is your HV secondary and that's the one that really powers the tubes and is only a couple of hundred mA for most amps
 
9/8/2000 12:21 AM
Daver

Eric,  
Sorry, fat fingers on the 3mA/30mA. I think it's 300mA anyway for heaters. I guess I wasn't clear. I know what is required for heaters. I need to know the current for the B+ needed total. I'm sure it must be peak current, not idle current, for the power tubes. I'm just not positive on how to correctly figure this. Thanks.  
Daver
 
9/8/2000 12:31 AM
eric

give me your tube lineup. and yes it is 300mA for the 12ax7 my bad. also, what voltage are you looking at your power tubes seeing  
eric
 
9/8/2000 12:35 AM
Daver

Eric,  
Two 6L6GC's @ 440VDC, 4X12AX7, 2X12AT7. Similar to a Super Reverb tube comliment, but a different circuit. I'd like to know how to figure this for any tube compliment. Thanks!  
Daver
 
9/8/2000 4:26 AM
Carl Z
Re: Calculate Total Power Xformer Current(sort of long)
Daver;  
 
Ideally what you need to do is do some load line calculations for a given power output. However this gets pretty involved as you need to take projected voltages into account, load impedances and bias points. What you're asking is how to design a power supply and this is probably the hardest thing you'll ever do when it comes to amp designs. The RIGHT way to do it is to do the design work, calculate the approximate current demand and then hook the amp up to a regulated power supply and measure it. If you're just shopping for a power transformer this will get you in the ballpark...enough to figure for the requirements at least.  
 
The preamp tubes will be running basically class A so the current demands will be fairly constant. Assuming you're biasing the tubes to around 1mA each section you're going to have about 12mA needed by the preamp.  
 
Where your big draw is going to come from is the power tubes. Assuming you're running them at about 35mA each at idle that comes to around 70mA. However we need to figure the requirements during full power operation. In a perfect world you'd only need 140mA at full power but tubes waste a lot of power as heat so we'll double that value to about 270 to 280mA. You can fudge a little here because you probably won't be running the amp full tilt for any extended period of time so this will give you a healthy safety margin.  
 
So, for the B+ winding you're looking at around 270mA + 12mA = 282mA  
 
Next thing is the heater winding. This is easy. each preamp tube needs 0.6mA and each power tube needs 1A. Just add them up. So your heaters need to be rated for 6A.  
 
Here are your transformer specs;  
B+ current rating of 320-0-320@280mA  
heater rating of 6.3vac @ 6A  
 
As a final note, if you've got a transformer and you want to figure out if it's under rated for your application, you can take a few quick measurements and find out.  
 
Make sure the transformer has not been used or plugged in for a good 24 hours to make sure the core's at room temperature. Then take a DC resistance measurement of the secondary. Now, hook up the transformer and run it under typical operating conditions...translation: thrash the hell out of it! Now unhook it and take another set of DC measurements on the secondary. At this point it's grade school arithemetic.  
 
[(R hot)/(R cold) * 257.5]-273 = winding temp in celcius  
If this value is lower than the insulation rating, then you're good to go.  
 
Carl Z  
http://www.geocities.com/summitamps
 
9/8/2000 12:50 PM
eric

you beat me to it, but you explained it much nicer than i ever would. The only question i have for you now carl, is how do you take into account the fact that he wants to do the amp at 440V, i could calculate the mA requirements at that voltage, do you then have to compensate for the lower voltage with a higher current with V=IR?
 

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